Polarographic Behavior of lead in the Presence of Gelatin JOHN KEENAN TAYLOR AND ROBERTA E. SMITH National Bureau of Standards, Washington, D . C .
AXY polarographic methods have been described for the determination of lead in various materials. In these methods, maxima of the waves must be suppressed, and gelatin is especially useful for this purpose. It has been pointed out (3) that gelatin in slight excess of that required to eliminate maxima greatly decreases the size of the wave. This phenomenon is discussed in detail in the present paper. EXPERIMENTAL DETAILS
Solutions for polarographic measurements were made by tr u i b (erring IO-nil. portions of a 2.5 millimolar solution of lead nitrate to 25-ml. volumetric flasks and, after addition of the materials necessary to make the proper supporting electrolytes, diluting to volume. All solutions were 0.1 N with respect to otassium chloride except as noted. The gelatin contents were afjusted by adding the required amounts of a freshly prepared gelatin solution to the flasks, and hydrochloric acid or potassium hydroxide was added to adjust the pH to the desired values. Measurements of pH nere made with a Beckman meter, Model H-2, using the Type E (blue glass) electrode in the alkaline range. A silver wire was used as the anode in all solutions containing chloride. Otherwise the mercury pool was used. Oxygen was removed from the solutions with purified nitrogen. The drop times were observed after each measurement and the rate of flow of mercury from the electrode (in the solution) was checked several times during the uorking (lay. All measurements mere made a t 25” * 0.1” (’. RESULTS
The results of the investigation are summarized in Figure 1: where the diffusion-current constants, calculated from the experi-
Table 1. Diffusion-Current Constant, I , of Lead i n Various Supporting Electrolytes I, Observed In In In 0.01% 0.1% 1% gelatin gelatin gelatin 3.50 3.35 3.25 2.10 2.31 1:iS 1:63 1.30 0.94 2.42
Supporting Electrolyte pH 1 S sodium hydroxide Alkaline t a r t r a t e 1i:5 Keutral t a r t r a t e 6.6 Acid t a r t r a t e 4.6 0.1 N potassium chlo3.55 5.1 ride 3.85 2.76 1.4 2.84 20% citric acid 1 N hydrochloric acid 3.85 3.83 .. 3.54 1 N nitric acidn 3.61 .. ’ Rlercury pool anode used in measurements.
1.65 2.69 3.77 3.42
I,
Literature (3) in 0.01%
Gelatin 3.39 2 39 2.30 2.37 3.80 3:86 3.67
mental data, are plotted as a function of pH for the three concentrations of gelatin used. In strongly acid solutions, the diminution of the waves is small, whereas in strongly alkaline solutions it is somewhat larger. In the p H range 3 to 13, the effect is very large. In the pH range 7 to 12, the diffusion current is very low and independent of the gelatin content. In this region, hydrolytic precipitation of the lead is probably the governing factor (1). Outside of this region, gelatin appears to remove lead from the solution in some manner. The behavior does not appear to hr related to the viscosity of the solution. In the region where diminution is appreciable, there is some variation of the wave height, depending upon the way in nfhich the gelatin is prepared. Gelatin solutions prepared by soaking the material in cold water and then warming to effect solution showed a smaller effect on the waves than those prepared by heating nearly to boiling. Those prepared by the first method tended to appear turbid on dilution with water while the latter remained clear. The polarographic behavior supports the conclusion that the solution of the gelatin is more complete by the second method. The diffusion-current constants found for lead in various supporting electrolytes are given in Table I. Here again, it is evident that the wave heights (diffusion-current constants) are great]) influenced by the gelatin content of the supporting electrolyte, except in very acid media. In all cases, except the alkaline-tartrate medium, the waves are well defined, even though in man! cases the waves are diminished by the gelatin. In alkaline tartrate, the waves for the solutions containing 0.01% gelatin are poorly defined, in that they are spread out and never quite exhibit a diffusion current plateau. Addition of alkali to the supporting electrolyte improves the form of the wave, but even xhen 1.5 S in alkali the wave form was not all that could be desired. I n the case of solutions with gelatin content 0.1% and greater, no acceptable waves were obtained in alkaline tartrate. Methylcellulose has been recommended for suppression of maxima. The authors have used a product under the trade name Tylose with some success. Because of the difficulty in dispersing this material, they have tried sodium carboxymethylcellulose prepared by E. I. du Pont de Nemours & Company under the trade name Sodium CMC. This material is easily dispersed in cold water and is stable, so that stock solutions may be prepared and stored for future use. It was found that concentrations of the order of 0.01% of Sodium CMC are effective in suppressing maxima but, like gelatin, it diminishes wave heights when present in larger amounts. However, because of its stability, it is attractive as a suppressor and should be investigated more completely. CONCLUSIONS
The results of this work emphasize that, when a choice is possible, strongly acid media are the most satisfactory for the polarographic determination of lead. When, because of the presence of interfering elements or other reasons, one of the other supporting electrolytes is required, the gelatin content of the solution must be controlled very carefully to avoid serious analytical errors. LITERATURE CITED
0
2
4
6
8
IO
(1) Gilchrist, Raleigh, J . Research Natl. BUT. Standards, 30, 89
12
14
(1943), R P 1519.
PH
(2) Kolthoff, I. M., and Lingane, J. J., ‘‘Polarography ” p. 121, New York, Interscience Publishers, 1941. (3) Lingane, J. J., IND.ENG.CHEM.,ANAL.ED., 15, 583 (1943).
Figure 1. Diffusion-Current Constant for Lead as a Function of Gelatin Content and pH of Solution 0
0.01% gelatin
X 0.1% gelatin
0 1.0% gelatin
RECEIVED M a y 31, 1949.
495
