I S D C S T R I A L A N D ENGINEERING CHEMISTRY
June. 1925
631
Electrometric Titration' By J. C. Brunnich DEPARTMENT OF
AGRICULTCRE AND
STOCK,
OLLOWING the method of electrometric titration, without the use of hydrogen and calomel cell, as suggested by van der Meulen and Wilcoxon12the writer carried out a series of tests using different electrodes with a Wendt electrotitration apparatus, and found that only platinum and graphite electrodes gave a suitable change from positive to negative deflections of the galvanometer when neutrality was reached. With various electrodes attached to the negative and positive sides of the apparatus, the following potentials were obtained:
F
-
+
C C C Pt Pt Pt
Zn cu Au
Pt
cu
C
Au
Zn
0 01 3 arid Volt > 1 00 0 42 0 11
0 0 >1 0
18 20 00 40
Neutral Volt > I 00 0 34 0 15 -0 005 0 10 0 83 0 20
N alkdll Volt > I 00 0 32 0 19 -0 13 0 07 0 48 0 26
(1 01
Only pure graphite could be used as electrodes; electric light carbons were auite unsuitable. Thk electrodes are easily affected and the potential of the cell is easily altered, but they are readily tested by successive immersion in three beakers, one containing distilled water, the second one drop of 0.1 N acid, and the third one drop of 0.1 N alkali in 40 cc. of distilled water. The galvanometer must give a strong deflection to the positive side with the acid, and to the negative side with the alkali, and show no deflection in the pure water. If kept in contact for any length of time the tapping key lowers the potential, 0.01 N acid being lowered from 0.18 to 0.10 volt, but again restored to normal after 70 seconds' rest. Temperature slightly alters the potential, 0.01 N acid showing 0.07G5 a t G o C., 0.180 a t 22' C., and 0.190 volt a t 42" C. The potentiometer was used in all the tests, but for practical purposes it is unnecessary, and the electrodes can be simply connected to a sensitive galvanometer giving distinct deflections with a potential of 2 to 3 millivolts. On dipping the electrodes into various solutions the galvanometer a t once indicates their reaction. The combined electrodes (Figure 1)were made from a graphite rod about 3 cm. long inclosed in a glass tube leaving about 2 cm. projecting. A ure 1 few drom of mercury inside the tube made the connection with-a copper w&e attached to the positive side of the galvanometer. The glass tube with the graphite rod was inclosed in a wider glass tube, at the lower end of which a few holes were blown to insure ready circulation of the liquid. Around this outer tube platinum wire was wound and connected with a copper wire to the negative pole of the galvanometer. The curves of the potentials obtained during titration are quite as characteristic and sharp as those obtained with the 1 2
Received April 10,1924. THISJOURNAL, 16, 62 (1923).
BRISBANE, QUEENSLAND. AUSTKALIA
hydrogen electrode and calomel cell, although the actual voltage is much lower. For the accurate determination of the true voltage near neutrality a millivoltmeter is required. The writer obtained a strong deflection with about 5 millivolts with his galvanometer. Approximately 0.1 N solutions of various acids were prepared and titrated with 0.1 N alkali. To 10 cc. of the acid to be tested were added 90 cc. of distilled water. I n several instances duplicate determinations made on different days are given, and although the actual potentials may be slightly different, the results of the titrations are not affected. (1) Hydrochloric and sulfuric acids titrated with 0.1 N alkali. 0.1 N alkali cc. 0 2 4 6 8 9.5 9.75 10
10.2 10.3 10.4 10.5 10.6 12
-Hydrochloric acidVolt Volt +0.19 +O. 17 +o. 19 +O. 17 10.19 ...
+o. is +o. 19 +o. 19
{ O : 17
... 4 0:09 +0.04
+O. 16
10.08 4-0.04 +0.02 -0.005 -0.02
+O. 16
40: 10 +0.045 io.015
-0.005 -0.01
-0.005 -0.01
...
-0.08
...
$ 0 : 18
+0.01
+o.oo
+o.oo
Volt
+o. 19 +o. 19
Sulfuric acid Volt +0.205 +0.200 f0.190 +O. 180 +O. 180 +O. 175 +O. 170 +0.130 +0.040
+o.oo ...
+o.oo
-0.005
... ...
-0.04
One of the results of the titrations of hydrochloric acid is plotted (Figure 2) and a t the same time the changes of various indicators near neutrality are shown. As a rule the galvanometer
t 0.26
+O.lO Ir) -k \
g o
- 0.K lo
9
lI
Cc. O.l,N alkali Figure 2
shows no deflection a t neutrality, but before phenolphthalein shows the faintest pink color the needle is strongly deflected. (2, 3, 4, and 5 ) Ten cubic centimeters 0.1 N solutions of potassium hydrogen phthalate, lactic acid, citric acid, and acetic acid were titrated in similar manner. The results are plotted in Figure 3. (6) Phosphoric acid made from sirupy phosphoric acid (Figure 4).
Vol. 17, No. 6
INDUSTRIAL AND ENGINEERING CHEXISTRY
632 .I N Alkali
0 2 3 4 4.5 4.75
0.1 N HC1
0.170 0.160 0.140 0.140 0.140
A
5.25 5.5 5.6 5.7 5.8 6 8 9 10 10.5 10.7 10.8 11
+O.ZOt
( 7 ),Borax was titrated with 0.1 N acid, and after addition of mannitol titrated with 0.1 N alkali (Figure 5).
Volt +o. 180 + O . 175 -0.170
cc
cc.
r
Volt
0.130 0.120 methyl orange change S 0.100 0.090 0.090 0,080 0.060 0.035 0.020 0.000 -0.005 -0.020 phenolphthalein faint pink
0.08 0.05 0.04 0.02 0.01 0.00 0.00 +0.005 methyl orange changes
10 grams mannitol added 0.1 X alkali cc.
r
0
+0.065 0.085 0.065 0.060 0.060 0.050 0.040 0.020 0.005 0.000 0.000 0,000 -0.005
2 4 6 8
I----.
-0.07
0 2 4 6 8 9 10 10.2 10.4
10
11 12 13 16 18 20 21 21.2 21.5 22
Volt
+o. 11
0.11 0.11 0.11 0.105 0,095 0.095 0.095 0.090 0,090 0,090 0.040 0.000 -0.005 -0,020 phenolphthalein faint pink
-0:030 -0,060
(8). Boric acid was titrated with 0.1 iV alkali, without andlwith addition of mannitol, and practically the same results were obtained. 0.1 N alkali
Volt-
cc.
Figure 3
This titration shows the characteristic broken curve of this acid on the formation of the primary and secondary salt. A further break a t about 4.5 cc. appears to indicate the existence of a still more acid salt. True neutrality is reached before phenolphthalein gives a decided pink color.
+o.
185 0.140 0.120 phenolphthalein 0.110 0.095 0.040 0,005 -0,030 phenolphthalein faint pink
0 4 6 8 9 10 10.2 10.4
to.20 .
Phosphoric acjd
$ t 0.10 2 0
Phenolphthalein -0.05
--+
t
i-0.10
0
I 0
I
2
*
I
I
I
4
6
8
IO
CC. OJgacid
I
0
I
2
I
4 Figure 5
I
I
6
8
1
1
1
l O f Z f 4
Cc. 0.11 alkali
1
1618
1
1
2022
