New Stoichiometry for Copper Dissolution in Nitric Acid Although the dissolution of copper in nitric acid was the object of numerous investigations, little attention has been paid to the stoichiometry of this reaction. We have found that nitrous acid, HNOI, is one of the products, and it plays an important role in the intermediate steps of copper dissolution as well.' We also found the quantities of HNOz and NO gas produced vary from one experiment t o another depending on the conditions and, thus, no definite stoichiometry could be predicted for this reaction. The explanation lies with the specific mechanism: the dissolution of copper in nitric acid takes d a c e a t first bv the reduction of NO1- toHNO?. then after a certain accumulation of HNO?. the reduction of HNO? to NO
Reaction (1) is very fast and is responsihle for the accumulation of HNOz in solution. We also found that the kinetics of the first stage of copper dissolution is first-order with respect t o the copper surface area [Cu] and to [HNOz]. During the second staee. the dissolution becomes zero-order with resoect to IHNOJ onlv. The possible products of the dissolution process are:.Cu(~ds)a,HNO~,-NO,and Hz0. Under certain conditions NO gas may he considered t o react completely with H N 0 3 in solution forming HN02. However, some of this gas may escape out of the solution, where i t oxidizes t o NOz. The overall chemical equation representing the CuIHNO3 interaction can, thus be written as:
+
-
+
+
+
o Cu b H N 0 3 m Cu(NO& x HNOZ y NO z H 2 0 Thus, NOz does not appear to be a primary product in the oxidation of copper metal by nitric acid.
(3)
' El-Cheikh, F. M., Khalil, S. A,, El-Manguch, M. A,, and Omar, Hadi A., Ann. Chim., 73, 75 (1983). Schmid. Gehard, and Baehr. Guenter. Z Physik. Chem., 41, (1964). Paranov. A. V., Pogrebnaya, V. L., and Msl'chikov, T. V.. Tr. Krasnodar. Politekh. Insf.. 29, 179 (1970).
F. M. El-Cheikh, S. A. Khalil, M. A. El-Manguch, and Hadi A. Omar Al-Fateh University Tripoli. Libya (L.A.J.)
Volume 62
Number 9
September 1985
781
