Having a supply of pure cadmium it was necessary to prepare pure oxalic acid.
Preparation of 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 and twice 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.
Preparation of Cadmium Oxalate.
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 acid was 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.
Mode of Procedure.
The crucibles were heated, tared, and weighed exactly as in the preceding 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 stage the 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.
The Drying and Weighing of the Oxalate.
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.