Treatment of Lyes—Evaporation to Crude Glycerine—Distillation—Distilled and Dynamite Glycerine—Chemically Pure Glycerine—Animal Charcoal for Decolorisation—Glycerine obtained by other Methods of Saponification—Yield of Glycerine from Fats and Oils.
Treatment of Lyes—Evaporation to Crude Glycerine—Distillation—Distilled and Dynamite Glycerine—Chemically Pure Glycerine—Animal Charcoal for Decolorisation—Glycerine obtained by other Methods of Saponification—Yield of Glycerine from Fats and Oils.
As pointed out in Chapter II. the fatty acids, which, combined with soda or potash, form soap, occur in nature almost invariably in the form of glycerides,i.e., compounds of fatty acids with glycerol, and as the result of saponification of a fat or oil glycerine is set free.
In Chapter V. processes of soap-making are described in which (1) the glycerine is retained in the finished soap, and (2) the glycerine is contained in the lyes, in very dilute solution, contaminated with salt and other impurities. These lyes, though now constituting the chief source of profit in the manufacture of cheap soaps, were till early in last century simply run down the drains as waste liquor.
Much attention has been devoted to the purification and concentration of glycerine lyes; and elaborate plant of various forms has been devised for the purpose.
Treatment of Lyes.—The spent lyes withdrawn from the soap-pans are cooled, and the soap, which has separated during the cooling, is carefully removed and returned to the soap-house for utilisation in the manufacture of brown soap. Spent lyes may vary in their content of glycerol from 3 to 8 per cent., and this depends not only upon the system adopted in the working of the soap-pans, but also upon the materials used. Although, in these days of pure caustic soda, spent lyes are more free from impurities than formerly, the presence of sulphides and sulphites should be carefully avoided, if it is desired to produce good glycerine.
The lyes are transferred to a lead-lined tank of convenient size, and treated with commercial hydrochloric acid and aluminium sulphate, sufficient being added of the former to neutralise the free alkali, and render the liquor faintly acid, and of the latter to completely precipitate the fatty acids. The acid should be run in slowly, and the point when enough has been added, is indicated by blue litmus paper being slightly reddened by the lyes.
The whole is then agitated with air, when a sample taken from the tank and filtered should give a clear filtrate.
Having obtained this clear solution, agitation is stopped, andthe contents of the tank passed through a filter press. The scum, which accumulates on the treatment tank, may be transferred to a perforated box suspended over the tank, and the liquor allowed to drain from it. The filtered liquor is now rendered slightly alkaline by the addition of caustic soda or carbonate, and, after filtering, is ready for evaporation.
The acid and alum salt used in the above treatment must be carefully examined for the presence of arsenic, and any deliveries of either article, which contain that impurity, rejected.
Lime, bog ore, and various metallic salts, such as ferric chloride, barium chloride, and copper sulphate have been suggested, and in some instances are used instead of aluminium sulphate, but the latter is generally employed.
Evaporation to Crude Glycerine.—The clear treated lyes, being now free from fatty, resinous, and albuminous matter, and consisting practically of an aqueous solution of common salt (sodium chloride) and glycerine, is converted into crude glycerine by concentration, which eliminates the water and causes most of the salt to be deposited.
This concentration was originally performed in open pans heated by fire or waste combustible gases. In the bottom of each pan was placed a dish in which the salt deposited, and this dish was lifted out periodically by the aid of an overhead crane and the contents emptied and washed. Concentration was continued until the temperature of the liquor was 300° F. (149° C.), when it was allowed to rest before storing.
This liquor on analysis gave 80 per cent. glycerol and from 9 to 10-1/2 per cent. salts (ash); hence the present standard for crude glycerine.
Concentration in open pans has now been superseded by evaporationin vacuo. The subject of the gradual development of the modern efficient evaporating plant from the vacuum pan, originated and successfully applied by Howard in 1813 in the sugar industry, is too lengthy to detail here, suffice it to say that the multiple effects now in vogue possess distinct advantages—the greatest of these being increased efficiency combined with economy.
The present type of evaporator consists of one or more vessels, each fitted with a steam chamber through which are fixed vertical hollow tubes. The steam chamber of the first vessel is heated with direct steam, or with exhaust steam (supplied from the exhaust steam receiver into which passes the waste steam of the factory); the treated lyes circulating through the heated tubes is made to boil at a lower temperature, with the reduced pressure, than is possible by heating in open pans.
The vapour given off by the boiling liquor is conveyed through large pipes into the steam chamber of the second vessel, where its latent heat is utilised in producing evaporation, the pressure being further reduced, as this second vessel is under a greater vacuum thanNo. 1. Thus we get a "double effect," as the plant consisting of two pans is termed. The vapours discharged from the second vessel during boiling are passed through pipes to the steam chamber of the third vessel (in a "triple effect"), and there being condensed, create a partial vacuum in the second vessel. The third vessel may also be heated by means of live steam. The vapours arising from the last vessel of the evaporating plant, or in the case of a "single effect" from the vessel, are conveyed into a condenser and condensed by injection water, which is drawn off by means of the pump employed for maintaining a vacuum of 28 inches in the vessel.
In the most recent designs of large evaporative installations, the vapours generated from the last vessel are drawn through a device consisting of a number of tubes enclosed in a casing, and the latent heat raises the temperature of the treated lyes proceeding through the tubes to supply the evaporator.
It will thus be observed that the object of multiple effects is to utilise all the available heat in performing the greatest possible amount of work. Special devices are attached to the plant for automatically removing the condensed water from the steam chambers without the loss of useful heat, and as a precaution against splashing over and subsequent loss of glycerine through conveyance to the steam chamber, dash plates and "catch-alls" or "save-alls" of various designs are fitted on each vessel.
In working the plant, the liquor in each vessel is kept at a fairly constant level by judicious feeding from one to the other; the first vessel is, of course, charged with treated lyes. As the liquor acquires a density of 42° Tw. (25° B.) salt begins to deposit, and may be withdrawn into one of the many patented appliances, in which it is freed from glycerine, washed and dried ready for use at the soap pans. Difficulty is sometimes experienced with the tubes becoming choked with salt, thereby diminishing and retarding evaporation. It may be necessary to dissolve the encrusted salt with lyes or water, but with careful working the difficulty can be obviated by washing out with weak lyes after each batch of crude glycerine has been run away, or by increasing the circulation.
It is claimed that by the use of the revolving heater designed by Lewkowitsch, the salting up of tubes is prevented.
The salt having been precipitated and removed, evaporation is continued until a sample taken from the last vessel has a density of 60° Tw. (33.3 B.) at 60° F. (15.5° C.). When this point is reached, the crude glycerine is ready to be withdrawn into a tank, and, after allowing the excess of salt to deposit, may be transferred to the storage tank.
The colour of crude glycerine varies from light brown to dark brown, almost black, and depends largely on the materials used for soap-making. The organic matter present in good crude glycerine is small in amount, often less than 1 per cent.; arsenic, sulphides and sulphites should be absent. Crude glycerine is refined in somecases by the producers themselves; others sell it to firms engaged more particularly in the refined glycerine trade.
Distillation.—Crude glycerine is distilled under vacuum with the aid of superheated steam. The still is heated directly with a coal or coke fire, and in this fire space is the superheater, which consists of a coil of pipes through which high pressure steam from the boiler is superheated.
The distillation is conducted at a temperature of 356°F. (180° C.). To prevent the deposition and burning of salt on the still-bottom during the distillation, a false bottom is supported about 1 foot from the base of the still. With the same object in view, it has been suggested to rotate the contents with an agitator fixed in the still.
Every care is taken that the still does not become overheated; this precaution not only prevents loss of glycerine through carbonisation, but also obviates the production of tarry and other bodies which might affect the colour, taste, and odour of the distilled glycerine. The vacuum to be used will, of course, depend upon the heat of the fire and still, but as a general rule good results are obtained with an 18 inch vacuum.
There are quite a large number of designs for still heads, and "catch-alls," having for their object the prevention of loss of glycerine.
The distillate passes into a row of condensers, to each of which is attached a receptacle or receiver. It is needless to state that the condensing capacity should be in excess of theoretical requirements. The fractions are of varying strengths and quality; that portion, with a density less than 14° Tw. (19.4° B.), is returned to the treated-lyes tank. The other portion of the distillate is concentrated by means of a dry steam coil in a suitable vessel under a 28 inch vacuum.
When sufficiently concentrated the glycerine may be decolorised, if necessary, by treating with 1 per cent. animal charcoal and passing through a filter press, from which it issues as "dynamite glycerine".
The residue in the still, consisting of 50-60 per cent. glycerine and varying proportions of various sodium salts—e.g.acetate, chloride, sulphate, and combinations with non-volatile organic acids—is generally boiled with water and treated with acid.
The tar, which is separated, floats on the surface as the liquor is cooling, and may be removed by ladles, or the whole mixed with waste charcoal, and filtered.
The filtrate is then evaporated, when the volatile organic acids are driven off; the concentrated liquor is finally mixed with crude glycerine which is ready for distillation, or it may be distilled separately.
Distilled Glycerine.—This class of commercial glycerine, although of limited use in various other branches of industry, finds its chief outlet in the manufacture of explosives.
Specifications are usually given in contracts drawn up between buyers and sellers, to which the product must conform.
The chief stipulation for dynamite glycerine is its behaviour in the nitration test. When glycerine is gradually added to a cold mixture of strong nitric and sulphuric acids, it is converted into nitro-glycerine, which separates as an oily layer on the surface of the acid. The more definite and rapid the separation, the more suitable is the glycerine for dynamite-making.
Dynamite glycerine should be free from arsenic, lime, chlorides, and fatty acids, the inorganic matter should not amount to more than 0.1 per cent., and a portion diluted and treated with nitrate of silver solution should give no turbidity or discoloration in ten minutes. The specific gravity should be 1.262 at 15° C. (59° F.) and the colour somewhat yellow.
Chemically pure glycerineor double distilled glycerine is produced by redistilling "once distilled" glycerine. Every care is taken to avoid all fractions which do not withstand the nitrate of silver test. The distillation is very carefully performed under strict supervision.
The distillate is concentrated and after treatment with animal charcoal and filtration should conform to the requirements of the British Pharmacopœia. These are specified as follows: Specific gravity at 15.5° C., 1.260. It should yield no characteristic reaction with the tests for lead, copper, arsenium, iron, calcium, potassium, sodium, ammonium, chlorides, or sulphates. It should contain no sugars and leave no residue on burning.
Animal Charcoal for Decolorisation.—The application of animal charcoal for decolorising purposes dates back a century, and various are the views that have been propounded to explain its action. Some observers base it upon the physical condition of the so-called carbon present, and no doubt this is an important factor, coupled with the porosity. Others consider that the nitrogen, which is present in all animal charcoal and extremely difficult to remove, is essential to the action. Animal charcoal should be freed from gypsum (sulphate of lime), lest in the burning, sulphur compounds be formed which would pass into the glycerine and contaminate it.
The "char" should be well boiled with water, then carbonate of soda or caustic soda added in sufficient quantity to give an alkaline reaction, and again well boiled. The liquor is withdrawn and the charcoal washed until the washings are no longer alkaline. The charcoal is then separated from the liquor and treated with hydrochloric acid; opinions differ as to the amount of acid to be used. Some contend that phosphate of lime plays such an important part in decolorising that it should not be removed, but it has, however, been demonstrated that this substance after exposure to heat has very little decolorising power.
Animal charcoal boiled with four times its weight of a mixture consisting of equal parts of commercial hydrochloric acid (free from arsenic) and water for twelve hours, then washed free from acid, dried, and burned in closed vessels gives a product possessed of great decolorising power for use with glycerines.
A good animal charcoal will have a dull appearance, and be of a deep colour; it should be used in fine grains and not in the form of a powder.
The charcoal from the filter presses is washed free from glycerine (which is returned to the treated lyes), cleansed from foreign substances by the above treatment and revivified by carefully heating in closed vessels for twelve hours.
Glycerine obtained by other Methods of Saponification.—French saponification or "candle crude" glycerine is the result of concentration of "sweet water" produced in the manufacture of stearine and by the autoclave process. It contains 85-90 per cent. glycerol, possesses a specific gravity of 1.240-1.242, and may be readily distinguished from the soap-crude glycerine by the absence of salt (sodium chloride). This glycerine is easily refined by treatment with charcoal.
The glycerine water resulting from acid saponification methods requires to be rendered alkaline by the addition of lime—the sludge is separated, and the liquor evaporated to crude. The concentration may be performed in two stages—first to a density of 32° Tw. (20° B.), when the calcium sulphate is allowed to deposit, and the separated liquor concentrated to 48° Tw. (28° B.) glycerine, testing 85 per cent. glycerol and upwards.
Yield of Glycerine from Fats and Oils.—The following represent practicable results which should be obtained from the various materials:—
Tallow9per cent. of 80 per cent. Glycerol.Cotton-seed oil10"Cocoa-nut oil12"Palm-kernel oil18"Olive oil10"Palm oil6"Greases (Bone fats)6-8"
The materials vary in glycerol content with the methods of preparation; especially is this the case with tallows and greases.
Every care should be taken that the raw materials are fresh and they should be carefully examined to ascertain if any decomposition has taken place in the glycerides—this would be denoted by the presence of an excess of free acidity, and the amount of glycerol obtainable from such a fat would be correspondingly reduced.
Fats and Oils—Alkalies and Alkali Salts—Essential Oils—Soap—Lyes—Crude Glycerine.
Fats and Oils—Alkalies and Alkali Salts—Essential Oils—Soap—Lyes—Crude Glycerine.
Raw Materials.—Average figures have already been given in Chapters III. and VIII. for the more important physical and chemical characteristics of fats and oils, also of essential oils; the following is an outline of the processes usually adopted in their determination. For fuller details, text-books dealing exhaustively with the respective subjects should be consulted.
It is very undesirable that any of these materials should be allowed to enter the soap pan without an analysis having first been made, as the oil may not only have become partially hydrolysed, involving a loss of glycerine, or contain albuminous matter rendering the soap liable to develop rancidity, but actual sophistication may have taken place. Thus a sample of tallow recently examined by the authors contained as much as 40 per cent. of an unsaponifiable wax, which would have led to disaster in the soap pan, had the bulk been used without examination. After observing the appearance, colour, and odour of the sample, noting any characteristic feature, the following physical and chemical data should be determined.
Specific Gravity at 15° C.This may be taken by means of a Westphal balance, or by using a picnometer of either the ordinary gravity bottle shape, with perforated stopper, or the Sprengel U-tube. The picnometer should be calibrated with distilled water at 15° C. The specific gravity of solid fats may be taken at an elevated temperature, preferably that of a boiling water bath.
Free acidityis estimated by weighing out from 2 to 5 grammes of the fat or oil, dissolving in neutral alcohol (purified methylated spirit) with gentle heat, and titrating with a standard aqueous or alcoholic solution of caustic soda or potash, using phenol-phthalein as indicator.
The contents of the flask are well shaken after each addition of alkali, and the reaction is complete when the slight excess of alkali causes a permanent pink coloration with the indicator. The standard alkali may be N/2, N/5, or N/10.
It is usual to calculate the result in terms of oleic acid (1 c.c. N/10 alkali = 0.0282 gramme oleic acid), and express in percentage on the fat or oil.
Example.—1.8976 grammes were taken, and required 5.2 c.c. of N/10 KOH solution for neutralisation.
5.2 × 0.0282 × 100—————————=7.72 per cent. free fatty acids, expressed as oleic acid.1.8976
The free acidity is sometimes expressed asacid value, which is the amount of KOH in milligrammes necessary to neutralise the free acid in 1 gramme of fat or oil.
In the above example:—
5.2 × 5.61—————=15.3 acid value.1.8976
Thesaponification equivalentis determined by weighing 2-4 grammes of fat or oil into a wide-necked flask (about 250 c.c. capacity), adding 30 c.c. neutral alcohol, and warming under a reflux condenser on a steam or water-bath. When boiling, the flask is disconnected, 50 c.c. of an approximately semi-normal alcoholic potash solution carefully added from a burette, together with a few drops of phenol-phthalein solution, and the boiling under a reflux condenser continued, with frequent agitation, until saponification is complete (usually from 30-60 minutes) which is indicated by the absence of fatty globules. The excess of alkali is titrated with N/1 hydrochloric or sulphuric acid.
The value of the approximately N/2 alkali solution is ascertained by taking 50 c.c. together with 30 c.c. neutral alcohol in a similar flask, boiling for the same length of time as the fat, and titrating with N/1 hydrochloric or sulphuric acid. The "saponification equivalent" is the amount of fat or oil in grammes saponified by 1 equivalent or 56.1 grammes of caustic potash.
Example.—1.8976 grammes fat required 18.95 c.c. N/1 acid to neutralise the unabsorbed alkali.
Fifty c.c. approximately N/2 alcoholic potash solution required 25.6 c.c. N/ acid..
25.6 - 18.95 = 6.65 c.c. N/1 KOH required by fat.1.8976 × 1000 / 6.65 = 285.3 Saponification Equivalent.
The result of this test is often expressed as the "Saponification Value," which is the number of milligrammes of KOH required for the saponification of 1 gramme of fat. This may be found by dividing 56,100 by the saponification equivalent or by multiplying the number of c.c. of N/1 alkali absorbed, by 56.1 and dividing by the quantity of fat taken. Thus, in the above example:—
6.65 × 56.1 / 1.8976 = 196.6 Saponification Value.
Theesterorether value, or number of milligrammes of KOH required for the saponification of the neutral esters or glycerides in 1 gramme of fat, is represented by the difference between the saponification and acid values. In the example given, the ester value would be 196.6 - 15.3 = 181.3.
Unsaponifiable Matter.—The usual method adopted is to saponify about 5 grammes of the fat or oil with 50 c.c. of approximately N/2 alcoholic potash solution by boiling under a reflux condenser with frequent agitation for about 1 hour. The solution is then evaporated to dryness in a porcelain basin over a steam or water-bath, and the resultant soap dissolved in about 200 c.c. hot water. When sufficiently cool, the soap solution is transferred to a separating funnel, 50 c.c. of ether added, the whole well shaken, and allowed to rest. The ethereal layer is removed to another separator, more ether being added to the aqueous soap solution, and again separated. The two ethereal extracts are then washed with water to deprive them of any soap, separated, transferred to a flask, and the ether distilled off upon a water-bath. The residue, dried in the oven at 100° C. until constant, is the "unsaponifiable matter," which is calculated to per cent. on the oil.
In this method, it is very frequently most difficult to obtain a distinct separation of ether and aqueous soap solution—an intermediate layer of emulsion remaining even after prolonged standing, and various expedients have been recommended to overcome this, such as addition of alcohol (when petroleum ether is used), glycerine, more ether, water, or caustic potash solution, or by rotatory agitation.
A better plan is to proceed as in the method above described as far as dissolving the resulting soap in 200 c.c. water, and then boil for twenty or thirty minutes. Slightly cool and acidify with dilute sulphuric acid (1 to 3), boil until the fatty acids are clear, wash with hot water free from mineral acid, and dry by filtering through a hot water funnel.
Two grammes of the fatty acids are now dissolved in neutral alcohol saturated with some solvent, preferably a light fraction of benzoline, a quantity of the solvent added to take up the unsaponifiable matter, and the whole boiled under a reflux condenser. After cooling, the liquid is titrated with N/2 aqueous KOH solution, using phenol-phthalein as indicator, this figure giving the amount of the total fatty acids present. The whole is then poured into a separating funnel, when separation immediately takes place. The alcoholic layer is withdrawn, the benzoline washed with warm water (about 32° C.) followed by neutral alcohol (previously saturated with the solvent), and transferred to a tared flask, which is attached to a condenser, and the benzoline distilled off. The last traces of solvent remaining in the flask are removed by gently warming in the water-oven, and the flask cooled and weighed, thus giving the amount of unsaponifiable matter.
Constitution of the Unsaponifiable Matter.—Unsaponifiable mattermay consist of cholesterol, phytosterol, solid alcohols (cetyl and ceryl alcohols), or hydrocarbons (mineral oil). Cholesterol is frequently found in animal fats, and phytosterol is a very similar substance present in vegetable fats. Solid alcohols occur naturally in sperm oil, but hydrocarbons, which may be generally recognised by the fluorescence or bloom they give to the oil, are not natural constituents of animal or vegetable oils and fats.
The presence of cholesterol and phytosterol may be detected by dissolving a small portion of the unsaponifiable matter in acetic anhydride, and adding a drop of the solution to one drop of 50 per cent. sulphuric acid on a spot plate, when a characteristic blood red to violet coloration is produced. It has been proposed to differentiate between cholesterol and phytosterol by their melting points, but it is more reliable to compare the crystalline forms, the former crystallising in laminæ, while the latter forms groups of needle-shaped tufts. Another method is to convert the substance into acetate, and take its melting point, cholesterol acetate melting at 114.3-114.8° C., and phytosterol acetate at 125.6°-137° C.
Additional tests for cholesterol have been recently proposed by Lifschütz (Ber. Deut. Chem. Ges., 1908, 252-255), and Golodetz (Chem. Zeit., 1908, 160). In that due to the former, which depends on the oxidation of cholesterol to oxycholesterol ester and oxycholesterol, a few milligrammes of the substance are dissolved in 2-3 c.c. glacial acetic acid, a little benzoyl peroxide added, and the solution boiled, after which four drops of strong sulphuric acid are added, when a violet-blue or green colour is produced, if cholesterol is present, the violet colour being due to oxycholesterol ester, the green to oxycholesterol. Two tests are suggested by Golodetz (1) the addition of one or two drops of a reagent consisting of five parts of concentrated sulphuric acid and three parts of formaldehyde solution, which colours cholesterol a blackish-brown, and (2) the addition of one drop of 30 per cent. formaldehyde solution to a solution of the substance in trichloracetic acid, when with cholesterol an intense blue coloration is produced.
Water.—From 5 to 20 grammes of the fat or oil are weighed into a tared porcelain or platinum dish, and stirred with a thermometer, whilst being heated over a gas flame at 100° C. until bubbling or cracking has ceased, and reweighed, the loss in weight representing the water. In cases of spurting a little added alcohol will carry the water off quietly.
To prevent loss by spurting, Davis (J. Amer. Chem. Soc., 23, 487) has suggested that the fat or oil should be added to a previously dried and tared coil of filter paper contained in a stoppered weighing bottle, which is then placed in the oven and dried at 100° C. until constant in weight. Of course, this method is not applicable to oils or fats liable to oxidation on heating.
Dregs, Dirt, Adipose Tissue, Fibre, etc.—From 10 to 15 grammes of the fat are dissolved in petroleum ether with frequent stirring, andpassed through a tared filter paper. The residue retained by the filter paper is washed with petroleum ether until free from fat, dried in the water-oven at 100° C. and weighed.
If the amount of residue is large, it may be ignited, and the proportion and nature of the ash determined.
The amount of impurities may also be estimated by Tate's method, which is performed by weighing 5 grammes of fat into a separating funnel, dissolving in ether, and allowing the whole to stand to enable the water to deposit. After six hours' rest the water is withdrawn, the tube of the separator carefully dried, and the ethereal solution filtered through a dried tared filter paper into a tared flask. Well wash the filter with ether, and carefully dry at 100° C. The ether in the flask is recovered, and the flask dried until all ether is expelled, and its weight is constant. The amount of fat in the flask gives the quantity of actual fat in the sample taken; the loss represents the water and other impurities, and these latter may be obtained from the increase of weight of the filter paper.
Starchmay be detected by the blue coloration it gives with iodine solution, and confirmed by microscopical examination, or it may be converted into glucose by inversion, and the glucose estimated by means of Fehling's solution.
Iodine Absorption.—This determination shows the amount of iodine absorbed by a fat or oil, and was devised by Hübl, the reagents required being as follows:—
(1) Solution of 25 grammes iodine in 500 c.c. absolute alcohol; (2) solution of 30 grammes mercuric chloride in 500 c.c. absolute alcohol, these two solutions being mixed together and allowed to stand at least twelve hours before use; (3) a freshly prepared 10 per cent. aqueous solution of potassium iodide; and (4) a N/10 solution of sodium thiosulphate, standardised just prior to use by titrating a weighed quantity of resublimed iodine dissolved in potassium iodide solution.
In the actual determination, 0.2 to 0.5 gramme of fat or fatty acids is carefully weighed into a well-fitting stoppered 250 c.c. bottle, dissolved in 10 c.c. chloroform, and 25 c.c. of the Hübl reagent added, the stopper being then moistened with potassium iodide solution and placed firmly in the bottle, which is allowed to stand at rest in a dark place for four hours. A blank experiment is also performed, using the same quantities of chloroform and Hübl reagent, and allowing to stand for the same length of time.
After the expiration of four hours 20 c.c. of 10 per cent. solution of potassium iodide and 150 c.c. water are added to the contents of the bottle, and the excess of iodine titrated with N/10 sodium thiosulphate solution, the whole being well agitated during the titration, which is finished with starch paste as indicator. The blank experiment is titrated in the same manner, and from the amount of thiosulphate required in the blank experiment is deducted the number of c.c. required by the unabsorbed iodine in the other bottle; this figure multipliedby the iodine equivalent of 1 c.c. of the thiosulphate solution and by 100, dividing the product by the weight of fat taken, gives the "Iodine Number".
Example.—1 c.c. of the N/10 sodium thiosulphate solution is found equal to 0.0126 gramme iodine.
0.3187 gramme of fat taken. Blank requires 48.5 c.c. thiosulphate.
Bottle containing oil requires 40.0 c.c. thiosulphate.
48.5 - 40.0 = 8.5, and the iodine absorption of the fat is—
8.5 × 0.0126 × 100—————————=33.6.0.3187
Wijs showed that by the employment of a solution of iodine monochloride in glacial acetic acid reliable iodine figures are obtained in a much shorter time, thirty minutes being sufficient, and this method is now in much more general use than the Hübl. Wijs' iodine reagent is made by dissolving 13 grammes iodine in 1 litre of glacial acetic acid and passing chlorine into the solution until the iodine is all converted into iodine monochloride. The process is carried out in exactly the same way as with the Hübl solution except that the fat is preferably dissolved in carbon tetrachloride instead of in chloroform.
Bromine absorptionhas now been almost entirely superseded by the iodine absorption, although there are several good methods. The gravimetric method of Hehner (Analyst, 1895, 49) was employed by one of us for many years with very good results, whilst the bromine-thermal test of Hehner and Mitchell (Analyst, 1895, 146) gives rapid and satisfactory results. More recently MacIlhiney (Jour. Amer. Chem. Soc., 1899, 1084-1089) drew attention to bromine absorption methods and tried to rewaken interest in them.
TheRefractive indexis sometimes useful for discriminating between various oils and fats, and, in conjunction with other physical and chemical data, affords another means of detecting adulteration.
Where a great number of samples have to be tested expeditiously, the Abbé refractometer or the Zeiss butyro-refractometer may be recommended on account of the ease with which they are manipulated. The most usual temperature of observations is 60° C.
TheTitreor setting point of the fatty acids was devised by Dalican, and is generally accepted in the commercial valuation of solid fats as a gauge of firmness, and in the case of tallow has a considerable bearing on the market value.
One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c. of a 25 per cent. caustic soda solution added, together with 50 c.c. of redistilled methylated spirit. The whole is stirred down on the water bath until a pasty soap is obtained, when another 50 c.c. of methylated spirit is added, which redissolves the soap, and the whole again stirred down to a solid soap. This is then dissolved in distilled water, a slight excess of dilute sulphuric acid added to liberate the fatty acids, and the whole warmed until the fatty acids form aclear liquid on the surface. The water beneath the fatty acids is then syphoned off, more distilled water added to wash out any trace of mineral acid remaining, and again syphoned off, this process being repeated until the washings are no longer acid to litmus paper, when the fatty acids are poured on to a dry filter paper, which is inserted in a funnel resting on a beaker, and the latter placed on the water-bath, where it is left until the clear fatty acids have filtered through.
About 10-15 grammes of the pure fatty acids are now transferred to a test tube, 6" × 1", warmed until molten, and the tube introduced through a hole in the cork into a flask or wide-mouthed bottle. A very accurate thermometer, graduated into fifths of a degree Centigrade (previously standardised), is immersed in the fatty acids, so that the bulb is as near the centre as possible, and when the fatty acids just begin to solidify at the bottom of the tube, the thermometer is stirred round slowly. The mercury will descend, and stirring is continued until it ceases to fall further, at which point the thermometer is very carefully observed. It will be found that the temperature will rise rapidly and finally remain stationary for a short time, after which it will again begin to drop until the temperature of the room is reached. The maximum point to which the temperature rises is known as the "titre" of the sample.
Care should be bestowed upon the sampling of solid caustic soda or potash as the impurities during the solidification always accumulate in the centre of the drum, and an excess of that portion must be avoided or the sample will not be sufficiently representative. The sampling should be performed expeditiously to prevent carbonating, and portions placed in a stoppered bottle. The whole should be slightly broken in a mortar, and bright crystalline portions taken for analysis, using a stoppered weighing bottle.
Caustic Soda and Caustic Potash.—These substances are valued according to the alkali present in the form of caustic (hydrate) and carbonate.
About 2 grammes of the sample are dissolved in 50 c.c. distilled water, and titrated with N/1 sulphuric acid, using phenol-phthalein as indicator, the alkalinity so obtained representing all the caustic alkali and one-half the carbonate, which latter is converted into bicarbonate. One c.c. N/1 acid = 0.031 gramme Na2O or 0.040 gramme NaOH and 0.047 gramme K2O, or 0.056 gramme KOH.
After this first titration, the second half of the carbonate may be determined in one of two ways, either:—
(1) By adding from 3-5 c.c. of N/10 acid, and well boiling for five minutes to expel carbonic-acid gas, after which the excess of acid is titrated with N/10 soda solution; or
(2) After adding two drops of methyl orange solution, N/10 acid is run in until the solution acquires a faint pink tint.
In the calculation of the caustic alkali, the number of c.c. of acid required in the second titration, divided by 10, is subtracted from that used in the first, and this difference multiplied by 0.031, or 0.047 gives the amount of Na2O or K2O respectively in the weight of sample taken, whence the percentage may be readily calculated.
The proportion of carbonate is calculated by multiplying the amount of N/10 acid required in the second titration by 2, and then by either 0.0031 or 0.0047 to give the amount of carbonate present, expressed as Na2O or K2O respectively.
An alternative method is to determine the alkalinity before and after the elimination of carbonate by chloride of barium.
About 7-8 grammes of the sample are dissolved in water, and made up to 100 c.c., and the total alkalinity determined by titrating 20 c.c. with N/1 acid, using methyl orange as indicator. To another 20 c.c. is added barium chloride solution (10 per cent.) until it ceases to give a precipitate, the precipitate allowed to settle, and the clear supernatant liquid decanted off, the precipitate transferred to a filter paper and well washed, and the filtrate titrated with N/1 acid, using phenol-phthalein as indicator. The second titration gives the amount of caustic alkali present, and the difference between the two the proportion of carbonate.
When methyl orange solution is used as indicator, titrations must be carried out cold.
Reference has already been made (p. 39) to the manner in which the alkali percentage is expressed in English degrees in the case of caustic soda.
Chloridesare estimated by titrating the neutral solution with N/10 silver nitrate solution, potassium chromate being used as indicator. One c.c. N/10 AgNO3solution = 0.00585 gramme sodium chloride.
The amount of acid necessary for exact neutralisation having already been ascertained, it is recommended to use the equivalent quantity of N/10 nitric acid to produce the neutral solution.
Sulphidesmay be tested for, qualitatively, with lead acetate solution.
Aluminatesare determined gravimetrically in the usual manner; 2 grammes are dissolved in water, rendered acid with HCl, excess of ammonia added, and the gelatinous precipitate of aluminium hydrate collected on a filter paper, washed, burnt, and weighed.
Carbonated Alkali (Soda Ash).—The total or available alkali is, of course, the chief factor to be ascertained, and for this purpose it is convenient to weigh out 3.1 grammes of the sample, dissolve in 50 c.c. water, and titrate with N/1 sulphuric or hydrochloric acid, using methyl orange as indicator. Each c.c. of N/1 acid required represents 1 per cent. Na2O in the sample under examination.
A more complete analysis of soda ash would comprise:—
Insoluble matter, remaining after 10 grammes are dissolved inwarm water. This is washed on to a filter-paper, dried, ignited, and weighed.
The filtrate is made up to 200 c.c., and in it may be determined:—
Caustic soda, by titrating with N/1 acid the filtrate resulting from the treatment of 20 c.c. (equal to 1 gramme) with barium chloride solution.
Carbonate.—Titrate 20 c.c. with N/1 acid, and deduct the amount of acid required for the Caustic.
Chlorides.—Twenty c.c. are exactly neutralised with nitric acid, titrated with N/10 AgNO3solution, using potassium chromate as indicator.
Sulphates.—Twenty c.c. are acidulated with HCl, and the sulphates precipitated with barium chloride; the precipitate is collected on a filter paper, washed, dried, ignited, and weighed, the result being calculated to Na2SO4.
Sulphides and Sulphites.—The presence of these compounds is denoted by the evolution of sulphuretted hydrogen and sulphurous acid respectively when the sample is acidulated. Sulphides may also be tested for, qualitatively, with lead acetate solution, or test-paper of sodium nitro-prusside.
The total quantity of these compounds may be ascertained by acidulating with acetic acid, and titrating with N/10 iodine solution, using starch paste as indicator. One c.c. N/10 iodine solution = 0.0063 gramme Na2SO3.
The amount of sulphides may be estimated by titrating the hot soda solution, to which ammonia has been added, with an ammoniacal silver nitrate solution, 1 c.c. of which corresponds to 0.005 gramme Na2S. As the titration proceeds, the precipitate is filtered off, and the addition of ammoniacal silver solution to the filtrate continued until a drop produces only a slight opacity. The presence of chloride, sulphate, hydrate, or carbonate does not interfere with the accuracy of this method. The ammoniacal silver nitrate solution is prepared by dissolving 13.345 grammes of pure silver in pure nitric acid, adding 250 c.c. liquor ammoniæ fortis, and diluting to 1 litre.
Carbonate of Potash (Pearl Ash).—The total or available alkali may be estimated by taking 6.9 grammes of the sample, and titrating with N/1 acid directly, or adding 100 c.c. N/1 sulphuric acid, boiling for a few minutes, and titrating the excess of acid with N/1 caustic soda solution, using litmus as indicator. In this case each c.c. N/1 acid required, is equivalent, in the absence of Na2CO3, to 1 per cent. K2CO3.
Carbonate of potash may be further examined for the following:—
Moisture.—From 2-3 grammes are heated for thirty minutes in a crucible over a gas flame, and weighed when cold, the loss in weight representing the moisture.
Insoluble residue, remaining after solution in water, filtering and well washing.
Potassiummay be determined by precipitation as potassium platino-chloride thus:—Dissolve 0.5 gramme in a small quantity (say 10c.c.) of water, and carefully acidulate with hydrochloric acid, evaporate the resultant liquor to dryness in a tared platinum basin, and heat the residue gradually to dull redness. Cool in a desicator, weigh, and express the result as "mixed chlorides,"i.e.chlorides of soda and potash. To the mixed chlorides add 10 c.c. water, and platinic chloride in excess (the quantity may be three times the amount of the mixed chlorides) and evaporate nearly to dryness; add 15 c.c. alcohol and allow to stand three hours covered with a watch-glass, giving the dish a gentle rotatory movement occasionally. The clear liquid is decanted through a tared filter, and the precipitate well washed with alcohol by decantation, and finally transferred to the filter, dried and weighed. From the weight of potassium platino-chloride, K2PtCl6, is calculated the amount of potassium oxide K2O by the use of the factor 94/488.2 or 0.19254.
Chlorides, determined with N/10 silver nitrate solution, and calculated to KCl.
Sulphates, estimated as barium sulphate, and calculated to K2SO4.
Sodium Carbonate, found by deducting the K2CO3corresponding to the actual potassium as determined above, from the total alkali.
Iron, precipitated with excess of ammonia, filtered, ignited, and weighed as Fe2O3.
This should be examined for the following:—
Actual Chloride, either titrated with N/10 silver nitrate solution, using neutral potassium chromate solution as indicator, or, preferably, estimated gravimetrically as silver chloride by precipitation with silver nitrate solution, the precipitate transferred to a tared filter paper, washed, dried and weighed.
Insoluble matter, remaining on dissolving 5 grammes in water, and filtering. This is washed, dried, ignited and weighed.
Moisture.—5 grammes are weighed into a platinum crucible, and heat gently applied. The temperature is gradually increased to a dull red heat, which is maintained for a few minutes, the dish cooled in a desicator, and weighed.
Sulphatesare estimated by precipitation as barium sulphate and calculated to Na2SO4.
Sodium.—This may be determined by converting the salt into sodium sulphate by the action of concentrated sulphuric acid, igniting to drive off hydrochloric and sulphuric acids, and fusing the mass until constant in weight, weighing finally as Na2SO4.
This should be examined, in the same way as sodium chloride, for chloride, insoluble matter, moisture, and sulphate. The potassium may be determined as potassium platino-chloride, as described under carbonate of potash.
The most important determinations for these are total alkali and silica.
Total alkaliis estimated by dissolving 2 grammes in distilled water, and titrating when cold, with N/1 acid, using methyl orange as indicator.
Silicamay be determined by dissolving 1 gramme in distilled water, rendering the solution acid with HCl, and evaporating to complete dryness on the water-bath, after which the residue is moistened with HCl and again evaporated, this operation being repeated a third time. The residue is then heated to about 150° C., extracted with hot dilute HCl, filtered, thoroughly washed, dried, ignited in a tared platinum crucible, and weighed as SiO2.
As already stated, these are very liable to adulteration, and an examination of all kinds of oil is desirable, while in the case of the more expensive varieties it should never be omitted.
Specific Gravity.—As with fats and oils, this is usually taken at 15° C., and compared with water at the same temperature. In the case of otto of rose and guaiac wood oil, however, which are solid at this temperature, it is generally observed at 30° C. compared with water at 15° C.
The specific gravity is preferably taken in a bottle or U-tube, but if sufficient of the oil is available and a high degree of accuracy is not necessary, it may be taken either with a Westphal balance, or by means of a hydrometer.
Optical Rotation.—For this purpose a special instrument, known as a polarimeter, is required, details of the construction and use of which would be out of place here. Suffice it to mention that temperature plays an important part in the determination of the optical activity of certain essential oils, notably in the case of lemon and orange oils. For these Gildemeister and Hoffmann give the following corrections:—
Lemon oil, below 20° C. subtract 9' for each degree below, above 20° C. add 8' for each degree above.
Orange oil, below 20° C. subtract 14' for each degree below, above 20° C. add 13' for each degree above.
Refractive Index.—This figure is occasionally useful, and is best determined with an Abbé refractometer, at 20° C.
Solubility in Alcohol.—This is found by running alcohol of the requisite strength from a burette into a measured volume of the oil with constant agitation, until the oil forms a clear solution with the alcohol. Having noted the quantity of alcohol added, it is well to run in a small further quantity of alcohol, and observe whether any opalescence or cloudiness appears.
Acid,ester, andsaponification valuesare determined exactly as described under fats and oils. Instead of expressing the result as saponification value or number, the percentage of ester, calculated in the form of the most important ester present, may be obtained by multiplying the number of c.c. of N/1 alkali absorbed in the saponification by the molecular weight of the ester. Thus, to find the percentage as linalyl acetate, the number of c.c. absorbed would be multiplied by 0.196 and by 100, and divided by the weight of oil taken.
Alcohols.—For the estimation of these, if the oil contains much ester it must first be saponified with alcoholic potash, to liberate the combined alcohols, and after neutralising the excess of alkali with acid, the oil is washed into a separating funnel with water, separated, dried with anhydrous sodium sulphate, and is then ready for the alcohol determination.
If there is only a small quantity of ester present, this preliminary saponification is unnecessary.
The alcohols are estimated by conversion into their acetic esters, which are then saponified with standard alcoholic potash, thereby furnishing a measure of the amount of alcohol esterified.
Ten c.c. of the oil is placed in a flask with an equal volume of acetic anhydride, and 2 grammes of anhydrous sodium acetate, and gently boiled for an hour to an hour and a half. After cooling, water is added, and the contents of the flask heated on the water-bath for fifteen to thirty minutes, after which they are cooled, transferred to a separating funnel, and washed with a brine solution until the washings cease to give an acid reaction with litmus paper. The oil is now dried with anhydrous sodium sulphate, filtered, and 1-2 grammes weighed into a flask and saponified with alcoholic potash as in the determination of ester or saponification value.
The calculation is a little complicated, but an example may perhaps serve to make it clear.
A geranium oil containing 26.9 per cent. of ester, calculated as geranyl tiglate, was acetylated, after saponification, to liberate the combined geraniol, and 2.3825 grammes of the acetylated oil required 9.1 c.c. of N/1 alkali for its saponification.
Now every 196 grammes of geranyl acetate present in the acetylated oil correspond to 154 grammes of geraniol, so that for every 196 grammes of ester now present in the oil, 42 grammes have been added to its weight, and it is therefore necessary to make a deduction from the weight of oil taken for the final saponification to allow for this, and since each c.c. of N/1 alkali absorbed corresponds to 0.196 gramme of geranyl acetate, the amount to be deducted is found by multiplying the number of c.c. absorbed by 0.042 gramme, the formula for the estimation of total alcohols thus becoming in the example given:—