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T H E JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMIhSTRY
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Quebracho Per cent Per Gnmbier cent Water.. 48.84 19.41 Insoluble. 7.58 9.50 Nontannin.. 36.14 27.06 Tnnninl.. 7.44 44.03 Wilson-Kern method, THISJOURNAL, 12 (1920),465.
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Solutions of these extracts were treated with tartaric acid to give a pH value of 2.5. Equal portions were then treated with sodium hydroxide to give a series of pH values the same as in the series of jellies. Each final liquor contained 1 g. of dry solids of the original extract per 100 cc. The gelatin dispersions were poured into test tubes and alIowed to set. On each was poured rc given volume of tan liquor having the same pH value as the jelly. Both the quebracho and gambier series were run in duplicate. They were kept in the ice box and examined a t intervals for 96 hrs. The extent of the diffusion of the tan liqdors into the jellies is shown in Fig. 1, the measurements being taken after 96 hrs, I n each case the duplicate series were practically identical. Gambier, which has a high ratio of nontannin to tannin, begins to penetrate at a pH value of 3.0 and reaches its maximum rate at pH=B.O. Quebracho, on the other hand, scarcely shows any penetration until pH=4.7, the isoelectric point of gelatin, is reached. At pH values greater than 9, however, the quebracho liquor penetrates a t the greater rate, possibly because of its higher tannin content. An interesting observation made during these experiments was that the shape of the interface between tan liquor and jelly also changes with the p1-I value. At pH ~ 2 . it 5 was concave downward, at pH=3.0 it was flat, but
Vol. 14, No, 1
with increasing pH value it became concave upward with the thickness of the meniscus increasing to a maximum of 7.5 mm. at pH=4.7 and then decreasing until the interface again became flat at pH=ll.O. The thickness of the meniscus as a function of the pH value is shown in Fig. 2. This change in shape of the interface appears to be related to the swelling of gelatin. The point of minimum in the curve in Fig. 2 occurs at the isoelectric point of gelatin, pH=4.7, while the curve slopes upward towards the points of maximum swelling of gelatin, pH=2.4 and pH = 11.7, for the external solution. The measurements recorded are those for the gambier series, but the quebracho series showed practically the same effect. Studies have also been made of the effect of change of acidity upon the rate of diffusion of tan liquors into cowhide. With increasing pH values up to about 8 there is a distinct increase in rate of diffusion, but because of the flaccid nature of hide at pH-8 it is difficult to make accurate measurements of the rate of diffusion. At pH values below 3 and above 11 the hide swells considerably and be. comes rubbery and distorted.
SUMMARY As ordinarily used in tanning, gambier and quebracho extracts show marked differences in the rate of tanning and of penetration into the hide. It is shown that the rate of penetration is a function of the hydrogen-ion concentration as well as of the nontannin content. The shape of the interface between a tan liquor ahd gelatin jelly was found also to be a function of the hydrogen-ion concentration.
The Electrometric Titration of Azo Dyes’ By D. 0.Jones and H.R. Lee THE NEWPORT CO., CARROLLVILLE, WISCONSIN
Electrometric methods of analysis have, in recent years, come into quite general use, particularly in the field of inorganic chemistry. Titanous chloride, originally suggested by Knecht2 as a reagent for the estimation of a large number of compounds, both organic and inorganic, has also in recent years found increased application, particularly in the field of dye chemi~try.~ Titanous chloride, combined with the electrometric method for determining the end-point of the titration, has been successfully applied in this laboratory to the analysis of azo dyes. The indirect titration was used in practically all analyses of azo dyes and nitro bodies. The reduction was made with an excess of standard titanous chloride and the excess was determined by titration with ferric alum. I n the former titanous chloride methods, involving the use of a thiocyanate indicator, the end-point was sometimes difficult to determine with accuracy on account of the turbidity of the solution, the presence of insoluble reduction products, or the masking of the end-point by the color of the solution itself. Thiocyanate was not satisfactory as an outside indicator, because the titrations were carried out in an atmosphere of carbon dioxide and because the solutions themselves were often of a pink color. The reduction products of some azo dyes produce, with ferric salts, characteristic colors, which sometimes furnished indications of the e n d - p ~ i n t . ~For example, diamino-H-acid 1 Presented before the Division of Dye Chemistry nt the 62nd Meeting of the American Chemical Society, New York, September 6 to 10, 1921. * Knecht and Hibhert, “New Reduction Methods in Volumetric Annlysis,” 1918. * F. English, THISJOURNAL, 12 (1920),994. 4 Arthur H. Green, “Analysis of Dyestuffs,” 2nd Ed., 117.
gave a blue color and amino-naphthionic acid a green color with ferric salts. These colors are not as a rule alike under all conditions and seldom furnish an accurate means of determining the end-point,
STANDARD SOLUTIONS A 0.25 N TiCla solution has been adopted for the analysis of azo dyes and a 0.05 N ferric alum solution for the back titration. The methods of preparing, storing and using the solutions were in general those of Knecht and Hibbert6with modifications similar in general to those of English.6 LaMotte’s titanous chloride was used in preparing the solutions and the boiling in the preparation was omitted.’
APPARATUB or the application of the electrometric method to oxidimetric reactions, reference is made to the work of Hostetter and Robertsa8 Wendt’s electrotitration apparatus, with slight modifications.in the calomel half cell and the addition of a reversing switch, was used in this work. Any modifications in the electrometric apparatus in its application to oxidimetric reactions as published by Robertsg will also find application here. The electrometric apparatus used in conjunction with titanous chloride is shown in the accompanying diagram. Carbon dioxide from a drum was used for maintaining an oxygen-free atmosphere in the reaction flask, which consisted of a 250-cc. Pyrex extraction flask with a 3-hole rubber stop-
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cit. LOC. cit. 7 J . A m . Chem. Soc., 43 (1921),91. 0 Ibid., 41 (1919), 1337. 8 I b i d . , 41 (1919), 1358. LOC.
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