THE RATE O F OXIDATION OF POTASSIUill IODIDE BY CHROMIC ACID BY RALPH E.
DeLURY
T h e present measurements, like those of the rate of oxidation of ferrous salts' by chrotiiic acid, were undertaken with a view of throwing light on the remarkable catalytic action of iron on the oxidation of iodides; and also i n order to ascertain whether the irregularities observed in the oxidation of ferrous sulphate' and of sodium thiosulphate3 were inherent to the use of chromic acid as a n oxidizing agent. With the exception of Series VI the rneasiirements were carried out a t 30" C. T w o of the reagents, suitably diluted, were mixed in a wide mouthed Erlenmeyer flask of 2 0 0 or 300cc capacity while the third (usually the substance present in least quantity) was pipetted into a test tube, together with enough water to make up 15 cc. Both vessels were left in the theriiiostat until the proper temperature was attained, when the testtube was taken out, quickly covered with a cloth to hinder loss of heat, and at the right moment emptied into the flask. T h e mixture was shaken and the last drops of liquid were removed from the tube by tapping it against the walls of the Erlenmeyer. When it was desired to stop the reaction, a "restrainer" was poured in, consisting of a solution of sodiuni acetate in which was dissolved sufficient sodium bicarbonate to neutralize somewhat less than nine-tenths of the sulphur ic acid initially present in the reacting mixtiire. T h e iodine liberated mas then determined with a/roo sodium thiosulpliate and iodine solutions, using starch4 as indicator ; a sharp end point was ensured by Jour. Phys. Chem. 7, I (1903). Ibid. 7, I O (1903). Ibid. 7, SI (1903). If a few crystals of menthol he left floating on the starch solution, it
keeps good for weeks ; the solution of sodium acetate was preserved from deconipositioii by the same means.
-
Ra@h E. DeLuvy
240
adding about one cubic centimeter of a normal solution of potassium iodide before titrating. T h e efficiency of the restrainer was tested by letting the mixture stand half an hour or so after titration ; no blue colour, indicating liberation of iodine, was observable. Blank experiments showed that (probably owing to the short duration of the experiments) there was no necessity for special precautions to exclude air or to prevent loss of iodine by evaporation ; the mouth oi the Erlenmeyer flask was simply closed by a watch glass. T h e stock solutions were :-Potassium bichroma2te, F/Io' (0.6 normal), made up by weighing the pure salt. Potassium iodide, approximately F/Io, standardized gravimetrically with silver. SuZfhuric acid, approximately 272, standardized gravimetrically with barium. T h e alkalinity of the iodide solution was determined, and has been allowed for in giving the initial compositions of the reacting mixtures. T h e volumetric solutions of iodine and thiosulphate were frequently compared with a solution of sodium arsenite whose titer was determined against freshly sublimed iodine. All pipettes used were calibrated by weighing the distilled water delivered, an interval of thirty seconds being allowed for draining. Classification of the Measurements T h e experiments may be grouped in six Series. I n Sevies I,the concentration of the bichromate was much less than those of the other reagents. Expts. 1-6. I n Series II the concentration of the iodide was much less than those of the other reagents. Expts. 7-11, In Series I11 bichromate and iodide were present in comparable amounts, the acid in excess. Expts. I 2-14. Series IVincludes experiments on the influence of each of the three reagents, by the Method of Constant Rates; Expts. 15 -1
7. 1
One formula weight in ten liters.
Oxidation o f Potassium Iodide by Chromic Acid
241
Sevies Y contains experiments on the catalytic action of a number of salts; Table 18. Finally the experiments of Sevies VI were carried out at o°C, to determine the influence of the temperature on the rate ; Expts. 19-27. Explanation of the Tables At the head of each table the total volume of the reacting mixture is given, and the quantities of the reagents initially present, in the following units. T h e letter A denotes the number of gramme-formula-weights of potassium bichromate (K2Cr207)multiplied by IOO,OOO ; B, the number of grammeformula-weights of sulphuric acid (H2S04>multiplied by IOO,OOO; and C the number of grarnme-formula-weights of potassium iodide, (KI) also multiplied by IOO,OOO. I/ is the volume in cubic centimeters. Under x is entered the amount of iodine liberated in B minutes, expressed in cubic centimeters of ~ Z / I O Othiosulphate. T h e amount of iodine dissolved in the potassium iodide of experiments i-iii, Table 18,is expressed in the same units. T h e value of x for 6 = 00 (that is, the number of cubic centimeters of ~ Z / I O O thiosulphate equivalent to the iodine that would be liberated if the reaction were allowed to proceed until none of the reagent present in least quantity remained) was obtained by calculation from the known initial composition of the reacting mixture; in the measurements of Series I this value was controlled by direct experiment ; in Series I1 a similar check was not feasible. RESULTS OF THE MEASUREMENTS Series I T h e first Series were carried out with potassium bichromate in small quantity, as it was thought that the iodine liberated by the reaction might exert less influence on the rate in presence of a large excess of potassium iodide, than if the quantity of the latter were small. Experiments i a n d ii, Table 18, show that the retardation due to this cause would not seriously affect the results of the measurements.
Ra@h E. DeLury
242
T h e constancy of K I 1 throughout each of the tables, and the fact that doubling the concentratioti of the bicliromate does not affect that cotistarit (Expts. I and 2 ; 3 and 4) sliows that the rate is proportional to the concentration of the bichromate. Doubling the concentration of the iodide trebles the rate (Expts. 5 and 2), and doubling that of the acid quadruples it. Thus, the rate is proportional to the concentration of the bichromate, and to the square of that of the acid. EXPTS. I AND 2. I. 2.
A , 1.968; B , 237.9; c, 142.4; V,100; hlaZg, 0.0065 ; R,0.18 A , 0.98 ; B , 237.5 C, 142.4; V,100 ; k, v,r> o 0056 ; X,0.088 X kl _ _ ~ _ __ _ _ _
(0.0057) 0.0063 0.0067 0.0064 0.0066
0.75 1.60 2.43
3.00 4.3' 11.81
8
2.
IO
0.75
15
1.21
1.50 1.90 2.18 5.90
20
25 30
-
(o.oojg) 0.0067 0.0064 0.0067 0.0067
01
-
EXPTS.3 A N D 4. A , 1.968; B , 478.3; c, 142.4; V,100; azg, 0.029; R,0.792 A ,0.984; B , 478 ; C, 142.4; V,100; , n ? x ,0.029; A,! 0.396 -
3. 4.
.-
I
X
__
.-
__
0.82 1.78 2.42 2.87 3.71 5.90
I .68
3.21 4.57 5.78
7.60
11.81
5.
0
5 IO
15 20
1
-2:
1
0.0260 0.0312 0.0306 0.0289 0.0287
-
-.
11
k,
1.18 2.16 2.74
0.0194 O.Oig7 0.0181
3.45
0.01gz -
l
1
6.
8
2
~
_____
4 _____
IO
0.24
0.00 I 80
4:-
0.48
0.00184
w
0.00204
I O 1
-
5.90
I
1
--
Oxidation o f Potassium Zodide by Chromic Acid
243
At tlie head of each Table of this Series, I have noted the average value of Rr (omitting those enclosed in brackets). Strictly speaking, R, should diminish as the reaction proceeds ; owing to the large excess of iodide and acid however, the diminution could never exceed a few percent, and is of the same order as the experimental errors. R is calculated from the average value of Rr by the relatioil R =dxIdO = 6 k I A . X log nat I O . Series I1 I n Series I1 the amount of the potassium iodide in the reacting mixture was niuch less than tliat of the bichromate or that of the acid. A few experinleiits in which iodine was dissolved in tlie potassium iodide before making up the reacting mixture led to the adoption of the following equation, K,Cr,O,+ 9kI -k j H , S O , =4K,SO,SCr,(SO4),+3KT, tjH,O whence the espressions, 6A - x , 6/7B,-x, 2i3C-zx, to represent the amounts of bichromate, acid and iodide reinaining when the iodine liberated was equivalent to z IO-: grammeformula-meiglits of sodium thiosulpliate. T h e details of tile measurements in question are not published in tlie present paper; it is hoped that the results of an investigation of the reaction between iodic and hydriodic acids which has been uiidertaken in this laboratory, may throw light on the retardation caxsed by iodine. EXPTS. j A N D 8.
A , 19 9 ; 0,240 3; C, 9.45; V,100; Kaug, 26 X IO-'";X, 0.083 8. A , 19.9; I',480.7; C, 9.35; L', 100. j.
...___
5
___
0.39
IO
0.73
I5
1.13 1.45
20
1.47 2.69 3.85 4.66 5.37 5.80
20
44
80 -
-
4 4.4 5
-
26 28 32 34 40 44
244
Ra@h E. DeLuvy
EXPTS.g AND IO. 9. A , 39.8; B, 240.3; C, 9.45; V,100;Kavg,2 2 x IO.
IO-'"; K , 0.141 A , 39.8; B , 240.0; C, 18.89; V , 100; Kavg,2 0 x IO-'"; R,0.257
IO
I5 20
25 30 40 50
I
'
-
-
1.10
I5
1.75
27 35
1.5 1.8
2.25 2.53
40 57 77
3.06
3.77 4-32
-
1.26 2.47
3.20
I .8
3.71 4.64 5.56
1.6 1.9 1.4
-
EXPT.11. A , 39.8; B , 480.7; C, 9.45; V,100; K,avg,
e
1
2.5
1
5.0
I
10.0
!
15.0
x
1
qs)
1-31 1 4.5 2.64 I I O 4.11 I I8 5.15
1 T(s)je II 2 1.8 .0
27
X
IO-'";
q g ) 1 ~ ( 9 j l e~
R,0.694 K x I O ~ O
1 2I 2 6 : ;
1 2 24 ;
!
I
46
I
4.6
27 29
Oxidation of Potassium Iodide by Chvomic Acid
--‘-I5 IO
,
I
1.11 2.24 I
I3 13
~I
5 15
+-, 0.22
(7)
0.96
11
I5
I
1.10
245
13
Ra&4 E. DeLury
246
were carried out by the “Method of Constant Rates” described i n a recent paper by Bray.‘ T h e restrainer being ?dded, and the solution analyzed before the concentrations of the reagents had been much reduced by the progress of the reaction, the quotient xi@ gives a first approximation to the rate ; the effect due to the slight decrease in the concentrations ( I 2 percent at most), was corrected by iiieaiis of equation I.
EXPT.15.
A , 17.79;
239.6 238.7 237.0 235.3 233.6 231.9 230.2 228. j 226.8 2 25 . I 223.4
c, 94.83 i
