We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
688 J. I. CRABTREB AND H. D. RUSSBU, [ J. S. M. P. B.
by the addition of sulfuric acid at intervals or by increasing the concentration of chrome alum. The addition of too much acid is apt to cause blistering and sulfurization and too much chrome alum increases the tendency of the bath to stain the film.
Since the addition of sodium sulfite to a chrome alum solution causes it to turn from a purple to a deep green color and also produces a decrease in hardening properties, an attempt was made to restore the hardening properties of such chrome alum solutions by treatment with various oxidizing agents such as potassium perchlorate, iodine, potassium persulfate, nitric acid, and potassium permanganate. The hardening was not revived and the original color was only obtained in solutions that were distinctly acid. The color change was therefore not due to a reduction process as might be expected in the presence of sodium sulfite.
By the addition of oxidizing agents together with sulfuric acid to an exhausted stop bath, it is possible to prolong the hardening life to a greater extent than when sulfuric acid is added alone, owing to the oxidation of the sodium sulfite. If the sodium sulfite in an exhausted bath is oxidized with sodium dichromate and sulfuric acid, the concentration of the chromium ion is increased and the hardening life is thereby prolonged. However, in the case of a bath exhausted with film developed in D-76 to 700 feet per gallon and revived with enough sodium dichromate to react with all of the sodium sulfite, the chrome alum content would be increased by 6 per cent. This is undesirable because a high concentration of chrome alum tends to stain the film green. The addition of oxidizing agents to exhausted chrome alum fixing baths is also undesirable, because a certain proportion of the hypo is thereby oxidized and its fixing properties destroyed.
Since potassium nitrite catalyzes the conversion of green chrome alum to the violet modification3 and oxidizes sodium sulfite, various exhausted chrome alum hardening baths containing 2 per cent sodium nitrite were tested for an increase in hardening properties. The color and hardening was only revived in baths whose acidity was adjusted between a pH of 4.0 and 3.0. The hardening of exhausted baths to which sulfuric acid alone had been added was increased, but the addition of nitrite greatly hastened the reaction. From the results obtained potassium nitrite was thought to be a suitable material for preventing the rapid decrease in hardening properties of chrome alum solutions but practical tests showed that the propensity of the