ADSORPTION O F POTASSIUM CHROMATE ON ZINC* BY R. F. REED AND S. C. HORNING
Chromic acid and its salts are commonly used in lithographic etches. These etches are supposed to reduce the affinity of the zinc lithographic plates for ink and to increase their affinity for water. This change in the character of a rnetal surface in the presence of various substances can be shown in many ways. Iron which has been immersed in concentrated nitric acid, or zinc which has been immersed in chromic acid will no longer react immediately with copper sulphate. These phenomena have received various explanations, but all of the theories depend on the formation of a surface film having new properties. The data obtained shows that potassium chromate reacts with a zinc surface, imparting to it new properties by depositing on it a compound of chromium. A qualitative measure of the extent to which the activity of a zinc surface is reduced by potassium chromate should be obtained by determining the effect of potassium chromate on the catalytic activity of zinc, adsorption on zinc, and on the chemical reactivity of zinc. Only the last two methods were investigated. The results obtained with these methods indicate that potassium chromate should act as a strong poison for zinc catalysts. The amount of corrosion of zinc over a period of twenty hours was determined in three separate solutions, one of distilled water, one of .00286 M potassium chromate, and one of .00286 bf chromic acid. These tests were made on two different samples of zinc, one sample containing 99.9 per cent zinc, and the other approximately 99.0per cent zinc. The metal in each case was completely covered with the solution and aeration was continued throughout the tests. The plates in the distilled water became covered with a deposit of basic carbonate and were pitted, while the plates in both the potassium chromate and chromic acid solutions remained clean and retained their polished appearance. The losses in weight of the plates in chromic acid and in potassium chromate sdutions were approximately equal, being about .OI mg. per square centimeter, while the loss in distilled water was more than .35 mg. per square centimeter. There was no noticeable difference in the rates of corrosion of the two grades of zinc of different purities. A comparison was made of the amount of moisture adsorbed by clean zinc surfaces and surfaces which had previously been treated with a solution of potassium chromate. The moisture adsorbed was determined by first passing an inert gas saturated with moisture over the plates and then passing dry gas over the plates and collecting the water in calcium chloride tubes. The clean zinc adsorbed .0013 mg. of water per square centimeter, while the treated plates adsorbed .0009 mg. of water per square centimeter.
* Institute of Scientific Research, University of Cincinnati, Department of Lithographic Research.
ADSORPTION OF POTASSIUM CHROUATE ON ZINC
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Three zinc plates were cleaned by rubbing with fine pumice powder and water. The first was allowed to dry and then immersed in a z per cent solution of methyl violet; the second was immersed, without drying, in the methyl violet solution; and the third was immersed, without drying, in a 15 per cent solution of potassium chromate and then in the methyl violet solution. The first was definitely colored a light blue, which could not be removed with water; the second acquired 5 deep blue bronze color, very similar to that of the methyl violet crystals, which changed on standing in air or on continued washing t o the original color of the dye; while the third was not colored at all. These experiments show conclusively that a zinc surface is changed in character through the action of potassium chromate. Since a reaction of some kind must occur, a determination was made of the amount of potassium chromate removed from solution by zinc and of the amount of chromium retained by the zinc surface. Small plates of zinc were cleaned under water with pumice, and without drying were immersed in a dilute solution of potassium chromate. The error caused by the introduction of the moisture adhering to the zinc plates is much less than the experimental error involved. Using .00004molar solutions, each square centimeter of zinc removed approximately .OOOI mg. of potassium chromate from solution. A clean zinc plate was treated with a 20 per cent solution of potassium chromate and the solution was allowed to dry on the plate. It was then washed in running water until all of the potassium chromate appeared to be removed. A determination of the chromium on the metal surface showed that as much as .003 mg. calculated as potassium chromate had been taken up per square centimeter. In each of the above cases the chromium was determined colorimetrically using diphenylcarbazide. It has been shown that potassium chromate reacts with a zinc surface, depositing on it a compound of chromium and making it less active. The adsorbed compound is probably either zinc chromate or potassium chromate. This question could be decided if the potassium, in addition to the chromium retained by the zinc, could be determined.
