Fig. 67.
Arsenites are distinguished by the volatility of the chloride; by decolorising a solution of permanganate of potassium, and by immediately giving a yellow precipitate with sulphuretted hydrogen. Arsenates are distinguished (after converting into soda salts byboiling with carbonate of soda and neutralising) by giving with nitrate of silver a red precipitate, and with "magnesia mixture" a white crystalline one.
Dry Assay.—There is no dry assay which is trustworthy. The following method is sometimes used to find the proportion of arsenious oxide in "crude arsenic":—Weigh up 5 grams of the dried sample, and place them in a clean dry test-tube about 6 inches long. Tie a small filter-paper over the mouth of the tube, so as to prevent air-currents. Heat the tube cautiously so as to sublime off the white arsenic into the upper part of the tube. Cut off the bottom of the test-tube by wetting whilst hot. Scrape out the arsenic and weigh it. The weight gives an approximate idea of the quantity, and the colour of the quality, of the white arsenic obtainable from the sample. Some workers (sellers) weigh the residue, and determine the white arsenic by difference. In determining the percentage of moisture in these samples, the substance is dried on a water-bath or in a water-oven.
Solution.—Where, as in crude arsenic, the substance is arsenious oxide (As2O3) mixed with impurities, the arsenic is best got into solution by warming with caustic soda, and neutralising the excess with hydrochloric acid; it will be present as sodium arsenite. Metals and alloys are acted on by means of nitric acid; or the arsenic may be at the same time dissolved and separated by distilling with a strongly-acid solution of ferric chloride, in the way described underVolumetric Methods.
With minerals, mattes, &c., solution is thus effected:—The finely-powdered substance is mixed (in a large platinum or porcelain crucible) with from six to ten times its weight of a mixture of equal parts of carbonate of soda and nitre. The mass is then heated gradually to fusion, and kept for a few minutes in that state. When cold, it is extracted with warm water, and filtered from the insoluble residue. The solution, acidified with nitric acid and boiled, contains the arsenic as sodium arsenate. With mispickel, and those substances which easily give off arsenic on heating, the substance is first treated with nitric acid, evaporated to dryness, and then the residue is treated in the way just described.
When the arsenic is present as arsenite or arsenide, distillation with an acid solution of ferric chloride will give the whole of the arsenic in the distillate free from any metal except, perhaps, tin as stannic chloride. With arsenates, dissolve the substance in acid and then add an excess of soda. Pass sulphuretted hydrogen into the solution; warm, and filter. Acidulate thefiltrate, and pass sulphuretted hydrogen. Decant off the liquid through a filter, and digest the precipitate with ammonic carbonate; filter, and re-precipitate with hydrochloric acid and sulphuretted hydrogen. Allow to stand in a warm place, and filter off the yellow sulphide of arsenic. Wash it into a beaker, clean the filter-paper (if necessary) with a drop or two of dilute ammonia; evaporate with 10 c.c. of dilute nitric acid to a small bulk; dilute; and filter off the globules of sulphur. The filtrate contains the arsenic as arsenic acid.
Having got the arsenic into solution as arsenic acid, and in a volume not much exceeding 50 c.c., add about 20 c.c. of dilute ammonia and 20 c.c. of "magnesia mixture." Stir with a glass rod, and allow to settle overnight. Filter, and wash with dilute ammonia, avoiding the use of large quantities of wash water. Dry, transfer the precipitate to a Berlin crucible, and clean the filter-paper thoroughly. Burn this paper carefully and completely; and add the ash to the contents of the crucible, together with 4 or 5 drops of nitric acid. Evaporate with a Bunsen burner, and slowly ignite, finishing off with the blow-pipe or muffle. Cool, and weigh. The ignited precipitate is pyrarsenate of magnesia (Mg2As2O7), and contains 48.4 per cent. of arsenic (As).
Instead of igniting the precipitate with nitric acid, it may be collected on a weighed filter-paper, dried at 100° C., and weighed as ammonic-magnesic arsenate (2AmMgAsO4.H2O), which contains 39.5 per cent. of arsenic. The results in this case are likely to be a little higher. The drying is very tedious, and is likely to leave behind more water than is allowed for in the formula. In a series of determinations in which the arsenic was weighed in both forms, the results were:—
Ammonic-magnesic Arsenate in grams.Arsenic in grams.Magnesium Pyrarsenate in grams.Arsenic in grams.0.00800.00320.00650.00310.04000.01580.03300.01600.07990.03160.06330.03060.16000.06320.12870.06230.40000.15800.32050.15510.79900.31560.64350.3114
There are two methods: one for determining the arsenic in the lower, and the other in the higher state of oxidation. In the first-mentioned method this is done by titrating with a standard solution of iodine; and in the latter with a solution of uranium acetate. Where the arsenic already exists as arsenious oxide, or where it is most conveniently separated by distillation as arsenious chloride, the iodine method should be used; but when the arsenic is separated as ammonic-magnesic arsenate or as sulphide, the uranium acetate titration should be adopted.
This is based on the fact that sodium arsenite in a solution containing an excess of bicarbonate of soda is indirectly oxidised by iodine to sodium arsenate,[103]and that an excess of iodine may be recognised by the blue colour it strikes with starch. The process is divided into two parts—(1) the preparation of the solution, and (2) the titration.
Preparation of the Solution.—For substances like crude arsenic, in which the arsenic is present as arsenious oxide, the method is as follows:—Take a portion which shall contain from 0.25 to 0.5 gram of the oxide, place in a beaker, and cover with 10 c.c. of sodic hydrate solution; warm till dissolved, put a small piece of litmus paper in the solution, and render acid with dilute hydrochloric acid. Add 2 grams of bicarbonate of soda in solution, filter (if necessary), and dilute to 100 c.c. The solution is now ready for titrating.
Fig. 68.
Where the arsenic has to be separated as arsenious chloride, the process is as follows:[104]—Weigh up 1 gram of the finely-powdered ore (metals should be hammered out into a thin foil or be used as filings), and place in a 16-ounce flask provided with a well-fitting cork, and connected with aU-tube, as shown in the drawing (fig. 68). TheU-tube should contain 2 or 3 c.c. of water, and is cooled by beingplaced in a jar or large beaker of cold water. The water used for cooling should be renewed for each assay.
Pour on the assay in the flask 50 c.c. of a "ferric chloride mixture," made by dissolving 600 grams of calcium chloride and 300 grams of ferric chloride in 600 c.c. of hydrochloric acid, and making up to 1 litre with water.
Firmly cork up the apparatus, and boil over a small Bunsen-burner flame for fifteen or twenty minutes, but avoid evaporating to dryness. Disconnect the flask, and pour away its contents at once to prevent breakage of the flask by their solidification. The arsenic will be condensed in theU-tube, together with the greater part of the hydrochloric acid; transfer the distillate to a beaker washing out the tube two or three times with water; add a small piece of litmus paper; neutralise with ammonia; render faintlyacidwith dilute hydrochloric acid; add 2 grams of bicarbonate of soda in solution; and dilute to 250 c.c. The solution is now ready for titrating.
The arsenic comes over in the early part of the distillation, as will be seen from the following experiment, made on 1 gram of copper precipitate; in which experiment the distillate was collected in separate portions at equal intervals, and the arsenic in each portion determined:—
Time Distilling.Iodine Required.Equivalent to Arsenic in the Distillate.5 minutes12.0 c.c.0.0450 gram5 "0.17 "0.0005 "5 "0.0 "5 "0.0 "To dryness0.0 "
The volume of each distillate was about 5 c.c.
In this operation the metals are converted into chlorides by the action of ferric chloride, which gives up a part of its chlorine, and becomes reduced to the ferrous salt. The calcium chloride does not enter into the chemical reaction, but raises the temperature at which the solution boils, and is essential for the completion of the distillation.[105]Two experiments with material containing 3.48 per cent. of arsenic gave—(1) with ferric chloride alone, 2.74 per cent.; and (2) with the addition of calcium chloride, 3.48 per cent.
It is always necessary to make a blank determination with 1 gram of electrotype copper, to find out the amount of arsenic in the ferric chloride mixture.[106]Unfortunately, a correction isalways required. This amounts to about 0.15 per cent. of arsenic on each assay, even when the mixture has been purified; and this constitutes the weakness of the method, since, in some cases, the correction is as much as, or even greater than, the percentage to be determined.
The acid distillate containing the arsenious chloride may be left for an hour or so without much fear of oxidation; but it is safer to neutralise and then to add the bicarbonate of soda, as the following experiments show. Several portions of a solution, each having a bulk of 100 c.c., were exposed for varying lengths of time, and the arsenic in each determined.
Time Exposed.Acid Solutions."Iodine" Required. Arsenic Found.Neutralised Solutions."Iodine"Required. Arsenic Found.—18.2 c.c. = 0.0136 gram18.1 c.c. = 0.0136 gram1 hour18.2 " = 0.0136 "18.2 " = 0.0136 "2 hours17.7 " = 0.0133 "18.0 " = 0.0135 "4 "17.5 " = 0.0131 "18.4 " = 0.0138 "5 "17.0 " = 0.0127 "18.3 " = 0.0137 "
The Titration.—Make astandard solution of iodineby weighing up in a beaker 16.933 grams of iodine and 30 grams of potassium iodide in crystals; add a few c.c. of water, and, when dissolved, dilute to 1 litre: 100 c.c. will equal 0.500 gram of arsenic.
A solution of starch similar to that used in the iodide-copper assay will be required. Use 2 c.c. for each assay. Variations in the quantity of starch used do not interfere; but the solution must be freshly prepared, as after seven or eight days it becomes useless.
To standardise the iodine solution, weigh up 0.3 gram of white arsenic; dissolve in caustic soda; neutralise; after acidulating, add 2 grams of bicarbonate of soda and 2 c.c. of the starch solution, and dilute to 200 c.c. with cold water. Fill a burette having a glass stop-cock with the iodine solution, and run it into the solution of arsenic, rapidly at first, and then more cautiously, till a final drop produces a blue colour throughout the solution. Calculate the standard in the usual way. White arsenic contains 75.76 per cent. of arsenic.
The following experiments show the effect of variation in the conditions of the titration:—
Make a solution of arsenic by dissolving 6.60 grams of white arsenic in 100 c.c. of sodic hydrate solution; render slightly acid with hydrochloric acid; add 10 grains of bicarbonate of soda, and dilute to 1 litre: 100 c.c. will contain 0.50 gram of arsenic.
Effect of Varying Temperature.—The reaction goes on very quickly in the cold, and, since there is no occasion for heating, all titrations should therefore be carried out cold.
Effect of Varying Bulk.—In these experiments, 20 c.c. of arsenic solution were taken, 2 grams of bicarbonate of soda and 2 c.c. of starch solution added, and water supplied to the required bulk. The results were:—
Bulk50.0 c.c.100.0 c.c.250.0 c.c.500.0 c.c."Iodine" required20.0 "20.0 "20.0 "20.0 "
Considerable variation in bulk does not interfere.
Effect of Varying Bicarbonate of Soda.—This salt must be present in each titration in considerable excess, to prevent the interference of free acid. The bicarbonate must be dissolved without heating, as neutral carbonates should be avoided.
Bicarbonate added1 gram2 grams5 grams10 grams"Iodine" required20.1 c.c.20.0 c.c.20.1 c.c.20.0 c.c.
These results show that large variation in the quantity of bicarbonate has no effect.
Effect of Free Acid.—In these experiments, the arsenic taken, the starch, and the bulk were as before, but no bicarbonate was added. In one case the solution was rendered acid with 5 c.c. of acetic acid, and in the other with 5 c.c. of hydrochloric acid; in both cases the interference was strongly marked, and no satisfactory finishing point could be obtained. This was much more marked with the hydrochloric acid.
Effect of Foreign Salts.—The process for getting the arsenic into solution will exclude all metals except tin, but the solution will be charged with sodium or ammonium salts in the process of neutralising, so that it is only necessary to see if these cause any interference. The alkaline hydrates, including ammonia, are plainly inadmissible, since no free iodine can exist in their presence. Monocarbonates similarly interfere, but to a much less extent; hence the necessity for rendering the assay distinctly acid before adding the bicarbonate of soda.
With 20 c.c. of arsenic solution; and with bulk, soda, and starch as before, the results obtained were:—
"Iodine" required.With 20 grams of ammonic chloride20.0 c.c." 20 grams of sodium chloride20.0 "" 20 grams of sodium acetate20.0 "" 0.050 gram of tin, as stannic chloride19.6 "Without any addition20.0 "
The interference of the stannic salt is probably mechanical, the precipitate carrying down some arsenious acid.
Effect of Varying Arsenic.—With bulk, starch, and soda as before, but with varying arsenic, the results were:—
Arsenic added1.0 c.c.10.0 c.c.20.0 c.c.50.0 c.c.100.0 c.c."Iodine" required1.1 "9.9 "20.0 "50.0 "100.0 "
Determination of Arsenic in Metallic Copper.—Put 1 gram of the copper filings, freed from particles of the file with a magnet, into a 16-oz.-flask; and distil with the ferric chloride mixture, as above described. Neutralise the distillate; acidify; add bicarbonate of soda and starch; dilute; and titrate with the standard solution of iodine.[107]Make a blank determination with 1 gram of electrotype copper, proceeding exactly as with the assay; and deduct the amount of arsenic found in this experiment from that previously obtained.
Working in this way on a copper containing 0.38 per cent. of arsenic and 0.80 per cent. of antimony, 0.38 per cent. of arsenic was found.
Determination of White Arsenic in Crude Arsenic.—Weigh out 1 gram of the dried and powdered substance (or 0.5 gram if rich), and digest with 10 c.c. of a 10 per cent. solution of soda; dilute to about 50 c.c., and filter. Render faintly acid with hydrochloric acid, and filter (if necessary); add 2 or 3 grams of bicarbonate of soda in solution, then 5 c.c. of starch, and titrate the cold solution with the standard solution of iodine.
The following is an example:—
1 gram of crude arsenic required 53.7 c.c. "Iodine;"100 c.c. "Iodine" = 0.6000 gram white arsenic;100 : 53.7 :: 0.6 : 0.3222, or 32.2 per cent.
With the test-tube method of dry assaying, this same sample gave results varying from 33 to 35 per cent. of white arsenic, which (judging from its appearance) was impure.
This may be looked upon as an alternative to the gravimetric method. It is applicable in all cases where the arsenic exists in solution as arsenic acid or as arsenate of soda. The process may be considered in two parts: (1) the preparation of the solution, and (2) the titration.
Preparation of the Solution.—If the arsenic has been separated as sulphide, it is sufficient to attack it with 10 or 15 c.c. of nitric acid, and to heat gently till dissolved, avoiding too high a temperature at first. Afterwards continue the heat till the separated sulphur runs into globules, and the bulk of the acid has been reduced to 3 or 4 c.c. Dilute with 20 or 30 c.c. of water; put in a piece of litmus paper; and add dilute ammonia until just alkaline. Then add 5 c.c. of the sodium acetate and acetic acid solution (which should make the solution distinctly acid); dilute to 150 c.c., and heat to boiling. The solution is ready for titrating.
When the arsenic exists in a nitric acid solution mixed with much copper, it is separated in the way described underExamination of Commercial Copper(Arsenic and Phosphorus), pages 208, 209.
If the arsenic has been separated as ammonium-magnesium arsenate, and phosphates are known to be absent; dissolve the precipitate (after filtering, but without washing) in dilute hydrochloric acid. Add dilute ammonia till a slight precipitate is formed, and then 5 c.c. of the sodium acetate and acetic acid solution; dilute to 150 c.c., and heat to boiling. Titrate.
If phosphates are present (which will always be the case if they were present in the original substance, and no separation with sulphuretted hydrogen has been made), the phosphorus will count in the subsequent titration as arsenic (one part of phosphorus counting as 2.4 parts of arsenic). It will be necessary to dissolve the mixed arsenate and phosphate of magnesia in hydrochloric acid. Add about four or five times as much iron (as ferric chloride) as the combined phosphorus and arsenic present will unite with, and separate by the "basic acetate" process as described underPhosphorusin theExamination of Commercial Copper, page 209. Obviously, when phosphates are present, it is easier to separate the arsenic as sulphide than to precipitate it with the "magnesia mixture."
The Titration.—Thestandard solution of uranium acetateis made by dissolving 34.1 grams of the salt (with the help of 25 c.c.of acetic acid) in water; and diluting to 1 litre. The water and acid are added a little at a time, and warmed till solution is effected; then cooled, and diluted to the required volume: 100 c.c. will equal 0.50 gram of arsenic.
Thesodic acetate and acetic acid solutionis made by dissolving 100 grams of sodic acetate in 500 c.c. of acetic acid, and diluting with water to 1 litre. Five c.c. are used for each assay.
The solution of potassic ferrocyanide used asindicatoris made by dissolving 10 grams of the salt in 100 c.c. of water.
To standardise the solution of uranium acetate, weigh up a quantity of white arsenic (As2O3) which shall be about equivalent to the arsenic contained in the assay (0.1 or 0.2 gram); transfer to a flask, and dissolve in 10 c.c. of nitric acid with the aid of heat. Evaporate to a small bulk (taking care to avoid the presence of hydrochloric acid); dilute with water; add a small piece of litmus paper; render faintly alkaline with ammonia; then add 5 c.c. of the sodic acetate mixture; dilute to 150 c.c.; and heat to boiling.
Fill an ordinary burette with the uranium acetate solution, and run into the assay a quantity known to be insufficient. Again heat for a minute or two. Arrange a series of drops of the solution of ferrocyanide of potassium on a porcelain slab, and, with the help of a glass rod, bring a drop of the assay solution in contact with one of these. If no colour is produced, run in the uranium acetate, 1 c.c. at a time, testing after each addition, till a brown colour is developed. It is best to overdo the assay, and to count back. It is not necessary to filter off a portion of the assay before testing with the "ferrocyanide," since the precipitate (uranic arsenate) has no effect.
The following experiments show the effect of variation in the conditions of titration. Make a solution of arsenic acid by dissolving 4.95 grams of arsenious acid (As2O3) in a covered beaker with 35 c.c. of nitric acid; evaporate down to 7 or 8 c.c.; and dilute with water to 1 litre: 100 c.c. will contain 0.375 gram of arsenic. Use 20 c.c. for each experiment.
Effect of Varying Temperature.—It is generally recommended to titrate the boiling solution, since it is possible that the precipitation is only complete on boiling. Low results are obtained in a cold solution, the apparent excess of uranium acetate striking a colour at once; on boiling, however, it ceases to do so; consequently, the solution should always be boiled directly before testing.
In four experiments made in the way described, but with 20 c.c. of a solution of arsenic acid stronger than that given (100 c.c. = 0.5 gram As), the results at varying temperatures were:—
Temperature15° C.30° C.70° C.100° C."Uranium" required18.0 c.c.18.5 c.c.18.5 c.c.18.7 c.c.
Effect of Varying Bulk.—These experiments were like those last mentioned, but were titrated boiling, and the volume was varied:—
Bulk50.0 c.c.100.0 c.c.200.0 c.c.300.0 c.c."Uranium" required14.0 "14.0 "14.5 "15.0 "
Considerable variations in bulk are to be avoided.
Effect of Varying Sodium Acetate.—These experiments were carried out like those last noticed, but the bulk was 150 c.c., and varying amounts of sodic acetate were added in excess of the quantity used in the experiments previously described:—
Sodic acetate added0 gram1 gram10 grams20 grams"Uranium" required14.5 c.c.14.5 c.c.16.0 c.c.18.0 c.c.
It is evidently important that the quantity of this salt present in each titration be measured out, so as to avoid variation.
Effect of Varying the Sodium Acetate and Acetic Acid Solution.—Acetic Acid also affects the results, but in the opposite direction, by preventing the precipitation of uranium arsenate. With varying volumes of the solution now under notice, the results were:—
Solution added0.0 c.c.5.0 c.c.10.0 c.c.15.0 c.c."Uranium" required14.5 "14.5 "14.5 "14.0 "Solution added20.0 "30.0 "40.0 "50.0 ""Uranium" required13.2 "10.0 "6.0 "2.0 "
These show that the quantity ordered (5 c.c.) must be adhered to.
Effect of Foreign Salts.—In these experiments, 10 grams of the salt (the effect of which it was desired to determine) were added to a solution in other respects resembling those previously used:—
Salt addedAmmonic sulphateAmmonic nitrateAmmonic chlorideMagnesium sulphate"Uranium" required15.5 c.c.15.5 c.c.15.3 c.c.15.3 c.c.
Without any addition, 15.0 c.c. were required; and in another experiment, in which 30 grams of ammonic salts were present,15.6 c.c. of uranium solution were required. Such variations in the amount of ammonic salts as occur in ordinary working are unimportant.
Phosphates, of course, interfere. In fact, the uranium acetate solution can be standardised by titrating with a known weight of phosphate, and calculating its equivalent of arsenic. Thus, in an experiment with 0.6 gram of hydric sodic phosphate (Na2HPO4.12H2O), equivalent to 0.05195 gram of phosphorus, or 0.1256 gram of arsenic, 23.25 c.c. of a solution of uranium acetate were required. The same solution standardised with white arsenic gave a standard of which 100 c.c. = 0.5333 gram arsenic. On this standard the 0.6 gram of sodic phosphate should have required 23.5 c.c.
Experiments in which 0.1 gram of bismuth and 0.1 gram of antimony were present with 0.1 gram of arsenic, showed no interference on the titration. Ferric or aluminic salts would remove their equivalent of arsenic, and, consequently, must be removed before titrating.
Effect of Varying Arsenic.—Varying amounts of metallic arsenic were weighed up and dissolved in nitric acid, &c., and titrated:—
Arsenic taken0.010 gram0.050 gram0.100 gram0.200 gramArsenic found0.010 "0.050 "0.100 "0.197 "
These experiments show that the method yields good results within these limits.
Determination of Arsenic in Mispickel.—Weigh up 1 gram of the dried and powdered ore, and evaporate to near dryness with 20 c.c. of dilute nitric acid. Make up to 100 c.c. with water, and pass sulphuretted hydrogen to reduce the ferric iron to the ferrous state, then add 20 c.c. of dilute ammonia, and again pass sulphuretted hydrogen. Warm, filter, and evaporate the filtrate to drive off the excess of ammonia; then add 10 c.c. of nitric acid, and boil down till the sulphide of arsenic at first precipitated is dissolved; neutralise; add 5 c.c. of sodium acetate and acetic acid solution; transfer to a pint flask, boil, and titrate.
For example, an impure sample of ore required, in duplicate assay of half a gram each, when treated in the above-mentioned way, 39.6 and 39.5 c.c. of the uranium acetate solution (100 c.c. = 0.537 gram of arsenic), equivalent to 0.2114 gram of arsenic, or 42.3 per cent.
An alternative method is as follows. Powder the ore very finely and weigh up .5 gram. Place in a 2-3/4 inch berlin dish and add strong nitric acid, one drop at a time until the action ceases; with care there need be no very violent reaction. Dry over awater bath. Cover with 2 grams of nitre and over this spread 5 grams of a mixture of equal parts of nitre and carbonate of soda. Fuse in a muffle or over a large gentle blow-pipe flame for 4 or 5 minutes. This will spoil the dish. Allow to cool and boil out in a larger dish with 100 c.c. of water. Filter and wash into an 8 oz. flask. Acidify the liquor with nitric and boil down to about 100 c.c. The acid should not be in too large excess, but an excess is needed to destroy nitrites. Neutralise with soda or ammonia. Add 5 c.c. of the mixture of sodium acetate and acetic acid. Titrate with uranium acetate.
Determination of Arsenic (As) in Crude Arsenic.—The method given under the iodine titration simply determines that portion of the arsenic which is present in the substance as arsenious oxide or white arsenic. The following method will give the total arsenic in the sample. It would be incorrect to report this as so much per cent. of arsenious oxide, although it may be reported as so much per cent. of arsenic equivalent to so much per cent. of white arsenic, thus:—
Arsenic30.0 per cent.Equivalent to white arsenic39.6 "
The equivalent of white arsenic is calculated by multiplying the percentage of arsenic by 1.32. The method of determining the percentage of arsenic is as follows:—-Boil 1 gram of the sample with 10 c.c. of nitric acid. When the bulk of the solution has been reduced to one-half, and red fumes are no longer evolved, dilute with a little water, and filter into a flask. Neutralise the filtrate, add 5 c.c. of sodic acetate solution, boil and filter. The precipitate (ferric arsenate) is transferred to a small beaker, treated with 5 c.c. of dilute ammonia, and sulphuretted hydrogen passed through it. The iron sulphide is filtered off, and the filtrate evaporated with an excess of nitric acid. When the solution is clear, it is neutralised, and 1 or 2 c.c. of sodic acetate solution having been added, is then mixed with the first filtrate. The solution is boiled and titrated.
A sample treated in this way required 49.2 c.c. of the uranium acetate solution (100 c.c. = 0.537 gram of arsenic), equivalent to 26.4 per cent.
Determination of Arsenic in Brimstone.—Take 10 grams of the substance, and powder in a mortar; rub up with 10 c.c. of dilute ammonia and a little water; rinse into a pint flask; pass a current of sulphuretted hydrogen; and warm on a hot plate for a few minutes. Filter, acidulate the filtrate with sulphuric acid; filter off the precipitate; attack it with 10 c.c. of nitric acid; and proceed as in the other determinations.
1. Mispickel contains 45.0 per cent. of arsenic, to how much white arsenic will this be equivalent?
2. How would you make a standard solution of iodine so that 100 c.c. shall be equivalent to 1 gram of white arsenic?
3. What weight of arsenic is contained in 1 gram of pyrarsenate of magnesia, and what weight of ammonic-magnesic arsenate would it be equivalent to?
4. The residue, after heating 10 grams of crude arsenic, weighed 0.62 gram. What information does this give as to the composition of the substance? If another 10 grams of the substance, heated on a water-bath, lost 0.43 gram, what conclusions would you draw, and how would you report your results?
5. If a sample of copper contained 0.5 per cent. of arsenic, and 1 gram of it were taken for an assay, how much standard uranium acetate solution would be required in the titration?
Phosphorus rarely occurs among minerals except in its highest oxidized state, phosphoric oxide (P2O5), in which it occurs abundantly as "rock phosphate," a variety of apatite which is mainly phosphate of lime. Phosphates of most of the metallic oxides are found. Phosphoric oxide in small quantities is widely diffused, and is a constituent of most rocks. Its presence in varying amounts in iron ores is a matter of importance, since it affects the quality of the iron obtainable from them.
Phosphorus occurs in alloys in the unoxidized state. It is directly combined with the metal, forming a phosphide. In this manner it occurs in meteoric iron. The alloy phosphor-bronze is made up of copper, tin, zinc, and phosphorus.
Phosphates are mined in large quantities for the use of manure manufacturers, and for making phosphorus.
Phosphorus and arsenic closely resemble each other in their chemical properties, more especially those which the assayer makes use of for their determination. Phosphorus forms several series of salts; but the phosphates are the only ones which need be considered. Pyrophosphate of magnesia, which is the form in which phosphoric oxide is generally weighed, differs from the ordinary phosphate in the proportion of base to acid. Metaphosphates differ in the same way. If these are present, it must be remembered they act differently with some reagents from the ordinary phosphates, which are called orthophosphates. They are, however, all convertible into orthophosphates by some meanswhich will remove their base, such as fusion with alkaline carbonates, boiling with strong acids, &c.[108]
Phosphides are converted into phosphates by the action of nitric acid or other oxidizing agents. Dilute acids, when they act on the substance, evolve phosphuretted hydrogen (PH3). The student should be on his guard against losing phosphorus in this manner.
There is no dry assay for phosphorus. All assays for it are made either gravimetrically or volumetrically.
The separation of phosphoric oxide is made as follows:—The ore or metal is dissolved in acid and evaporated, to render the silica insoluble. It is taken up with hydrochloric acid, diluted with water, and treated with sulphuretted hydrogen. The filtrate is boiled, to get rid of the excess of gas, and treated with nitric acid, to peroxidize the iron present. If the iron is not present in more than sufficient quantity to form ferric phosphate with all the phosphorus present, some ferric chloride is added. The iron is then separated as basic acetate. The precipitate will contain the phosphorus, together with any arsenic acid not reduced by the sulphuretted hydrogen. The precipitate should have a decided brown colour. The precipitate is washed, transferred to a flask, and treated first with ammonia, and then with a current of sulphuretted hydrogen. The filtrate from this (acidulated with hydrochloric acid, and, if necessary, filtered) contains the phosphorus as phosphoric acid. This method is not applicable in the presence of alumina, chromium, titanium, or tin, if the solution is effected with nitric acid. The precipitate obtained by the action of nitric acid on tin retains any phosphoric or arsenic oxide that may be present.
A method of separation more generally applicable and more convenient to work is based on the precipitation of a yellow phospho-molybdate of ammonia,[109]by the action of an excess of ammonic molybdate upon a solution of a phosphate in nitric acid. Dissolve the substance by treatment with acid, and evaporate to dryness. Take up with 10 c.c. of nitric acid, and add 20 grams of ammonic nitrate, together with a little water. Next put in the solution of ammonium molybdate solution in the proportion of about 50 c.c. for each 0.1 gram of phosphoric oxide judged to be present. Warm to about 80° C., and allow to stand for an hour. Filter, and washwith a 10 per cent. solution of ammonic nitrate. It is not necessary that the whole of the precipitate be placed on the filter; but the beaker must be completely cleaned. Dissolve the precipitate off the filter with dilute ammonia, and run the solution into the original beaker. Run in from a burette, slowly and with stirring, "magnesia mixture," using about 15 c.c. for each 0.1 gram of phosphoric oxide. Allow to stand for one hour. The white crystalline precipitate contains the phosphorus as ammonium-magnesium phosphate.
Phosphate of lead is decomposed by sulphuric acid; the lead is converted into the insoluble lead sulphate, and the phosphoric acid is dissolved. Phosphate of copper and phosphate of iron may be treated with sulphuretted hydrogen; the former in an acid, and the latter in an alkaline, solution. Phosphate of alumina is generally weighed without separation of the alumina, since this requires a fusion. In all cases the aim is to get the phosphoric oxide either free, or combined with some metal whose phosphate is soluble in ammonia.
Joulie's method of separation is as follows:—One to ten grams of the sample are treated with hydrochloric acid, and evaporated to dryness with the addition (if any pyrites is present) of a little nitric acid. The residue is taken up with hydrochloric acid, cooled, transferred to a graduated flask, and diluted to the mark. It is then shaken up, filtered through a dry filter, and a measured portion (containing about 0.05 gram of phosphoric acid) transferred to a small beaker. Ten c.c. of a citric-acid solution of magnesia[110]is added, and then an excess of ammonia. If an immediate precipitate is formed, a fresh portion must be measured out and treated with 20 c.c. of the citrate of magnesia solution and with ammonia as before. The beaker is put aside for from two to twelve hours. The precipitate is then filtered off and washed with weak ammonia; it contains the phosphorus as ammonium-magnesium phosphate.
If the phosphate is not already in the form of ammonic-magnesic phosphate, it is converted into this by the addition to its solution of an excess of ammonia and "magnesia mixture." In order to get the precipitate pure, the "magnesia mixture" is run ingradually (by drops) from a burette, with constant stirring. A white crystalline precipitate at once falls, if much phosphorus is present; but, if there is only a small quantity, it may be an hour or two before it shows itself. The solution is best allowed to rest for twelve or fifteen hours (overnight) before filtering. The presence of tartaric acid should be avoided; and the appearance of the precipitate should be crystalline. The solution is decanted through a filter, and the precipitate washed with dilute ammonia, using as little as may be necessary. The precipitate is dried, transferred to a weighed Berlin or platinum crucible; the filter-paper is carefully burnt, and its ash added to the precipitate, which is then ignited, at first gently over a Bunsen burner, and then more strongly over the blowpipe or in the muffle. The residue is a white mass of magnesium pyrophosphate containing 27.92 per cent. of phosphorus, or 63.96 per cent. of phosphoric oxide.
Instead of separating and weighing this compound, the phosphoric oxide in it can be determined by titration. In many cases the ore may be dissolved and immediately titrated without previous separation. It is better, however, to carry the separation so far as to get phosphoric acid, an alkaline phosphate, or the magnesia precipitate. It may then be prepared for titration in the following way:—The precipitate in the last case (without much washing) is dissolved in a little hydrochloric acid, and the solution in any case rendered fairly acid. Dilute ammonia is added till it is just alkaline, and then 5 c.c. of the sodic acetate and acetic acid mixture (as described under the Arsenic Assay). This should yield a clear distinctly-acid solution. It is diluted to 100 or 150 c.c., heated to boiling, and titrated with the uranium acetate solution, using that of potassic ferrocyanide as indicator.
Thestandard solutionrequired is made by dissolving 35 grams of uranium acetate in water with the aid of 25 c.c. of acetic acid, and diluting to 1 litre.
Anequivalent solution of phosphoric oxideis made by dissolving 25.21 grams of crystallised hydric disodic phosphate (HNa2PO4.12H2O) in water, and making up to 1 litre. 100 c.c. will contain 0.5 gram of phosphoric oxide (P2O5), or 0.2183 gram of phosphorus. In making this solution, transparent crystals only must be used. The uranium acetate solution is only approximately equivalent to this, so that its exact standard must be determined.
Sodic Acetate and Acetic Acid Solution.—It is the same as that described underArsenic.[111]Use 5 c.c. for each assay.
The following experiments show the effect of variation in the conditions of the titration:—
Effect of Varying Temperature.—The solution should be titrated while boiling. This is especially necessary for the last few c.c. in order to get a decided and fixed finishing point.