POLAROGRAPHIC EFFECTS OF GELATIN
Jan., 1951 [CONTRIBUTIONFROM
THE STERLING
I77
CHEMISTRY LABORATORY O F YALE UNIVERSITY ]
Some Polarographic Effects of Gelatin and Other Maximum Suppressors BY LOUISMEITESAND THELMA MEITES The recent literature has contained a number of reports that such materials as methyl red and gelatin can radically alter the characteristics of a polarographic wave.1-8 The present paper is a continuation of some earlier preliminary work on the effects of gelatin.’
4.5
Experimental The polarographic apparatus will be described elsewhere. All measurements were made a t 25.00 * 0.05”. Dodecyl- and hexadecyltrimethylammonium bromides were prepared from trimethylamine and the corresponding alkyl bromide. Sodium laurate and myristate were kindly furnished by the Research Department of the ColgatePalmolive Peet Co. “Bacto” gelatin (Difco Laboratories, Inc., Detroit), made by acid hydrolysis and clarifi~ation,~ was swelled in water a t 25’ for one hour, then dissolved by warming briefly to 60” and diluted t o volume. Triton X-100, a polyethylene glycol ether of monoisooctylphenol, was considered to contain 100% active material as received from the Rohm and Haas Co. “Bacto” agar (Difco Laboratories, Inc.) was dissolved by boiling briefly with water and cooling rapidly. Methyl red (General Chemical Co., New York City) was dissolved in very dilute sodium hydroxide and neutralized t o pH 9 with sulfuric acid. Gum arabic, U. S. P. XI11 (Mallinckrodt Chemical Works), was dissolved in warm water. All PH measurements are referred t o a saturated solution of recrystallized potassium hydrogen tartrate, whose pH was taken as 3.57.1°
Results The effect of gelatin on the electrocapillary curve11~12 in a typical case is shown in Fig. 1. No effect is observed with 0.001% gelatin, but 0.01% gelatin decreases the drop time over the entire range of potentials by about 5y0,and 0.1% gelatin causes a further 5% decrease. Since this effect differs from those of most other capillary-active substances in that the latter are substantially without effect on the electrocapillary curve a t very negative potentials, we instituted a systematic study of the effect of gelatin on the drop time a t -0.5 v., which was near the electrocapillary maximum in all the solutions studied. In no case was there a maximum in any c.-v. curve a t or near -0.5 v.; such a maximum would, of course, have invalidated the data. Although they show the relation between interfacial tension and drop time to be a complicated (1) K. Wiesner, Coll. Cscchoslov. Chem. Communs., la, 594 (1947). (2) R . H . Coe and L. B. Rogers, THISJOURNAL, TO, 3276 (1948). (3) H. A. Laitinen, J. C.Bailar, Jr., H. F. Holtzclaw, Jr., and J , V. Quagliano, ibid., 70, 2999 (1948). (4) C. A. Reynolds and L. B. Rogers, Anal. Chcm., Bl, 176 (1949). (5) C. A. Reynolds, ibid., 21, 759 (1949). (6) J. J . Lingane, THISJOURNAL, 65, 886 (1943). (7) L. Meites, ibid., 71, 3269 (1949). (8) F. Buckley and J. R. Taylor, J . Research Natl. Bur. Standards, 34, 97 (1945). (9) H. W. Schoenlein (Director, Difco Laboratories, Inc.), private communication. (10) J. J. Lingane, I n d . Eng. Chcm., Anal. Ed., 19, 810 (1947). (11) A. Frumkin, Ergeb. erakl. Naturw., 7, 235 (1928). (12) I. M. Kolthoff and J. I. Lingane, “Polarography,” Interscience Publishers, Inc., New York, N. Y., 1941, pp. 96-104.
4.0
j u‘
3.6
3.c
2.E
-
A -
-1.0 -1.5 -2.0 S. C. E., V. Fig. 1.-Electrocapillary curves in 0.10 F potassium 0-0.5
E d . e . US.
chloride containing (a) 0 (open circles), (b) 9.80 X IO-‘ (solid circles), (c) 1.06 X IO-* (half-solid circles), and (d) 1.20 X 10-1 (double circles) % gelatin.
one, the Harkins-Brown equationla and the fact that m is nearly independent of the interfacial tension14show that t is a function of 0 alone with any given driving pressure and any one capillary. Therefore a discontinuity in the interfacial tension-concentration curve for a surface-active material can be located by finding the point at which a discontinuity occurs in the drop time-concentration curve. Figure 2 shows several typical curves for gelatin in various media. Each point was secured by timing several sets of five or ten drops, and the individual values generally agreed to within 0.005 second. The measurements with methyl red must be made at potentials between f0.15 v. and -0.3 v., where the dye is neither oxidized nor reduced. (The reduction at the dropping electrode is well known,I5 but the oxidation, for which the halfwave potential is +0.206 * 0.002 v. in 0.1 F phosphate, PH 7.0, seems not to have been reported.) At potentials outside this range, the methyl red is destroyed as it reaches the drop surface, and no discontinuity is observed in the drop time-concentration curve (Fig. 3 I). 41, 499 (13) W. D . Harkins and F. E. Brown, THISJOURNAL, (1919). (14) Ref. 12, p. 64. (15) Ref. 12, p. 120.
LOUISMEITES AND THELMA
178
-4
1.00
L’ol. 73
MEITES
log (C, %). -3.0
-2.5
12
18
-3.5
1.00 c
s- 0.95
h
h
E
G zi ‘If 0.95
.*
0.90 0.9 160
1.00
-s0
120
0.95
s
h 0
u
80 40
0.90
-3 0
-2.1 1% (C,
-1.2
%I
-3.0
-2.1 log (C,
-1 2
%).
Fig 2
Effect of gelatin concentration on drop time at Ei e = -0 3 v. in solutions containing (a) 4 rnM nickel (11) in F potassium thiocyanate, (b) 3 5 m l l vanadium (IV) in F potassium oxalate, pH 5.5, (c) 1 mJf cyanide in 0 1 F potassium hydroxide, ~ n t (ti) l 1 m.ll bismuth (111) in 0 7 F perchloric acid
Paralleling the discontinuity in the interfacial tension-concentration curve, we have found a sharp decrease in the diffusion coefficient of methyl red occurring a t the same concentration. Figure 3 I1 shows the anodic diffusion current of methyl red in 0.1 F phosphate, pH 7.0, as a function of concentration. At concentrations the slope of the line (according above 6.5 X to the IlkoviE equation Ai 5 days No polymer
.. 1 ,,
5 (4%) 1 (AN) . .
Remarks
Color fades Two samples +5
CC.
Hz0
f 5 cc. HsO; two samples
-12 -2 days
..
TABLE I1 ENZYMATIC DEHYDROGENA0.4% FORMALDEHYDE SOLUTIONS, t = 22’, SOLUTION NOT DEGASSED
POLYMERIZATION INDUCED BY TION OF 73”m ,e
soln., cc.
4 4 4 4
HCHO s o h , MMA, cc. cc.
5 5 5
..
. .. 0.2 .2
...
MB soln., cc.
1 .. 1 1
Remarks
Color does not fade Some polymer after 5 days No polymer after 10 days $5 cc. H20, color does not fade
Discussion Iiroiii the results of Table I it caii be coiicluded
