Glass and Other Electrodes for Measuring pH Values of Very Dilute Buffers and of Distilled Water JOHN 0. BURTON,HARRYMATHESON,AND S. F. ACREE, Bureau of Standards, Washington, D. C.
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N THE e l e c t r o m e t r i c The s e n s i t i v i t y of this measurement of the pH modified equipment with the values of buffered soluLeeds and Northrup Type K tions more dilute than 0.0001 potentiometer and 2500-e galM by means of quinhydrone vanometer and the Thompson and hydrogen electrodes the glass e l e c t r o d e of about 60 resistance of the s o l u t i o n , m e g o h m s resistance, made escape of carbon d i o x i d e , from Corning No. 015 glass acid properties of the quinis about 0.004 millivolt per hydrone, and polarization of mm. s c a l e d i v i s i o n w i t h L .........._........ I............ the electrodes are so disturb0.001 M sodium, acid phthalnI I ing that the e. m. f. readings ate (designated NaHPh in may vary as much as 5 to Table I), 0.007 millivolt with 30 millivolts or 0.1 to 0.5 pH 0.0001 M solution and 1milliunit. The isohydric indicavolt for d o u b 1e - d i s t i 11e d tor technic (1) is apparently water. Standard cells can be applicable within 0.1 pH to checked to within 0.1 millisuch solutions and distilled volt. W i t h the Thompson water, but has not heretofore FIGURE1. WIRINGDIAGRAM OF GLASS ELECTRODE- glass electrode the readings been c o m p a r e d with a reliare easily made to within 0.1 POTENTIOMETER SYSTEM m i 11i v o 1t , are reproducible a b l e e. m. f. method. By Upper circuit as modified in this work Lower: schematic Partridge circuit adding Varley s h u n t s to a with double-distilled water to modification of the vacuum within 2 or 3 millivolts, and tube potentiometer used by Partridge ( 5 ) ,keeping the grid become better as the buffer concentration increases. attached to the circuit when balancing it, and using a ThompTable I gives some illustrative pH data on dilute solutions. son (2, 3, 4, 7 ) glass electrode, e. m. f. readings can be made The results show that the hydrogen electrode (1 x 1 cm. within 0.1 to 2.0 millivolts on weakly buffered solutions and gold covered with palladium sponge) gives drifting readings distilled water, and the pH values agree with those obtained in very dilute solutions, especially tap water and distilled by the isohydric indicator method. This combination is also water containing traces of carbon dioxide. The quinhydrone suitable for measuring the pH of solutions containing active electrode (1 x 1 cm. gold) gives e. m. f. values which decrease oxidizing or reducing agents, such as chlorine or tannins (6,8) steadily for 20 to 30 minutes as the weakly acid solid quinwhere the hydrogen and quinhydrone electrodes and indicator hydrone dissolves. Chlorine attacks the quinhydrone and methods might fail. alters the e. m. f. values. The glass electrode and isohydric Figure 1 shows a diagram of the circuit used with a special indicator methods give approximately the same pH values Duovac triode tube. Recent work by the authors shows in very dilute buffers and distilled water. I n general, either that the F. P. 54 G. E. tube is especially good with this method is satisfactory for such solutions. For other samples circuit. which are colored, turbid, or possess oxidizing or reducing properties a glass electrode is, of course, to be preferred. TABLEI. PH VALUES OF SOLUTIONS OF VARIOUSCONCENTRATIONS OBTAINED BY DIFFERENT METHODS LITERATURE CITED GLASS ISOHYDRIC
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HYDROQENQUINHYDRONE ELEC- INDICATOR Fawcett, E. H., a n d Aoree, S. F., J . Bact., 17, 163 (1929); IND. ELECTRODE ELECTRODE TRODE METHOD ENG.CHEM..Anal. Ed.. 2, 78 (1930). 0.1 M NaHPh 3.93 3.93 3.92 3.90 0.01 M NaHPh Hughes, W. S., J . Am. Chek. Soc., 44, 2860 (1922); J . Chem. 4.05 4.05 4.06 4.04 0.001 M NaHPh 4.34 4.37 to4.33 4.30 4.35 Soc., 1928, 491. 0.0001 M NaHPh 4.85 4.82 to 4.72 4.90 4.84 MacInnes, D. A., a n d Belcher, D., J . Am. Chem. Soc., 53, 3315 0.00001 M NaHPh 5.68 to 5.95 5.34 to 5.20 5.70 5.61 Tap water (1931). 8.42 to 8.98 7.70 7.58 7.11, Boiler water 11.74 11.52 11.70 MacInnes, D. A., and Dole, M., Ibid., 52, 29 (1930); J. Gen. Double-distilled water 6.18 to 7.66 6.05 ii 5.71 6.57 6.50 Physiol., 12, 805 (1929). Leather extract 5.25 5.40 5.28 ..b 0.01 M buffer containPartridge, H. M., J. Am. Chem. Soc., 51, 1 (1929); Microchemie, ing 0.07 gram of chlo11, 326, 337 (1932). literc 4 59 4.66 4.72 ..d Pleass, J . SOC.Chem. Ind., 48, 152T (1929). uffer containThompson, M. R., Bur. Standards J . Research, 9, 852 (1932). ing 1 gram of chlorine per litere * .f 0.20 1.90 ..d Wallace, E. L., and Beek, John, Jr., Ibid., 4, 737 (1930). 5 Solution too alkaline for quinhydrone electrode. b Solution too deeply colored €or indicator method. RECEIYED June 10, 1933. Presented before the Division of Water, Sewage, C Solution pre ared by diluting 50 ml. of 0.05 M KHPh-NaOH solution and Sanitation Chemistry a t the 85th Meeting of the American Chemical = 4.80) m 5c! ml. of saturated chlorine water (PH = 1.71) t o 250 ml. (P?Indicator faded quickly. Society, Washington, D. C., March 26 to 31, 1933. Publication approved 6 Solution prepared by passing chlorine gas into 0.01 M KHPh-NaOH by the Director of the Bureau of Standards of the U. S. Department of solution (PH 4.80). Commerce. f Solution removed palladium sponge from electrode.
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