JUNCTION POTENTIALS BETWEEN SOLUTIONS OF SUGARS AND POTASSIUM CHLORIDE JOHN M. ORTI
AND
MARTIN H. ROEPKE*
Division of Physics and Biophysical Research, The Mayo Foundation, Rochester, Minnesota Received January 18, 1986
The measurement of electrode potentials in sugar solutions has been frequently and advantageously employed by many workers to determine the p H of such solutions and to gain further insight into the oxidationreduction behavior of the sugars. In making such measurements, potassium chloride salt bridges are almost universally used to connect the electrode in the sugar solution with the reference electrode,-a calomel half-cell, for instance. Such procedure is not invariably safe. That is, if the sugar solution is too alkaline, or has been heated too much, or both, the solution may have present in it sp high an ionic concentration that even a saturated potassium chloride salt bridge can not completely “smother out” the liquid junction potential. Urban and Shaffer have fully demonstrated that the junction potential between a saturated solution of potassium chloride and an even moderately alkaline solution of glucose is not a negligible quantity. The purpose of this note is to present evidence that if the pH of the sugar solution does not greatly exceed 10, a saturated potassium chloride salt bridge may be relied upon to eliminate effectually all significant liquid junction potentials, no matter how concentrated the sugar solution may be. No attempt is made to explain the ionic mechanism involved in liquid junction potentials in general. For the purposes of comparison, platinized platinum electrodes were immersed in two kinds of solutions, namely, normal hydrochloric acid and concentrated glucose. Both kinds of these solutions were connected t o saturated calomel electrodes by potassium chloride salt bridges. Readings are given in table 1. Columns 3 and 5 give the differences between the readings of the cells which included the stronger potassium chloride bridges and the reading of the cell with a 0.05 N bridge. In the case of the solutions of N hydrochloric acid, there is seen to be a Present address, Rockville Centre, Long Island, New York. Present address, Toronto, Ontario, Canada. 941
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JOHN M. ORT AND MARTIN H. ROEPKE
difference of 0.0167 volt between the readings of the cells with salt bridges of N and of 5 N potassium chloride. Such a difference may possibly be construed to mean that a greater concentration of potassium chloride than 5 N , if it could be prepared, would eliminate still more junction potential; namely, that even a saturated potassium chloride salt bridge fails appreciably to eliminate all the junction potential. Fales and Mudge, however, have expressed the belief that saturated potassium chloride bridges actua.lly eliminate all but negligible junction potentials under these conditions. I n considering column 5 in table 1, it is seen that the junction potentials involved in even concentrated glucose solutions of pH as high as 10 are much less than those for N hydrochloric acid. The reading for the saturated bridge is but little different from that of a N potassium chloride bridge. Since a fivefold increase in the’concentration of potassium chloride produces only a small difference in the over-all reading of the cell, it seems TABLE 1 Over-all readings, in volts, of the various cells at 90°C. (1) NORMALITY OF K c l I N BRIDGE
0.05 0.10
0.50 1 .oo 5 (saturated)
APPROXIMATELY NORMAL
HCl
’
SOLUTIONS OF 60 GI. GLUCOSE IN 1clcl CC. B U F F E R P H 10
(2)
(3 1
(4 )
(5)
Readings
Differences
Readings
Differences
0.3102 0.3000 0.842 0.2633 0.2466
0 0.0102 0.0360 0.0469 0.0636
0.8527 0.8506 0.8464 0.8459 0.8456
0 0,0021 0.0063 0 ,0068 0.0071
reasonable to suppose that even a N potassium chloride bridge eliminates most of the junction potential and that a saturated bridge eliminates practically all of it. Likewise we may assume that a solution of an infinite concentration of potassium chloride, if such were possible, could eliminate absolutely all such potential difference. I n considering these data in the light of this assumption, we noticed that the values in column 5 fulfilled, within the limit of experimental error, the relationship demanded by the following equation :
z -0.05 --R
Y = Y’10
where Y = junction potential eliminated in excess of whatever is eliminated by a 0.05 N potassium chloride bridge. Y = Y’ when x,the concentration of potassium chloride, becomes infinitely large. K = a constant. Since the above formula holds quite accurately throughout the range
JUNCTION POTENTIALS B E T W E E N SOLUTIONS
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of all concentrations of potassium chloride that can be prepared, it is interesting to note that, according to this formula, the difference between the reading for a saturated bridge and the reading for a bridge of a. concentration of potassium chloride of infinity is only 0.00016 volt. We therefore conclude that for the purposes for which most potential measurements are made on glucose solutions, the solution junction potential may be ignored when saturated potassium chloride salt bridges are used and the pH of the glucose solution does not greatly exceed 10, provided also that the sugar solution has not been heated or treated in any way that produces a large amount of ionized decomposition products. REFERENCES (1) FALES, H.A., AND MUDGE,W. A.: J. Am. Chem. SOC.42, 2434-53 (1920). A study of the saturated potassium chloride calomel cell. (2) URBAN,FRANK,AND SHAFFER,P. A.: J. Biol. Chem. 94, 697-715 (1932). The acidic property of sugars. .
