To find the titer value of the sodium thiosulfate solution, weigh accurately 30 to 50 mg. of pure copper foil into a 250-ml. Erlenmeyer flask. Dissolve in 5 ml. of concentrated nitric acid and treat as above. Calculate the titer value in terms of milligrams of Cu per milliliter of thiosulfate reagent. DISCUSSION
If water is used instead of methanol to wash the sample in the copper procedure, the diluted nitric acid Fill allow iron to dissolve causing copper to electrodeposit back onto the plate. I n addition, the presence of iron can be troublesome because large amounts of ferric hydroxide obscure the blue color of the cupric ammonia complex
formed during neutralization. However, the introduction of significant amounts of iron salts is avoided by using concentrated nitric acid to dissolve the copper and passivate iron a t the same time. This method demonstrated that printing plates which had performed poorly had as little as 10 microinches of chromium and 80 microinches of copper on them, considerably less than the desired thickness. ACKNOWLEDGMENT
The authors thank H. J. Kolbert for his helpful information on problems pertaining to the graphic arts field.
LITERATURE CITED
(1) Banks, C. V., O’Laughlin, J. IT-., ~ A L CHEY. . 28. 1338 11956). ( 2 ) Diehl, H., Snii(h
