Acidity of Karaya Gum Solutions
TABLEIV. PH OF K ~ ~ a . r .GUM 4 DISSOLVED IN 0.05 N ACETIC Grams Gum per 100 Grams Solvent 0.0
W. E. THRUN
'
0.2 0.4 0.6
Q T
H E volatile acid of karaya gum may be determined by distillation over phosphoric (1) or sulfuric acid. According t o Tschirch and Fluck (9) the acid number of the gum varies from 13.4 to 21.3. In this laboratory the acid number of four samples of the gum was determined by a method similar to the official (8) except that sulfuric acid was used and results from 17.4 to 22.7 were obtained. The volatile acid is in part free acetic acid and probably mostly combined acetic acid. The acid that could be washed from 1 gram of a sample that smelled strongly of acetic acid with absolute alcohol was equivalent to 3 cc. of 0.1 iV alkali. The acidity of the gum was reported by Gabel (4) to be equivalent t o 4.5 cc. of 0.1 AT alkali per gram. The gums were mixed with a small amount of glycerol, dissolved in water a t room temperature, and then titrated with 0.1 Ai sodium hydroxide, using phenolphthalein as indicator. From what follows, the results obtained by this method must vary somewhat according to the time the gum has been in solution. Thebase neutralizes the free acid
ACID
uinhydrone Grams Gum Quinhydrone 'PH at per 100 Grams pH at 25" c. Solvent 25' C. 3.10 3.50 3.56 3.58
0.8
3.64 3.69 3.76 3.84
1.0
1.2 1.4
TABLE v. QUINHYDROXE PH OF SOLUTIONS CONTAINING 0.665 MILLIEQUIVALENT OF AVMONIAPER GRAM OF GUM Grams Gum per 100 Grams Solvent 0.2 0.4 0.6 0.8 1.0 1.2
NHa Added to Solvent HzO, pH 7.90 8.03 8.Oi i.50 i.74 7.25
NHa Added 4 Hours after Soln., PH 8.15 8.35 8.29 8.02
8.21
7.90
the indicator solution in which the gum solution is dissolved, the quinhydrone and the colorimetric pH will drop as the age of the solution increases, running somewhat parallel for a time, and then the colorimetric p H will drop below the electrometric pH. Some data are given in Table 111. This divergence is explained by assuming that the gum particles absorb the indicator and are themselves more acid than t'he solution in which they are suspended; the quinhydrone value gives the pH of the solution, while the indicator after sufficient time gives the' PH of the particles. Because the acid isadsorbed' from the TABLE 111. COLORIMETRIC AND QUINHYDRONE PH VALVES solution by the particles, the quinhydrone pH rises (1 7% soiutions; indicator added t o water before solution) after some time. Further substantiation of this exPresent -No Ammonia -----.--$mmonia planation is shown by the quinhydrone pH values m-omocresol Green Phenol Red Phenol Red Cresol Red obtained from 24-hour solutions prepared by dissolvcolori- quinhy- colori- quinhy- colori- quinhy- colori- quinhymetric drone metric drone metric drone metric drone ing the gum in a 0.05 N solution of acetic acid. Time pH pH pH pH pH pH pH pH The results,. given in Table IV, show that the gum, Haws although acid itself, lowers the hydrogen-ion con0.8 4.64.63 9.42 8.7i 8.56 centration of the acetic acid solution. 5.59 8.5 8.50 3.2 4.6 4.62 9.03 7.99 23.5 4.6 4.64 8.3 8.72 7.7 8.02 7.7 The quinhydrone pH of slightly ammoniacal solu29.5 4.64.71 8.1 8.50 7.4 7.53 7.37.22 5.03 7.8 8.14 6.8 7.10 1.07.00 50.0 4.6tions drops because of the gradual release of the 76.0 4.65.13 7.7 8.14 acid by the particles which themselves remain more acid for some time. Table V gives the quinh y d r o n e p H of 24-hour s o l u t i o n s . One set was prepared by dissolving the gum in water containing not absorbed in the particles and probably the exposed parts ammonia; the other set was prepared by dissolving the gum of the gum colloid which is undoubtedly an acid. in water first and adding the same quantity of ammonia Peyer (8) mentioned that the gum sometimes smells of solution 4 hours afterward. The pH of the second set of trimethylamine. Since no report of the amount of volatile solutions is in all cases higher, showing that the gum particles bases present in the gum could be found, they were deterhad adsorbed acid in the first 4 hours and that this acid had mined in four samples as follows: One-gram samples Tvere not immediately been given up to neutralize the ammonia. dissolved in 100 cc. of Aater in Kjeldahl flasks by shaking The differences in pH of the particles and the surrounding with glass beads. Then 150 cc. of 20 per cent sodium hysolution after the suspensions have become older and the fact droxide solution were added, and the distillation was carried that the rather large particles swell in water point toward the on with a Kjeldahl apparatus until about 150 cc. had gone establishment of a Donnan equilibrium. over. The distillate was titrated with 0.01 N acid, using methyl orange as indicator. Blank determinations were also Literature Cited made. The volatile base of three samples was found to be equivalent t o 1.2 cc. of 0.1 N alkali per gram, and of the other Assoc. Oj'icial Agr. Chem., Methods of Analyses, 2nd ed., p. sample 1.1 cc. per gram. 405 (1925); 3rd ed., p. 475 (1931).
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B e d , E., a n d Umstatter, H., Kolloid-Beihefte, 34, 1 (1931) Cannon, J. H., J . Assoc. Oficial A g r . Chem., 17,468 (19343. Gabel, L.F.,J . Am. Pharm. Assoc., 23,341 (1934). Goester, L.E.,Phawn. Weekblad, 66,1041 (1929). Jacobs, M.B.,a n d Jaffe, L., IND. ENQ.CHEJI.,Anal. Ed., 3, 210
Hydrogen-Ion Concentration of Solutions Since no work on the pH of karaya gum and its solutions has been reported in the literature, a study of the pH values obtained under various conditions was undertaken. It was found that the pH of the gum solutions determined electrometrically (quinhydrone) becomes higher after about 24 hours. The apparent pH taken colorimetrically agrees with the quinhydrone value a t first, but, if the indicator has been added to the water used in preparing the solution, the color remains constant. If a small amount of ammonia is added to
(1931). Patrick, W. A., i n H. S. Taylor's "Treatiae o n Physical Chemistry," 2 n d corrected printing, p. 1314, New York, D. V a n Nostrand Co., 1925. Peyer, W., Apoth.-Ztg., 40,667 (1925); Scddeut. Apoth.-Ztg., 65,
569 (1925). (9) Tschirih, A.', a n d Fluck, H., Phann. Acta Helv., 3, 151 (1928) RECEIVED
1218
March 29, 1935.
