1. The reactions involved in this preparation cannot be expressed quantitatively in a simple equation. The process has been investigated by Gröger.¹

2. Barium nitrate is sparingly soluble in cold water and even less soluble in copper nitrate solution. It is therefore necessary to heat the mixture in order to bring both salts into solution together.

3. The warm, freshly prepared ammonium chromate solution may be used for the precipitation at once or may be allowed to cool to room temperature. However, the solution prepared as indicated is supersaturated at room temperature and deposits crystals on standing. If a stock solution of ammonium chromate is to be held over from day to day, a portion of the ammonia should be withheld and added immediately before precipitation. Chromic acid may be used for the preparation of the ammonium chromate solution provided that the acid solution is kept cold while a sufficient quantity of ammonia is introduced below the surface.

4. The amount of ammonia used is calculated to give a mother liquid that is neutral to litmus, allowance being made for the basic character of the precipitate. While a deviation either way affects the yield adversely, it is better not to try to adjust the end point after precipitation, but to make a slight correction on the amount of ammonia to be used for the next batch. The mother liquor serves as its own indicator: if acid, the solution is yellow; if alkaline, a deep green.

5. A convenient-sized pan, 6 cm. deep, 19 cm. long, and 10 cm. wide, can be made from a sheet of nickel 1/32 inch thick.

6. The ignition of copper ammonium chromate causes a spontaneous exothermic reaction which proceeds with a rapid evolution of gas. Although the size of the charge ignited at one time is unimportant, the retaining vessels should not be too full, else there will be considerable mechanical loss of product. It is also inadvisable to break up the lumps of dried precipitate, as the lumpy condition diminishes spraying of the chromite on ignition.

7. The unextracted catalyst should be a bluish black, friable powder. It is a satisfactory catalyst for the dehydrogenation of alcohols and for the less difficult hydrogenation reactions, such as the reduction of nitro compounds. This mixture of copper chromite and copper oxide is somewhat less active and more susceptible of reduction to metallic copper than the catalyst from which the copper oxide has been removed by acid extraction.

8. If the washing is unduly prolonged, the catalyst tends to become colloidal and is difficult to separate either by decantation or filtration.

9. No special precautions are necessary in handling or storing the copper chromite catalyst, since it is unharmed by exposure to air or moisture.

10. Barium has been included as a catalyst component on account of its protective action against sulfate poisoning and its reported stabilization of the catalyst against reduction. Alternatively, the above procedure may be used for the preparation of a copper chromite catalyst containing no barium. In this case the barium nitrate is omitted and 242 g. (1 mole) of copper nitrate is used. All other details are the same as given above.

11. An alternative procedure described by Adkins² is as follows: Thirty-one grams of barium nitrate is dissolved in 820 cc. of distilled water which has been heated to 80°. To this hot solution is added 260 g. of copper nitrate trihydrate, and the mixture is stirred and heated until solution is complete. Meanwhile, 151 g. of ammonium dichromate is dissolved in 600 cc. of distilled water and 225 cc. of 28 per cent aqueous ammonia added. The hot nitrate solution is poured in a thin stream with stirring into the ammonium chromate solution. The orange precipitate is collected on a filter, pressed, and sucked as dry as possible. It is dried in an oven at 75–80° for twelve hours, pulverized, and divided into three portions. Each portion is decomposed separately in a large porcelain casserole (15-cm. diameter) by heating over a free flame. The mass should be heated just hot enough to cause decomposition to take place at the minimum temperature. During the decomposition the powder is stirred continuously with a steel spatula, and the heating is regulated so that the evolution of gases does not become violent. This is accomplished by heating only one side of the casserole and by increasing the rate of stirring when the decomposition starts to spread throughout the mass. During this process the color of the powder changes from orange to brown and finally to black. When the entire mass has become black the evolution of gases ceases, and the powder is removed from the casserole and allowed to cool. The combined product is then stirred for thirty minutes with 600 cc. of 10 per cent acetic acid solution, collected on a filter, washed by suspending it six times in 100-cc. portions of water, dried for twelve hours at 125°, and pulverized. The yield of catalyst is 170 g.