JULY 15, 1939
ANALYTICAL EDITION
( 5 ) Kauko, Y . ,Angew. Chem., 47, 164 (1934); 48, 539 (1935); Acta Chem. Fennica, 5B,54 (1932). (6) Michaelis, L., "Die Wasserstoffionen-Konzentration", 2nd ed., p. 40, Berlin, Julius Springer, 1922. (7) Parker, G . H., J . Gen. Physiol., 7, 641 (1925). (8) Thiel, A,, Dassler, H., and Wulfken, F., Fortschr. Chem. Physik physik. Chem., 18, 1 (1924).
375
(9) Wilson, F. W., Orcutt, F. S., and Peterson, W. H., IND.ENO. CEBM.,Anal. Ed., 4 , 357 (1932). CONTRIBUTION from the Physiology Laboratory, Stanford University. Work supported in part by a grant from the Rockefeller Foundation and by the National Youth Administration.
Volumetric Oxidation of Iodide to Iodate by
Sodium Chlorite L. F. YNTEMA
AND
THOMAS FLEMING, St. Louis University, St. Louis, Mo.
T
H E iodide ion may be oxidized to iodate by a chlorite when the iodide solution is buffered with sodium acetate and acetic acid ( 1 ) . The potential of the couple HCIOz
+ 3H+ + 4e = C1- + 2H20
is given by Latimer (4) as 1.56 volts, and that of the couple IOs-
+ 6H+ + 6e = I- + 3Hz0
as 1.085 volts. There is enough difference in the EOvalues so that a quantitative oxidation of iodide to iodate by chlorous acid may be expected. This investigation was undertaken to ascertain under what conditions the reaction may be use3 for a quantitative volumetric determination of the iodide ion and to examine some of its limitations. Obviously, other oxidizing and reducing agents would interfere. According to the equation 3HC102 21- = 21033C13H+
+
+
+
it is apparent that six equivalents of chlorous acid, measured as an oxidizing agent, will be required to oxidize one mole of iodide. Accordingly, the ratio of equivalents of sodium
chlorite to moles of potassium iodide should be 6 to 1, if the method is valid. There are few references in the literature to the use of sodium chlorite as a volumetric reagent. Levi and Ghiron (6) employed it for the volumetric determination of permanganate, and Jackson and Parsons (3) developed a method in which it is used in the estimation of sulfite.
Reagents The sodium chlorite was obtained from The Mathieson Alkali Works, Inc., by whose analysis it consisted of 98.5 per cent sodium chlorite, with traces of sodium chlorate and sodium hydroxide. All other reagents used were A. R. quality. Distilled water for preparation of solutions was freshly boiled. The sodium chlorite solutions, about 0.13 N , were prepared and standardized essentially by the method described by Jackson and Parsons (5). Their observations and those of Levi and Natta (6) concerning the stability of the standardized chlorite solutions were confirmed in this investigation. The potassium iodide solutions, about 0.02 M , were standardized by precipitating and weighing the iodide as silver iodide.
Experimental
Two buffer systems, acetic acid-sodium acetate and monosodium orthophosphatedisodium orthophosphate, were investigated in detail (Tables TABLEI. ACETATEBUFFERSOLUTIONS I and 11). The pH measurements were made with a comRatio of M1 Moles of Equiv. of NaClOz Equiv. of 2 M HCzHsO; H ~ O , KI x 103 Ratio, Moles of XI Average Ratio NaClOz x mercial glass electrode aspH 108 Amber Blue Amber Blue Amber Blue 2 M NaCrHaOz M1. sembly. All titrations were 0.5 carried out a t room tempera9.5 25 .. 4.70 0.784 0.786 6.00 ( 8 ) 5 5 . 9 7 (8) 6.00 5.99 ture. A measured volume of 0 . 5 __ the standardized sodium chlo,. 3.69 0.618 0,620 5.98 5.96 5,98'(5) 5 . 9 6 (5) 9.5 50 rite, about 30 ml., was run from 0.5 5.80 3.71 0.620 0.622 5 . 9 8 (5) 5 9 5 (5)b 100 5.98 5.96 9.5 a buret into a flask containing measured volumes of buffer "5 5.95 (2), 5 . 9 0 (2)b)C .. 3.66 0.616 0,620 9.5 200 5.94 5.89 solutions and water. Starch I solution was added and the .. 3.69 0,614 0.617 9 25 6.00 5.99 6 . 0 0 (4) 5 . 9 9 (4) standardized potassium iodide 1 added from a buret. The blue .. 3.71 0.618 0.620 5 . 9 9 (8) 5 , 9 7 (8) 5.99 5.97 9 50 color of the iodine-iodide-starch 1 __ complex appeared first, and 9 100 5.37 4.48 0.746 0,747 6.00 5.99 6 . 0 0 (10) 5.98 (IO) then disappeared as the iodine -.I 9 200 ,. 4.25 0.708 0.714 6.01 5.96 6 . 0 0 (3) 5 . 9 6 (3) was oxidized to iodate. It was 2 found that, upon the first ad__ 8 25 .. .. ... ... .. .. .... ..... dition of iodide, there was a n 2 appreciable interval of time be.. 3.68 0.603 0.611 6.11 6.03 8 50 6 . 0 8 (2) 6.02 (2)c)e fore any reaction took place. 2 __ However, when the first few 5.02 4.25 0.697 0.708 6.10 6.01 S 100 6 . 1 1 (2) 6 . 0 4 (2)C milliliters of iodide had been 2 oxidized, t h e reaction pro8 200 *. .. ... .. .... .... d ceeded rapidly, provided the a Number of experiments on whioh averages are based indicated by digit in parenthesis. 1, Reaotion slow. End point not sharp. d No end point. e C1Oz evolved. acidity was high enough. According to Bray (1)the reaction I
...
..
INDUSTRIAL AND ENGINEERING CHEMISTRY
376
VOL. 11, KO. 7
basic zinc salts or zinc oxide were used as buffers. Britton Equiv. of NaC10z (2) reports about 5.9 as the Ratio of Ml., Equiv. of Moles of 1 M NaHaPOi NaC102 X KI X 10s Ratio' Moles of K I Average Ratio HZO, pH a t which basic zinc salts Amber Blue -4mber Blue Blue 103 Amber M1. 1 M NazHPOi PH precipitate upon the addi6 tion of alkali. Fifteen milli6 . 0 1 (4)O .... 6.01 0.611 3.67 ... 25 .. 4 liters of 0.2 iM zinc sulfate 6 5 . 9 5 (2)Q .... were diluted with 50 ml. of 0,617 .. 5 . 9 6 ... 50 .. 3 . 6 8 4 water and sufficient dilute 6 5 , Q l(110 .... 0.613 5.91 .. 3.62 ... 100 6.50 4 p o t a s s i u m h y d r o x i d e was 6 added to precipitate a small .... b .... .. ... .. .. ... .. 4 200 amount of basic zinc sulfate. 8 Such solutions were so alka6 . 0 0 (4) 25 .. 4 . 2 2 0 . 7 0 2 0 . 7 0 3 6 . 0 1 6 . 0 1 6 . 0 1 (4)C 2 line that the reaction pro8 ceeded too slowly. Then the 5 . 9 8 (4) 5.98 0.690 5 . 9 9 (4) 5.99 4.13 0.689 50 .. 2 procedure n-as tried of suspend8 ing 1 gram of zinc oxide in 5.96 (4) 5 . 9 s (4) 0.706 5.97 4.12 5.98 100 0.704 6.08 2 50 ml. of water and adding 2 8 5 . 9 4 (2)"pJ 5 . 9 6 (2) 5.95 4.26 0.716 5.96 0.714 200 2 .. or 3 ml. of 30 per cent acetic acid. The chlorite solution 9 6 . 0 2 (7) 25 1 .. 4 . 2 3 0 . 7 0 0 0 , 7 0 3 6 . 0 4 6 . 0 2 6 . 0 4 (7) was added and titrated with 9 iodide. When the reaction 5,QQ (4) 1 50 .. 3 . 7 3 0 . 6 2 1 0 , 6 2 3 6 . 0 1 5 . 9 9 6 . 0 1 (4) rate mas too slow, 1 or 2 9 d r o p s of a c e t i c a c i d w e r e 5 . 9 s (3) 5 . 9 9 (3) 5.9s 5.99 4.23 0.708 100 0,706 1 5.71 added, immediately causing a 9 discharge of color. When the 5 1 9 (3)d 5 . 9 6 (3) 5.91 5.96 0.721 200 .. 0.714 4.26 1 blue was permanent, after 9 2 6 . 0 4 (4) 6.03 6 . 0 5 (4) 0.608 6.04 3.66 0.606 25 addition of 0.5 ml. of acetic .. 0.5 9.5 acid, the ratio of chlorite to 6 . 0 4 (4) 6 . 0 2 (4) 6.03 0.617 6.04 50 3.72 .. 0.615 0.5 iodide was 6 to 1. I n another 9.5 series dilute hydrochloric acid 5 . 9 7 (2) 5.97 5 . 9 9 (2) 5.99 0.709 100 4.23 0.707 5.28 63 (10 per cent) was used instead 9.5 of acetic acid with equally 5 , 9 2 (2) 200 0.5 .. 4 . 2 7 0 . 7 1 5 0 . 7 2 2 5 . 9 7 5 . 9 2 5 . 9 9 (2) satisfactory results. This pro9 8 cedure is, however, not so con6.06 ( 2 ) e 25 .. 3 . 6 7 0 . 5 9 7 0 . 6 0 4 6 . 1 4 6 . 0 6 6 . 1 3 (2) 0.2 venient as those using soluble 9.8 6 . 0 1 (3) buffers. 50 0.2 .. 3 . 7 0 0 , 6 1 0 0 . 6 1 2 6 . 0 7 6 . 0 4 6 . 0 6 (3) I n order to determine the 9.8 6 . 0 1 (2) 4.20 0.703 5.98 5.99 (2) 100 5.06 6.01 0.699 0 3 influence of nitrates, sulfates, 8 9 and chlorides on the reaction, 5 . 9 6 (2) 200 .. 4 . 2 3 0 . 7 0 5 0 . 7 0 9 6 . 0 0 5 . 9 7 5 . 9 8 (2) 0.2 a s e r i e s of d e t e r m i n a t i o n s 10 (Table 111) was carried out. .... .... b j e 50 .. ... ... .. 0 .. ... The procedure was the same as 10 that employed in securing the 4.20 0.591 5.91 6 . 1 1 (1) 5.91 (l)d,e 100 .. 0.688 6.11 0 data for Tables I and 11,except 10 that an amount of the added 200 . . . ./ 5.95 .. 4 . 2 1 0.706 ... * . 5 . 9 5 (2) 0 substance, to give the indicated Number of experiments on which averages are based indicated by digit b No end point. a Reaction slow. in parenthesis. d End point not sharp. e C102 evolved. i Amber fades t o blue. molarity a t the end point, was dissolved in the buffered solution before addition of the chlorite. Potassium nitrate or sulfate did not interfere. As is favored by the presence of I,, 13-, and H+. When the more potassium chloride retarded the reaction, in each case an exalkaline solutions were used, the procedure was followed of adding 4 or 5 ml. of iodide a t a time and then agitating cess of iodide was added before a color change took place. The until the blue color, first formed, had disappeared. As the effect of the presence of bromides was also investigated. It end point was approached, the iodide was added a few drops was found that free bromine was liberated. a t a time with subsequent shaking. Discussion Under the optimal conditions, two distinct end points were noted, first an amber and then the blue of the starchExamination of Tables I and I1 shows that the reaction is iodine complex. The amber end point did not appear in the quantitative in properly buffered solutions having a pH of more alkaline solutions, nor the blue in the more acid. Usually 5.3 to 5.7. The best results were obtained in an acetate the amber was followed, after the addition of 0.10 ml. of iosolution having a ratio of acetic acid to sodium acetate of 1 dide, by the first appearance of blue. Considerable excess, to 9. I n dilute solutions an excess of iodide was required. 0.50 ml., mas necessary to give an intense blue. When the pH was less than 5.2 there was evolution of chlorine The reverse titration, adding the chlorite to the iodide, was dioxide, and when the pH was greater than 6.0 the reaction also studied, but consistent results could not be obtained. An proceeded very slowly. excess of chlorite was usually needed to discharge the blue color It was found that titration to a blue end point required an that formed first. However, it was found possible to backexcess of iodide. Better results were obtained by taking titrate the excess chlorite with iodide and secure good results. as an end point the amber which is obtained when starch is Another series of experiments was carried out in which added to very dilute iodine solutions. TABLE11. PHOSPHATE BUFFERSOLUTIONS
I
C
.
JULY 15, 1939
ANALYTICAL EDITION
377
has suggested that the iodideiodine pair plays a role in (50 ml. of HzO, 1 ml. of 2 M H C ~ H ~ Oand Z , 9 ml. of 2 M N~CzH309) t’he reaction of chlorites with Equiv. of Moles of Ratio, Equiv. of NaC10z sulfites. Concn. of NaClOz X KI x 103 Moles of K I Average Ratio The accuracy of the method Additions loa Amber Amber Blue Amber Blue Blue 0 . 2 M KNOa 4.13 0.689 0.691 6.00 5.98 6.00 ( 2 ) . 5,98 (2) for analytical use is indicated 1.0 M KNOs 4.13 0.688 0.690 6.00 5.98 6.00 (2) 5.98 ( 2 ) by Table IV, in which some of 0 . 2 M KzS04 4.14 0.689 0.690 6.01 5.99 6.00 (2) 5.99 ( 2 , 0 . 2 M KC1 4.47 0.747 0.749 5.98 5.96 5.98 ( 2 ) 5.95 (2) the data of Table I have been 0 . 5 M KC1 4.38 0.732 0.734 5.98 5,97 5.99 (2) 5.97 ( 2 ) b converted from moles and 1 . 0 M KC1 4.15 0.695 0.697 5.98 5.96 5.97 (2) 5.96 ( 2 ) b equivalents to milligrams. The a Number of experiments on which averages are based indicated by digit in parenthesis. b Reaction slow. deviation of the calculated amount of iodine from the TABLE IV. ACCURACY OF METHOD EXPRESSED IN MILLIGRAMS amount taken is within the OF DEVIATION range of experimental error in each case. TABLE 111. EFFECTOF NITRATE,SULFATE,AND CHLORIDE IONS
(Solution:
1 ml. of 2 M HCaHaOz,,9 ml. of 2 M NaCzHsOz, and Hs0 as
indicated)
NaC10z Taken Mo. 83 3 83: 8; 83.7 83.4:
Hz0 M1. 25 50
I - Taken
MQ. 77.99 78.43 78.48 78.04
101.1 95.0;
100 200
96.10 97.94
I - Calcd. from NaClOz Mg. 77,98 78.47 78.3 78.0;
Deviation
MQ. +O.Ol -0.04 fO.0
+o. o!
94.6 88.8;
94.688.8;
-0 ,Os -0.02
89.82 91.72
89.9 91.6;
- 0 . o8 +0.10
Recommended Procedure Dissolve a weighed portion of the sample, calculated to contain 0.04 to 0.07 gram of iodide, in freshly boiled distilled water, cooled to room temperature. Buffer with 1 ml. of 2 M acetic acid and 9 ml. of 2 M sodium acetate and add starch solution. Titrate with 0.1 or 0.2 N sodium chlorite solution until a discharge of the blue color indicates that all the iodine, first formed by oxidation of the iodide, has been converted to iodate. Add a small excess of chlorite. Back-titrate with 0.02 M potassium iodide to a permanent amber. One milliliter of 0.2 N sodium chlorite is equivalent to 0.004231 gram of iodine. Summary
No interference by small concentrations of chlorides is to be expected, since the potential of the couple l/zC1z
+e
=
C1-
is 1.3583 volts. High concentrations of chloride ion, on the other hand, would retard the reduction of the chlorite. Since the potential of the couple Brz
+ 2e
=
2Br-
is 1.087 volts, a value very close t o that of the iodate-iodide, 1.085 volts, the liberation of bromine can be expected. The observation of Bray (1) that the oxidation of iodides to iodates by chlorite is favored by the presence of H+, as well as I2 and 18-, is confirmed. The reaction is very slow until the first few milliliters of iodide have been oxidized and then is very rapid if the hydrogen-ion concentration is sufficiently high. It is possible that a higher oxidation state of iodine acts as a catalyst for the reaction. Parsons (7)
A sodium chlorite solution may be employed for the volumetric determination of iodides in solutions buffered to a pH of 5.3 to 5.7 with either acetates or phosphates. Less satisfactory results were obtained using basic zinc salts as buffers. The reaction involves six equivalents of chlorite for each mole of iodide. Nitrates and sulfates do not interfere. Chlorides retard the reaction and bromides must be absent. Literature Cited (1) Bray, Z. phys. Chem., 54, 741 (1906). (2) Britton, “Hydrogen Ions”, p. 263, New York, D. Van Nostrand Co., 1929. (3) Jackson and Parsons, IND. ENG.CHEM.,Anal. Ed., 9, 14 (1937). (4) Latimer, “Oxidation States of the Elements and Their Potentials in Aqueous Solutions”, pp. 297-8, New York, Prentice Hall, Inc., 1938. ( 5 ) Levi and Ghiron, Atti accad. Lincei, 31, 370 (1922). (6) Levi and Natta, Gam. chim. ital., 53, 532 (1923). (7) Parsons, IND. ENG.CHEM.,Anal. Ed., 9, 250 (1937).
Formula for Determining the Viscosity of Latex. N THE authors’ paper, L‘Examination of Rubber Latex and
Rubber Latex Compounds. I. Physical Testing Methods” [IND.EN^. CHEM.,Anal. Ed., 9,182-9 (193711, the following formula on page 187 is in error: q’ .=
Ka dHg(WZP1
WlPZ) f dl(w2hl - w l h ) w2w1
A dl tz - tl
where q’
R
=
= = = g dl = = P I and PZ =
L
A Correction
hl and hz = average heights in cm. of top of latex column above bottom of capillary at pressures PI and P z , respec-
w1 and wz = weights tively in grams of liquid delivered at pneumatic pressures PI and P z , respectively, in time: tl and tz, respectively
This error has been pointed out by B. M. Kedrov [Caoutchouc and Rubber (U. S. S. R.), 1938, Xo. 7, 28-31]. In his paper, Mr. Kedrov includes the correct formula, which is as follows:
limiting coefficient of viscositv radius of capillary tube in cm. length of capillary tube in cm. acceleration of gravity in cm. per sq. sec density of latex in grams per cc. density of mercury in grams per cc. pneumatic pressures in cm. of mercury
H. F. JORDAN P. D. BRASS C. P. ROE UNITEDSTATESRUBBERCOMPANY PASSAIC, N. J. April 14, 1939
