The Oxalate Method.
The method consists in taking a weighed amount of cadmium oxalate, decomposing it by heat, when a mixture of oxide and metal are said to be formed, dissolving this mixture in nitric acid, converting the nitrate into oxide and weighing the oxide.
Lenssen[8]obtained results by this method which agree very closely with those recorded in the earlier part of this dissertation.
Working with the same method, Partridge[9]arrived at a value about one fourth of a unit lower than that of Lenssen.
It appeared desirable that this method should be repeated with the greatest care to ascertain what result it would give under the most favorable conditions.
Having a supply of pure cadmium it was necessary to prepare pure oxalic acid.
The commercial acid was crystallized three times from cold water to separate it from acid oxalates. It was then boiled for two days with a 15 per cent solution of hydrochloric acid, to remove any mineral matter present. The acid which crystallized from the hydrochloric acid solution was recrystallized twice from hot, redistilled alcohol andtwice from pure ether. It was finally boiled with water to decompose any ethyl oxalate and twice crystallized from pure water. The acid was dried in the air at ordinary temperatures. This acid left no residue on ignition.
A piece of cadmium was dissolved in pure nitric acid. On carefully evaporating the solution cadmium nitrate was obtained. Twenty-five grams of the nitrate were dissolved in 750 c.c. of redistilled water. Somewhat less than an equivalent of the oxalic acid was dissolved in an equal volume of water, and slowly added to the solution of the nitrate with constant shaking. A little less than an equivalent of oxalic acidwas used to avoid any tendency to form acid oxalates. Cadmium oxalate was precipitated on standing a few minutes as a white crystalline compound, well adapted to washing. The oxalate was filtered off and washed until the wash water was free from all traces of nitric acid. It was then washed ten times with water which had been twice redistilled and dried in an air-bath for twenty hours at 150°C.
The arrangement of the crucibles which were weighed was in all respects like that in the preceding method.
The crucibles were heated, tared, and weighed exactly as in thepreceding method. The oxalate was weighed in ground-stoppered weighing tubes from which it was transferred to the inner of the two porcelain crucibles. The pair of crucibles, (1 and 2 fig. 3) was placed in a third porcelain crucible and the whole system introduced into an upright air-bath. The outer crucible was supported on a porcelain triangle about an inch from the bottom of the bath and was not allowed to touch its walls at any point. The top of the bath was covered with a sheet of iron over which was placed an asbestos board. The exterior was also covered with a lining of asbestos. A thermometer was introduced well into the bath. The temperature was allowed to rise slowly until the oxalate began to show a brown color around the edge. From this stagethe temperature was kept as low as possible in order to effect the decomposition. When the oxalate was decomposed the bath was allowed to cool and the contents of the crucible completely dissolved in nitric acid. The nitrate was evaporated to dryness and decomposed as in the method first described. The end of the decomposition was determined in the same manner and the oxide, free from all impurities, weighed.
It was necessary to dry the oxalate before weighing from fifteen to twenty hours at 150°C. in addition to the twenty hours drying of the whole preparation. At this temperature the last traces of moisture were removed by prolonged heating.
The weighing of the oxalate was made in the weighing glasses in which it was dried. Two of these glasses had been previously tared against each other, using the lighter as the tare and adding fragments of glass to it until the difference in weight was a small fraction of a milligram. The oxalate having been dried to constant weight, was weighed. It was then poured as carefully and completely as possible from the weighing glass into the crucible and the glass again weighed against its tare. The difference in the two weights gave the amount of oxalate. The glass and its tare were dried and reweighed to determineif the few milligrams of oxalate adhering to the walls of the glass had absorbed any moisture during the transfer of the oxalate. In one experiment a slight difference was detected when a second drying and weighing were made.
The weight of the cadmium oxalate as obtained from the balance was corrected for the difference in specific gravity between the cadmium oxalate and the weights.
The values assigned to carbon in the last two columns were found thus—
Calculating the atomic weight directly from all the oxalate used and oxide found it would give:
There seems about equal evidence for the two values assigned to carbon when oxygen = 16. The value of cadmium as given by this method is therefore 112.025 or 112.032.
As will be seen at a glance this figure agrees much more closely with that of Lenssen than with that of Partridge.
It also agrees fairly well with the figure 112.0706 which I obtained by the first method described.
The method possesses no advantage whatever over the one which involves starting with the element itself. The oxalate can however be obtained pure having pure metal. The salt is of definite composition when perfectly dry.
The method as carried out avoided the contact of any foreign material with the salt after it was weighed.
1 The avidity with which the dried oxalate takes up moisture from the air is an objection to its use for the determination of atomic weights. Even with the greatest care there is a slight element of uncertainty introduced from this source.2 The oxalate is stated to decompose into a mixture of the oxide and metal. The temperature required for thisdecomposition is somewhat higher than the melting point of cadmium. The metal heated above its melting point possesses a vapor-tension and loss in weight must result, whatever precaution is taken in heating. This is the probable explanation why the results obtained by this method are lower than those of the preceding.
1 The avidity with which the dried oxalate takes up moisture from the air is an objection to its use for the determination of atomic weights. Even with the greatest care there is a slight element of uncertainty introduced from this source.
2 The oxalate is stated to decompose into a mixture of the oxide and metal. The temperature required for thisdecomposition is somewhat higher than the melting point of cadmium. The metal heated above its melting point possesses a vapor-tension and loss in weight must result, whatever precaution is taken in heating. This is the probable explanation why the results obtained by this method are lower than those of the preceding.
A comparison of the two methods leads me to attach much more importance to the results of that one which establishes the relation between cadmium and cadmium oxide directly and I therefore regard the atomic weight of cadmium as very closely expressed by the figure 112.07 when oxygen = 16.