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 b y 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 b y 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 t h a t 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 T H E 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
