A Durable Glass Electrode DUNCAN A. MACINNES AND DONALD BELCHER T h e Laboratories of The Rockefeller Institute for Medical Research, New York, N. Y.
T
HE glass electrode of the novel form shown by AB in Figure 1 has been found convenient for use in pH
determinations, and is no more fragile than any other instrument made of glass. It has the form of a spiral condenser, the spiral of which is made of special (Corning 015) glass. For the electrodes now in use the spiral, which has fourteen turns of about 2.5 cm. diameter, is made of 92 cm. of the commercial tubing, with an external diameter of 5 mm. and walls about 0.6 mm. thick. The precise dimensions are not important. The rest of the apparatus AB is made of Jena thermometer glass, the spiral being joined to the inlet tube A and the outlet tube B at the points a and b, with Corning seal glass H. The arm L is fitted to hold a silversilver chloride electrode, P’, and the space between the jacket J and the spiral is filled with 0.1 N hydrochloric acid. (Any solution having a definite pH and chloride ion concentration may be used.) The solution, the pH of which is to be measured, is placed inside the spiral, which has a volume of about 7 cc., and is connected through a liquid junction with saturated potassium chloride a t the point K with a reference calomel electrode .I I n connection with their study of this electrode the authors have developed a convenient method for making the liquid junction between the solution the pH of which is to be measured and saturated potassium chloride solution. The operation of the device is as follows: With the stopcock S closed the solution is poured into the upper portion of the vessel AB. The stopcock is then placed in position to connect the tube B and the reject tube T,but is turned clockwise before the solution has passed point a. Rotating the stopcock further in the same direction connects the tubes R and T and also permits the flow of saturated potassium chloride from the reservoir R through the extra bore g in the rear end of the stopcock and out through the reject tube T. This flow will be clear from a study of the plan of the apparatus in the lower part of Figure 1. Another clockwise turn of the stopcock through 120” forms a liquid junction a t the point e between the solution in AB and the saturated potassium chloride solution, the latter being in electrical connection with the saturated calomel electrode P. (E. Machlett and Son, 50 William St., Long Island City, N. Y . ,are prepared to make the apparatus as shown in Figure 1.) Although the glass used in the spiral has six hundred times the thickness of that used in the membrane type of glass electrodes described by MacInnes and Dole (6)and MacInnes and Belcher (4), the area exposed to the solution is greater by approximately that factor, so that the (apparent direct current) resistance is approximately the same as that of one of the thinner membrane electrodes. Seventeen spiral electrodes made as described had resistances ranging from 19 to 30 megohms with an average of 25 megohms. The “asymmetry-potentials”--i. e., the more or less permanent potentials existing in the glass-are, however, much greater than those observed with the membrane electrodes. For the latter they are usually of the order of one or two millivolts. 1 A variant of this design which has been found useful in certain cases is t o have the external surface of the spiral bathed with the solution the pH of which ia to be determined. It appears desirable to have the active surface of glass completely immersed, on account of an effect, described by Kahler and De Eds (S),on the measured potential, due to a film of solution on an exposed surface.
With the spiral type electrodes their initial values are as high as 100 mv. However, if the electrodes are kept filled with water these asymmetry potentials slowly decrease. For instance, one electrode showed a decrease of asymmetry potential of 8 mv. the first day. This rate of change steadily decreased until after a month the asymmetry potential showed no further change. The variations observed are always in the direction of a decrease of the asymmetry potential. In practice the sum of the asymmetry potential and that of the reference electrode, which we will call EO’, can be obtained by a measurement involving a convenient buffer solution whose pH value is accurately known. The relation between the measured potential E, the p H value, and EO‘is, of course, given by the usual equation pH =
E -Eo‘ 2.303 RT/F
That comparatively thick glass can be used for glass electrodes if the surface is also increased in proportion to the thickness has recently’ been shown in an interesting paper by T h o m p s o n (8). T h i s worker has studied electrodes of test-tube shape, with a metal coating, to which c o n t a c t is made, on the exterior surface. With such e l e c t r o d e s the potential which corresponds to EO‘as defined above is, however, subject to small but u n p r e d i c t a b 1e changes, pro b a b 1y due to r e p l a c i n g a reversible electrode, such as the s i l v e r silver chloride electrode P’, by a glassmetal contact at which, so far as we k n o w , no d e f i n i t e electrochemical reaction takes place. Measurements, using the spiral type of e l e c t r o d e , on a series of s o l u t i o n s covering a wide range of p H values a r e given i n T a b l e I. T h e p H v a l u e of the potassium a c i d phthalate buffer was used to o b t a i n the value of EO’of Equation l, and the other 3 measured values FIGURE1. DIAGRAM OF GLASS given in t h e t h i r d ELECTRODE
199
ANALYTICAL EDITION
200
column of the table are computed from the measured potentials. The two figures for the pH of each solution indicate the reproducibility obtained on refilling the spiral. It will be seen that there is agreement within about 0.01 p H unit between these measured values and the published “accepted” values given in column 4 within the pH range 1 to 8. Above the latter value, however, there is the familiar deviation of the glass electrode. The accepted values are all on the basis that the potential of the tenth normal calomel electrode is 0.3376, The value for the pH of 0.1 N hydrochloric acid is from Clark ( I ) , that of the phthalate buffer from work of the authors and the pH values of the diethylbarbituric acid buffers from Michaelis (6). The pH of the 0.01168 N hydrochloric acid has been found by interpolating a value of the activity coefficient from the values given by Scatchard ( 7 ) . It seenis probable that within the range in which the glass electrode is effective the new measured values are as accurate‘as the published ones. The potential measurements recorded in Table I were obtained in a constant-temperature room a t 25” * 0.1” with the potentiometer and Compton electrometer described In addition many routine by MacInnes and Belcher measurements have been made in this laboratory on biological solutions using for the potential determinations the vacuum tube apparatus described by Hill ( 2 ) . With both these potential-measuring equipments insulation and electrostatic screening have been carefully studied. Although the authors do not suggest any relaxation of care with respect t o these factors, their importance is not as great as it sometimes is in work of this kind, because the glass elec-
(e),
(e).
Vol. 5, No. 3
trodes described in this article have comparatively low resistances. TABLEI. COMPARISON OF ACCEPTED AND MEASURED PH VALUES O F BUFFERSOLUTIONS MEASURED E. M. F. PH VALUE Measured Accepted -0,1360 1.06 1.06 -0,1362 1.06
SOLUTION 0.10 N Hydrochloric acid 0.01168 N Hydrochloric acid
-0,0807
-0.os12
1.99 1.99
1.9s
0.05 N Potassium acid phthalate
0.0370 0.0367
(3.98)
3.98
Diethylbarbiturate buffer
0.2143 0.2137
6.98
6.98
7.00
Diethylbarbiturate buffer
0.2753 0.2743
8.00
8.01
8.00
Diethylbarbiturate buffer
0.3297 0.3289
8.93 8.92
9.00
Diethylbarbiturate buffer
0.3601 0.3614
9.44 9.45
9.60
LITER.4TURE
CITED
(1) Clark, W. M., ”Determination of Hydrogen Ions,” 3rd ed., Williams and Wilkins. 1928.
( 2 ) Hill, S. E., Science, 73, 529 (1931). (3) Kahler, H., and De Eds, F., J . Am. Chem. Soc., 53, 2998 (1931). (4) MacInnes, D. A., and Belcher, D., Ibid., 53, 3315 (1931). ( 5 ) MacInnes, D. A., and Dole, M . , IND.ENG.CWEM., Anal. Ed., 1, 57 (1929); J. Gen. Physiol., 12, 805 (1928-29); J . Am. Chem. Soe., 52, 29 (1930). (6) Michaelis, L., J. Biol. Chem., 87,33 (1930). (7) Scatchard, G., J . Am. Chem. Soc., 47,648 (1925). (8) Thompson, M. R., BUT.Standards J . Research, 9, 833 (1932). RECEIVEDFebruary 11, 1933. ~
Estimation of Dextrin in the Presence of Glue JEROME ALEXANDER,50 East 41st St., New York, N. Y.
A
DMIXTURE of dextrin powder with glue may be readily detected by microscopic examination, using mineral oil or some high-boiling alcohol as a mounting medium. However, if the dextrin has been dissolved in the glue liquor prior to drying the glue, the addition is not so obvious. Some dextrins will not respond to the iodine test, especially if the glue is highly colored; but any dextrin may with certainty be detected by hydrolyzing the mixed glue with dilute hydrochloric acid and then testing the liquid for dextrose with Fehling’s or Benedict’s qualitative solution. The protective colloid action of the protein and its hydrolysis products tends to make the copper oxide precipitate so fine that it appears yellow, and if all the copper is not reduced, the solution may appear to be green. For the precise estimation of dextrin in glue, the following method proved satisfactory: . Soften 0.6 gram of the glue under test by soaking in 10 cc. of dilute hydrochloric acid (5 parts of concentrated hydrochloric acid t o 100 parts of water). Hydrolyze the mixture slowly at low tem erature (overnight a t about 60” C.). After neutralization mafe up the solution to exactly 50 cc., and determine the dextrose content by Benedict’s quantitative method. BENEDICT’S QUANTITATIVE SOLUTION (2). Dissolve 200 grams of sodium citrate, 200 grams of sodium carbonate, crystallized (equivalent to 75 grams of anhydrous sodium carbonate), and 125 grams of potassium thiocyanate (sulfocyanide) in enough hot distilled water to make up t o about 800 cc. Dissolve separately 18 grams of purest copper sulfate crystals (air dried) in about 100 cc. of distilled water, and pour into the first solution with constant stirring. Add 5 cc. of a 5 per cent solution of potassium ferrocyanide, and then make up to 1000 cc. with distilled water. The solution is stable, and 25 cc. are reduced by 0.05 gram of dextrose.
To make the analysis, a 150-cc. flask is held in a stand over a Bunsen burner, a t such a height that the reagent solution can be kept briskly boiling by a small flame. In this flask are placed 3 to 4 grams of anhydrous sodium carbonate and 25 cc. of the reagent, the mixture being heated until most of the carbonate is in solution. The glue solution is now run in slowly, until a chalk-white precipitate is formed and the blue color lessens perceptibly in intensity. The glue solution is now run in still more cautiously, with constant boiling, until disappearance of the last trace of blue indicates the end point. If less than 5 cc. of glue solution has been used, the solution is accurately diluted so that about 10 cc. will be necessary, and the titration repeated with this diluted solution. Should the mixture in the flask become too thick, boiled distilled water may be added to replace that lost by evaporation. In using this method, no trace of reduction was shown by a glue known to be pure, which was run as a check or blank in the quantitative test, after having been proved free from reducing substances by qualitative test. Mixtures of this glue with several commercial dextrins gave concordant results. In converting the dextrose figure to dextrin, it should be remembered that commercial dextrins contain variable percentages of water, dextrose, etc. The dextrose figure multiplied by the factor 0.9 gives a close approximation to the amount of dextrin present (1). (1) Allen, “Commercial Organic Analysis,” 5th ed., Vol. I, p. 530, Blakiston, 1923. (2) Cole, S. W., “Practical Physiological Chemistry,” 6th ed., Simpkin, Marshall, Hamilton, Kent and Co., London, 1920. RBCEXYEDFebruary 10, 1933.
