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.
488 J. I. CRABTREE AND H. D. RUSSBU, [J. S. M. p. E.
screened to the color of noonday sun with a No. 79 Wratten filter. A comparison between the color of each solution gave a relative measure of the acidity of the solutions. The color change observed with brom phenol blue under these conditions is indicated in Table I.
The pH values bearing asterisks were checked with other indicators such as methyl orange (pH range 2.9-4.0), methyl red (pH range 4.2-6.4), and brom cresol purple (pH range 5.2-6.8). The pH values between these points were estimated by judgment.
This colorimetric method gives relative pH values which are sufficiently accurate for practical purposes. Blectrometric methods were not used because they did not give satisfactory results in the presence of sodium sulfite.
Effect of Acidity on the Hardening Action of Chrome Alum Solutions.— Using the above procedure, the effect of concentration and the acidity on the hardening properties of plain chrome alum stop baths was determined. The results are indicated in Fig. 1 from which it is seen (curve A) that for neutral film the hardening increases to a maximum at a concentration of 0.2 per cent and then decreases rapidly as the concentration of chrome alum increases. For alkaline film the hardening does not reach a maximum until a concentration of 0.4 per cent is reached and then it remains at a maximum throughout the range of concentrations tested. In view of the fact that melting points above 212°F. were not determined, it cannot truthfully be said that the hardening reaches a maximum if the film does not melt at 212°F. For the purpose of this investigation, however, it was considered unnecessary to make melting points in water under pressure.
The acidity of the solutions indicated in curve B by pH values was measured in the manner outlined above. Curve B shows that for maximum hardening the acidity of the bath for neutral film should be about a pH of 4.0 and that for alkaline film between 3.8 and 3.0. The degree of hardening observed in curve C was obtained after the acidity of the baths had been so adjusted by adding either sulfuric acid or caustic soda. For neutral film the pH was 4.0 and for alkaline film, 3.2. It is seen that when the acidity is adjusted to these values, the hardening properties of the baths are constant, irrespective of the concentration of the chrome alum. The inaccuracy of judging the acidity of the solutions with pH indicators and the extreme change in hardening properties resulting from slight changes in the acidity of the solutions accounts for the