A COMPARISON OF THE GLASS AND QUINHYDRONE ELECTRODES FOR T H E MEASUREMENT OF T H E ACTIVITY OF T H E HYDROGEN ION I N SUCROSE SOLUTIONS H. P. CADY AND J. D. INGLE Department of Chemistry, University of Kansas, Lawrence, Kansas Received February $1, 1986 INTRODUCTION
In the determination of the activity of the hydrogen ion in acidified sucrose solutions by electromotive force methods, the hydrogen electrode has been used by a number of investigators, including W. C. M. Lewis and his associates (1,3, 6 ) , Taylor and Bomford (Q),who found that the hydrogen-ion activity increases during the inversion of sucrose by acid, and b y Scatchard (8), who found that the hydrogen electrode does not give reliable values of the hydrogen-ion activity in sucrose solutions. We thought that the glass electrode (2, 5 ) might be used to measure accurately the activity of the hydrogen ion in sucrose solutions. We used the quinhydrone electrode for comparison. EXPERIMENTAL PROCEDURE
Silver-silver chloride electrodes were used as standard reference electrodes. They were prepared as suggested by MacInnes and Beattie (4). The glass electrodes were prepared as recommended by MacInnes and Belcher (5). The quinhydrone electrodes used were flat pieces of platinum, 2 x 1 cm. All of the solutions used in the measurements were 0.1 molal with respect to hydrochloric acid and 0.1 molal with respect to the other constituent, either sucrose, dextrose, or levulose. All of the measurements were carried out at 30°C. APPARATUS
The electrical measurehents made using the quinhydrone electrode were made with a Leeds and Northrup Type K potentiometer and a Leeds and Northrup high sensitivity Type R galvanometer. The electrical measurements using the glass electrode were made with a circuit including a space charged grid tube, the G.E.F.P. 54 Pliotron. A diagram of this circuit is shown in figure 1. The glass cell used in making measurements of E.M.F. between the silver837
FIQ.1. Diagram of the circuit S
FIG.2. Glass cell used in making measurements of
E . M. P . between the silver-silver chloride half-cell and the glass electrode
FIQ.3. The half-cell used in the experiments involving a number of electrodes a38
839
ACTIVITY OF THE HYDROQEN ION IN SUCROSE SOLUTIONS
silver chloride half-cell and the glass electrode is shown in figure 2. The first series of measurements with the quinhydwne electrode was made in this cell. The half-cell used in the experiments involving a number of electrodes is shown in figure 3. TABLE 1
E. M . F . measurements with the cell 0.1 M Sucrose 0.1 M HC1 0.1 M HCl 0.1 M HCl Quinhydrone
1
1
TIME
E.M.F
1
Ag-AgCl
TIME
-
E.M.F.
his.
mans.
uolis
hw.
mins.
0.3456 0.3455
1 3 6 18 24
10 30 30 30 30 30 30
27 33 43 47 53 66 72
30 30 30 30 30 30
0.3454 0.3454 0.3454
0.3448 0.3446
Volts
0.3446 0.3443 0.3436 0.3434 0.3432 0.3423 0.3419
TABLE 2 Measurements made between pairs of platinum electrodes immersed in 0.1 molal hydrochloric acid containing guinhydrone
TIME
I
I1
I11
________-A.IF.
IV
v
VI
VI1
VI11
1x
~ - - _ _ _ _ _
A . l L . A.ID. A . / ' ~ d ' . ' ~ d ' I F . ' ~ d ' / D . ' ~ d . /D L ,.I L . D . l F .
x L.lF.
__________-__________ h m . mins
1 1
2 2 3 4
6 16 23 30 42
30 5 30 -0.035 0.062 10 30 -0.094 0.065 45 25 0.029 45 0.380 45 0.246 45 1.465 25 1.393 45 2.962
0.487 0.177 0.180 0.209
0.287 0.175 0,144 0.130
0.248 0.039 0.239 0.321 -0.146 -0.144 0.303 -0.512 -0.159 -0.123 0.303 -0.364 -0.172 -0.094
0.200 0.002 0.036 -0.179 -0.215 0,079 -0.068 -0.147
0,211 0.164
0.303 -0.277 -0.149 -0.092
0.057 -0.089 -0.146
0.224 0.213
0.304 -0.038 -0.091 -0.080
0.011
0.167
0.156
0.173 0.115
0.164
1.166-0.049
0.009 0.058
1.350
1.292
2.820
0.276 0.080
2.836
2.756
0.206 0.1% -0.070
0.196
DATA
The data in table 2 represent measurements made between pairs of platinum electrodes immersed in 0.1 molal hydrochloric acid containing
840
H. P. CADY AND J. D. INGLE
TABLE 3 Measurements of E. M . F . E. M. F. IN YOLTE 'TIME
I F.
hrs.
1 3 7 21 31 55 72
~
I1
~
F.
.4g
I11 A.
vd.8.
~
v
IV
Ae
rg.
1
s.
A.
4g
1
Ag
mins.
20 50 50 20 35 35 35 35
0.34377 0.34409 0.34431 0,34429 0.34450 0.34463 0.34494 0,34495 0.34516
-0.00100 -0 00058 -0,00033 -0,00023 0,00021 0.00120 0.00225 0.00491 0.00719
VI A.
1 3 7 21 31 55 72
j
20 50 50 20 35 35 35 35
0.34521 0.34507 0.34496 0.34488 0.34469 0.34395 0.34338 0.34116 0.33973
VI1
I Ivd. 5.
0.00045 0.00040 0.00040 0.00039 0.00044 0.00057 0.00078 0.00120 0.00183
A.
0.34304 0.34491 0,34480 0.34473 0.34459 0,34387 0.34323 0,34104 0.33948
VI11
i Ig. S.
0.00028 0,00024 0.00024 0.00024 0.00029 0.00049 0.00063 0.00108 0.00158
.g. S.
0.34476 0.34467 0.34456 0.34449 0.34425 0,34338 0.34260 0.33996 0.33790
IX
1 Ivd. S.
Ig. S.
0.00017 0.00016 0.00016 0.00015 0,00010 0.00008 0.00015 0.00012 0.00025
1
X
F.
3rd. S.
-0.00127 -0.00082 -0.00049 -0.00044 -0.00009 0.00076 0.00171 0.00391 0.00568
TABLE 4
E. M . F. measurements using a glass electrode
T I Y D IN HOUR8
0.5 1.5 3.5 7.5 21.5 31.5 43.5 51.5 72.0
I
E. Y.B. IN YILLIVOLTS
Trial I
0.44 0.45 0.42 0.42 0.43 0.36 0.42 0.44 0.40
I
Trial11
0.45 0.41 0.40 0.42 0.48 0.43 0.40 0.46 0.43
1
F.
-0,00144 -0.00098 -0.00065 -0.00059 -0.00019 0.00068 0,00156 0,00379 0.00543
ACTIVITY OF THE HYDROGEN ION IN SUCROSE SOLUTIONS
841
quinhydrone. All pairs of electrodes were connected by liquid bridges of 0.1 molal hydrochloric acid. Substances added to the cells are indicated at the top of each column. F stands for “freshly prepared electrode”, D for “dextrose”, L for “levulose”, Ivd. S. for “inverted sucrose”, and A indicates “the original acid and quinhydrone with nothing added”. All substances added had concentrations of 0.1 molal. The symbols used in table 3 are the same as those in table 2. The additional symbol Ig.S., meaning “inverting sucrose”, is also used. Ag is used to indicate a Ag-AgC1 electrode in 0.1 molal hydrochloric acid, in place of a quinhydrone electrode. DISCUSSION
The data in table 1 indicate a large apparent decrease in the hydrogenion activity during the acid inversion of sucrose. Such a large decrease in the activity, however, appears improbable. The data in tables 2 and 3 show that the potential of a quinhydrone electrode in old hydrochloric acid solution changes with the passage of time. The data in tables 2 and 3 also show that the inverting sucrose, the inverted sucrose, the dextrose, and the levulose all have about the same effect upon the potential of a quinhydrone electrode. In fact they seem to affect the potential of the electrode very little. The data in columns 111,IV, and V of table 3 indicate that the potential of the quinhydrone electrodes in the old hydrochloric acid solution, the inverting sucrose, and the inverted sucrose solutions, all become less positive with respect to the silver-silver chloride half-cell as time passes. Now since the potential of the quinhydrone electrode depends solely upon the activity of the hydrogen ion and the ratio of the activity of the quinone to the activity of the hydroquinone, and since it is not probable that the activity of the hydrogen ion has changed in this closed system, it is evident that the drop in potential must be due to a decrease in the above-mentioned ratio to some value less than unity. Thus either the activity of the quinone must have decreased or the activity of the hydroquinone must have increased. Now the quinone is known to be a fairly strong oxidizing agent; it can add hydrochloric acid in a concentrated solution of hydrochloric acid. Biilmann has shown that the change in potential of the quinhydrone electrode in dilute hydrochloric acid solutions is due to some action of the hydrochloric acid on quinone. This being the case, the activity of the quinone would be reduced and the voltage of the electrode would be reduced. Since the decrease in potential for the electrodes in the inverting and inverted sucrose solutions is so very nearly the same, we can conclude that the formation of the inversion products of sucrose has little or no effect on the potential of the quinhydrone electrode. This also indicates
842
N. P. CADY AND J . D. INGLE
that it makes no difference whether the inversion products are all present a t once or whether they are formed in the acid solution as time goes by. Since the only other substance present is the hydrochloric acid, we must assume that it is the hydrochloric acid that is causing the change in potential of the electrode. This is also borne out by the fact that the old hydrochloric acid solution which was in contact with the quinhydrone for three days showed an even more pronounced effect upon the potential of the electrode than did the solutions which contained the inversion products of sucrose. The fact that the electrode in the inverting and inverted sucrose solutions gave a constant difference of potential during the wholc seventy-two hours, would indicate that the rate of change of potential of both electrodes is the same. The data in column VI11 of table 3 show the truth of the above statement. Columns VI and 1-11of table 3 indicate that the potential of the electrode in the old hydrochloric acid solutions becomes less positive more rapidly than does the potential of either the electrode in the inverting or that in the inverted sucrose solutions. The data in column VI11 of table 2 indicate that the rate of change of potential of the quinhydrone electrode in a solution containing levulose is the same as that for an electrode in a solution containing dextrose. Thus the data of tables 2 and 3 indicate that the products of inversion of sucrose have little or no effect, either separately or collectively, upon the potential of the electrode. The data in column I of table 3 confirms the conclusions of Morgen, Lammert, and Campbell ( 7 ) , Le., the potential of the quinhydrone electrode can be accurately reproduced only when care is taken t o clean and dry th(2 electrode before use. In our experiments the electrode after use in the acidified quinhydrone solution was left in the solution until time for the next measurement on a fresh solution, a t which time it was washed with distilled water and transferred to the freshly prepared quinhydrone solution. It may be seen that the potential of the electrode, used in the above manner, showed an increase. This m-ould indicate that the amount of oxidation, Le., the oxidizing agent, in the cell had increased. Now each time the electrode was placed in a fresh solution, the electrode came in contact with the air; also the solution was thoroughly shaken to dissolve the quinhydrone quickly. If oxygen from the air were absorbed on the platinum surface there is a possibility that some hydroquinone was oxidized to quinone and thus the potential of the electrode was slightly raised. The data in table 4 show that the E.M.F. of the glass electrode with respect t o the silver-silver chloride half-cell remains constant throughout the acid inversion of sucrose. Since the E.Y.F. remained constant throughout the inversion, it follows that the activity of the hydrogen ion remained constant also. Thus it is seen that the hydrogen ion is a true catalyst in
ACTIVITY OF THE HYDROGEN ION IN SUCROSE SOLUTIONS
843
this reaction. The work of Taylor and Bomford (9) showed an increase in the activity of the hydrogen ion during the inversion process, but this is undoubtedly due t o the hydrogenat,ion of the sucrose, as suggested by Scatchard (8). SUMMARY
1. The quinhydronc electrode has been used to measure the activity of the hydrogen ion in inverting sucrose solutions and has been found to give a changing potential when the electrode is left in contact with the acidified quinhydrone solution for long periods of time. 2. The glass electrode has been used to measure the activity of the hydrogen ion in inverting sucrose solutions and has been shown to give reproducible results. 3. By the use of the glass electrode i t has been shown that the activity of the hydrogen ion remains constant throughout the inversion of sucrose by hydrochloric acid. REFERENCES (1) CORRAN A N D LEWIS:J. Am. Chem. Sac. 44, 1673 (1922). (2) DOLE:J. Am. Chem. Sac. 64, 3095 (1932). (3) JONESAND LEWIS:J. Chem. Sac. 117, 1120 (1920). (4) MACINNESAND BEATTIE:J. Am. Chem. SOC.42, 1117, 1455 (1920). ( 5 ) MACINNES AND BELCHER: J. Am. Chem. Sac. 63, 3315 (1931). (6) MORANAND LEWIS:J. Chem. SOC. 121, 1613 (1922). (7) MORQEN,LAMMERT, AND CAMPBELL: J. Am. Chem. Sac. 63, 454 (1931). (8) SCATCHARD: J. Am. Chem. SOC.48, 2026 (1926). (9) TAYLOR AND BOMFORD: J. Chem. SOC.126, 2016 (1924).
