John Emery Bucher

“ The foil was always kept submerged in the acid liquid. The moist crystals of cadmium chloride were transferred to a combustion tube passing through an asbestus covered air-bath, and dried in a current of hydrochloric acid gas for several hours at 300°C. The hydrochloric acid gas had been passed through a long calcium chloride tube to dry it, although calcium chloride probably does not do this very thoroughly. The hydrochloric acid gas was then replaced by 40a current of nitrogen prepared as has already been described under the sulphide method. ”

“ It is well to put the oxide into the bulb before the tube is bend. If the hydrochloric acid is too strong, it must be cooled before entering the bulb as otherwise the reaction is too violent, and the experiment may be lost. This experiment shows that there is no excess of cadmium present in the oxide employed for no gas was found. If three grammes of the oxide contained enough metal to take up .00126 grms. of oxygen, .00016 grms of hydrogen should have been set free, and its volume under ordinary conditions of temperature and pressure would have been about 1.9 114cubic centimetres. ”

“ Carbonate of cadmium was 102made by dissolving pure cadmium in nitric acid, adding an excess of ammonia and a small quantity of ammonium carbonate. After standing for some time the cadmium carbonate was filtered off and rejected. The filtrate was treated with an excess of ammonium carbonate and the precipitated cadmium carbonate allowed to digest in it for some time. After washing by decantation several times the carbonate was transferred to a funnel containing a porcelain filter-plate, covered with a piece of ashless filter paper of slightly larger diameter, and washed thoroughly. ”

“ No metal was observed on taking up in nitric acid. To be certain of what the cause of error is would 118require some very carefully conducted experiments, but as there are a number of much more reliable methods for determining the atomic weight of cadmium, it does not seem desirable to spend the time required in making them. It should be mentioned that Lenssen, in 1860, first employed this method. He made three determinations. 1.5697 grms of cadmium oxalate giving 1.0047 grammes of oxide, which gives a value of 112.043 for the atomic weight of cadmium . ”

“ Having started with an acid which was probably pure and subjected it to these operations with reagents free from chlorine, there can be no doubt as to the purity of the resulting acid. The hydrogen sulphide used was prepared from potassium hydrosulphide as in the sulphide method, and washed first with a solution of the hydrosulphide, then very thoroughly 78with pure water. From the hydrobromic acid obtained, a specimen of cadmium bromide was prepared as before and sublimed twice in a vacuum. ”

“ The silver chloride obtained always darkened on heating and contained cadmium, as was shown in the following manner: The lump of silver chloride was attached to the negative pole of a cell and electrolyzed in a bath containing dilute sulphuric acid. The resulting metal was then dissolved in nitric acid and the silver precipitated by adding hydrochloric acid. The filtrate was evaporated to expel the nitric acid and the residue taken up with water 65and tested for cadmium with hydrogen sulphide. An appreciable quantity was always found. ”

“ The crucible was then dried slowly in an air-bath. After this it was transferred to a porcelain crucible and slowly heated until decomposition was complete. In this manner a layer of platinum felt is obtained which acts as a very efficient 50filter. Another layer of double chloride was then decomposed as before so that if there were any imperfections in the first layer they would be covered by the second layer. The surface was smoothed down by means of a glass rod. To prepare a good filter the drying and subsequent heating should be very slow. ”

“ The heating must not be at too high a temperature, otherwise the felt becomes very compact and is useless for filtering purposes. Pressure produces the same effect. The filters were always treated with strong nitric acid, washed and 51reheated before being used, but in no case was chlorine detected in the nitric acid after the washing, nor any loss in weight of the crucible. ”

“ The more important ones will now be given. Nearly every filtrate including the corresponding wash water was examined for chlorine after the silver and cadmium had been precipitated by hydrogen sulphide. The excess of hydrogen sulphide was expelled by boiling, after the addition of some nitric acid. In two cases an inverted condenser was used. On adding silver 61nitrate a precipitate was always obtained showing the presence of chlorine. Care was always taken to filter off sulphur formed by the oxidation of hydrogen sulphide, before adding the silver nitrate. ”

“ This was accomplished by boring out the stopcocks of the blast lamp so that a larger supply of gas was furnished. The two oxides in the platinum crucibles seemed to be constant in weight, but that in the porcelain crucible seemed to lose in weight slowly. The weight given was taken after four hours blasting, which is 108longer and at a higher temperature than was used in any of the five determinations made by the oxide method. If the cadmium oxide prepared from the carbonate retained any carbon dioxide, it would lose weight in being dissolved and reconverted into oxide. ”

“ The current of hydrogen sulphide was then replaced by one of nitrogen, in which the boats were cooled, but while still warm they were transferred to their weighing tubes and allowed 20to cool in a dessiccator containing caustic potash and weighed. Before weighing, the stoppers of the weighing tubes were loosened for a moment in order to equalize the internal and external pressure. ”

“ The author wishes to acknowledge his indebtedness for advice and instruction to Professor Morse at whose suggestion and under whose guidance this work has been carried on. He also wishes to express his thanks for instruction to Professor Remsen in Chemistry, Professor Williams in Mineralogy, Dr. Ames in Physics and Mr. Hulburt in Mathematics. ”

“ In the first four determinations, there may have been loss of cadmium by reduction and subsequent volatilization, but in the later determinations this is not probable. It is believed that the greater part of the variation 15was due to imperfect dehydration of the oxalate. This and other sources of error in this method will be referred to later. The nickel crucible used gave a slight sublimate on heating, even after fifteen hours’ blasting. ”