T H E INFLUESCE OF TEMPERATURE ON T H E OXIDATIOS POTENTIALS OF MIXTURES OF FERRIC AND FERROUS CHLORIDES I S HYDROCHLORIC ACID SOLUTIOS BY S T D S E Y R A Y M O S D CARTER AND THOMAS JOSEPH GLOVER
I t has been shown by Carter and Clews’ that the oxidation-reduction potentials of mixtures of ferric and ferrous chlorides in hydrochloric acid are so modified by the acid that its presence would probably account for the anomalies observed by Wardlaw and Clews2 in the oxidising and reducing reactions of these mixtures in hydrochloric acid with sulphur dioxide. In order, however, to institute a closer comparison between the sulphur dioxide observations which were made at IOO’C and the potential measurements, which were taken at room temperature, it was necessary to determine how the latter values would be affected by change in temperature. For this object the following investigation was undertaken.
Experimental Preparatzon of solutzons. The ferrous chloride solutions were prepared from pure iron and hydrochloric acid, whilst those of ferric chloride were obtained by dissolving the anhydrous solid in this acid. The solutions were made up as nearly as possible to the composition required and the actual composition then determined by analysis. The ferric iron was estimated by reduction with titanous chloride using an excess of potassium thiocyanate as an indicator.3 The total iron was determined by oxidising the ferrous chloride present with sodium peroxide and titrating as before with titanous chloride. The ferrous iron was then obtained by difference. The total chlorides were estimated as silver chloride and the free hydrochloric acid calculated. Apparatus. The oxidation half element, which was furnished with four recently platinised platinum electrodes, consisted of a glass vessel containing about roo ccs. of the acid ferrous-ferric chloride solution over which was enclosed an atmosphere of carbon dioxide. The solution was connected with a normal calomel electrode by a chain of three intermediate vessels, the one next the calomel electrode containing normal potassium chloride and the other two the same solution as the oxidation half element. The connecting syphons had tubes attached resembling handles to tuning forks. The syphons were filled and any bubbles which collected during the progress of the experiment were removed by suction through these tubes which were afterwards closed with glass plugs. In order to keep down the electrical resistance of the Knecht and Hihbert : “New Reduction Methods in Volumetric Analysis.” J. Chem. Soc., 125, 1880 (1924). J. Chem. Soc., 117, 1093 (1920).
680
SYDNEY RAYMOND CARTER AND THOMAS JOSEPH GLOVER
cell the syphon tubes were of wide bore (ca. 7 mm.) The limbs of the syphon connecting the normal potassium chloride vessel with the ferrous-ferric vessel next to it, were made about five inches apart to permit of the two parts of the cell being placed in two different thermostats simultaneously when so required. This syphon was fitted with a tap which was open only whilst potential readings were being taken. Potential Measurements. The cell was allowed to stand in the zo°C thermostat for about half an hour, after which, the e.m.f. was observed about every five minutes until a constant value was attained. If any one of the platinum electrodes continued to disagree appreciably from the others it was replaced by a new one. A Tinsley potentiometer and galvanometer were used to measure the potentials, observations of which were taken to the nearest tenth of a millivolt. Constant e.m.f. readings were as a rule easy to obtain except when the cell appeared to have become polarised. The e.m.f. having been measured a t 20°C the cell was transferred to the 6ooC thermostat and the e.m.f. measured in the same way as a t the lower temperature. Finally the calomel electrode and adjacent vessel were placed in the thermostat at 20°C whilst the oxidation half element and the other two vessels were allowed to remain in the 6ooC thermostat. The e.m.f. of the cell as thus arranged was again measured. The concentrations of the ferrous and ferric iron are denoted by [Fe"] and [Fe"'] respectively and they refer to salt concentrations. The e.m.f.'s of the oxidation cells a t zo°C, 60°C and 20"C-60°C as above described, are denoted by E b , E d , and Er respectively. The Influence of Temperature on the E.M.F. of the Oxidation Cells. Table I gives the potentials of a number of cells in which the hydrochloric acid of the oxidation half element is a t varying concentrations. Slight corrections are applied to the observed e.m.f.'s to bring the acid concentrations to the rounded values given in Table I, and they are tabulated in columns 3 and 4 for the temperatures zo°C and 60°C respectively. I t will be seen that for any given acid concentration the potential increment for this temperature interval, increases with a rise in percentage of ferric salt, which is in accordance with the logarithmic formula stated below. I t will be also observed that for a ferrous-ferric mixture of given proportion the potential rise due to the increase of temperature falls off with diminishing acid concentration. Variatzon of Potentzal ut ZO°C and 60°C' with Proportion of Ferric Iron. The potentials when [Fe"] = [Fe"'], which are denoted by E, have been calculated by the formula E,, = E RT/nF.log [Fe"]/[Fe"'] = E 0.0001983 T log [Fe"]/[Fe'"]. The numerical value of the factor R T / n F has been taken as o.ooo1983T or o.oj81 and 0.0661 for 2ooC and 6ooC respectively, and the constancy of the potential E,, in columns j and 6, Table 1 shows that the logarithmic formula holds a t both temperatures for the acid concentrations stated when the values of the ferric and ferrous salt concentrations are substituted in this formula. Previous work' had only verified this relation for a lower temperature 17°C. J. Chem. SOC., 124, 1881 (1924).
+
+
681
OXIDATION POTENTIALS OF FERRIC A N D FERROUS CHLORIDES
TABLE I Solution
t
NO.
El, 20°C
t
=
=
E (I 60°C
I
25.78
[HCl] = o loo S o 4056 0 4355
2
50.66
0
3 4
6.34 26.42
o 4652
4339
[HCl] = 0 0
3219 3640 3923 4179
3
50 00
0
6
74.49
0
7 8 9
15.79
0
5 0 . 78
0
IO
75.33 87.62
0
I1
21.03
0
I2
42.27
o 2958
'3 I4
I
Ehu
t = 2ooc 0.4323 0,4332
E,io
t = 60°C 0 0
4657 3645
ooo S 0
0 0 0
3291 3805 4118 4417
0.3899 0.3899 0.3923 0,3909
o 4067 0 4102
o 4118 4109
0
[HCl] = 3 O O S
0
o
3037 3181 3783 3971
3022
3528 o 3876 0
0
4055
0,3459 0.3173 0.3499 0,3477
0 0 0 0
3503 3519 3553 3494
[HCl] = 4 9 0 s
IS
5 2 .06 64.7 1 68.11
16
77
I i
0
2 0
28.92 34.96 52.43 78.29
21
40.84
0
22
57 59 80.54
50
3090 3203 o 3272 0 3368 0
0
[HCl] I8 '9
o 2651 o 2926 0 3084 o 3206 o 3285 0 338;
2724
=
2109 0 2557 o 2679 0 2977
[HCl] =
23 I
2
I777 o 1912 o 2219
0.3058 0.3037 0.3068 o 3050 0.3056
3030 o 3026 o 3060 0 3033 o 3068 o 3032
2313 2473
0 . 2636
0 2571
0.2608
0
0 2572
0.2654 o 2653
0.3081
0
6665 0
0
0
2934
253' 2514 o 2j66 0
I O 02.Y
0 2075
0 . 1871 0.1836 0.1861
4
5
0
1757
0
I733
3
o 1681
1615 o 1667 0
h
T h e D
