Part 6

These results agree more closely with those of von Hauer and Lenssen than with those of any other experimenter. The following table gives a comparison of the work of these investigators with that herein described:

von Hauer. Lenssen. Work here described. 9 determinations. 3 determinations. 10 determinations. (O = 16) (O = 16) (O = 16) Mean 111.940 112.067 112.0705 Max. 112.121 112.304 112.086 Min. 111.796 111.911 112.053 Diff. .325 .393 .033

A difference of three or four tenths of a unit between the different results of a series leaves considerable doubt as to the accuracy of the method employed and to the value obtained.

The figure selected by Ostwald, as most probable for the atomic weight of cadmium is 112.08. This is the mean of the results on von Hauer and Huntington. My own work leads me to believe that this number is very close to the true value when oxygen is taken as 16.

Lehrb. d. Allg. Chem. I, 60.

Objections to the method.

Marignac offered the objection to this method for determining the atomic weight of zinc that the zinc oxide dissociated when heated in platinum over the blast-lamp. The same objection might be urged against this method for determining the atomic weight of cadmium, had it not been shown that the objection does not hold for zinc. What took place was a reduction of the zinc oxide by the highly heated hydrogen which passed through the hot platinum.

Archives des Sciences Phys. et Nat. (3) 10, 193.

Amer. Chem. Journ. X, 148.

It was shown that zinc oxide can be heated in a platinum vessel in a muffle furnace, to the melting point of steel, without undergoing any dissociation, or in any wise losing in weight. This source of error was avoided by using porcelain vessels, which were not brought into contact with the free flame.

The statement of Marignac that the oxide of zinc derived from the nitrate retains oxides of nitrogen even when heated to the temperature at which it begins to undergo dissociation, was shown by the same authors to be without foundation. The basis of this objection is doubtless to be found in the imperfect method of testing for such oxides.

It might be urged as an objection to this method that the difference in weight between the metal and oxide is not very great, therefore any error in weighing would be multiplied in the result. At first sight this objection may appear valid, but since the substances weighed were so well adapted to that purpose and the weighings could be made with such a high degree of accuracy no appreciable error could have resulted from this source.

A crucible with its contents was repeatedly weighed against its tare and weights to ascertain the difference between successive weighings under the conditions employed. A number of weighings agreed to .00002 gr. and in some instances to half this amount.

Advantages of the Method.

1 The great advantage of the method is its extreme simplicity. From the beginning of an experiment until the end the contents of the crucible are not brought into contact with any foreign substance. By this means small errors resulting from incomplete precipitation, and filtration and all other errors incident to ordinary processes of analysis were avoided.

2 The nature of the metal and its oxide rendered them well adapted to weighing. The specific gravity of the metal and oxide approached so closely to that of the weights, that it was unnecessary to reduce the weighings to a vacuum standard.

3 The advantages derived from weighing by tares have been pointed out.

4 The closely agreeing results speak strongly in favor of the accuracy of the method.

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 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 arrived at a value about one fourth of a unit lower than that of Lenssen.

Journ. f. prakt. Chem. 79, 281.

Amer. Journ. Science XL, 377.

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.

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.