ANALYTICAL EDITION
lune, 1945
Finally a set of data was obtained (Table 111) in which the panels were merely cleaned and given finishing coats without prior application of primer.
393
metal, in which case a situation is reached where the electrical gage will no longer function. DISCUSSION AND SUMMARY
Table Ill. Comparison of Thickness Measurement Methods over Solvent Cleaned Metals without Primer Thickness by Micrometer Mils 2.6 2.2 1.6 1.0
Thickness by Electrical Gage Mils 3.0 2.5 1.5 1.2
24ST-21 24ST-22 24ST-23 24ST-24
2.8 2.1 1.8 1.4
2.8 2.2 2.1 1.5
17ST-21 17ST-22 17ST-23 17ST-24
2.5 2.1 1.8 1.2
2.8 2.3 1.8 1.5
B-21 B-22 B-23 B-24
2.3 2.3 1.8 1.3
2.9
M-21 M-22 1\1-23 11-24
2.3 2.1 1.5 1.1
2.5 2.3 1.5 1.1
Panel No AL-21 AL-22 AL-23 AL-24
0
3.0
1.7 1.5
Panel designations same as in Table I.
The first instrument was designed to measure the thickness of an insulating layer over a nonmagnetic conductor. I t seemed t o be of interest to determine the effect of a slightly conducting coat applied t o a nonmagnetic conductor. Some clear lacquer was pigmented to produce a film displaying conductivity. The thickness of the film was first determined with the micrometer and finally by the electrical gage. I n every instance the results agreed as closely as those presented in the tables for nonconducting films. This appeared surprising a t first but, when it is considered that the paint is an extremely poor conductor compared to the metal underneath, the result is somewhat understandable. However, a point can be reached where the film whose thickness t o be measured displays a conductivit,y approaching that of the
From an examination of the data it is evident that the average readings made by the electrical gage are on the average slightly higher than those obtained by the micrometer. Reasons for this differential have been pointed out. I n a very few instances micrometer readings are higher. More uniform agreement was obtained on unprimed panels (Table 111), which may be explained by the fact that the primer pigment was not so well dispersed as were the top coat pigments; this resulted in a slight unevenness in the primer coat and was accentuated as the film was built up by additional coats. I n one or two instances where the film was thin, wider discrepancies exist, but these readings fall below the 0.5-mil range where calibration of the instrument is much less accurate. As film thickness increases, the accuracy of the measurement is greater. Among the advantages of the gage is the fact that measurements can be made rapidly and with reproducible accuracy. S o limit of its usefulness is imposed by the type of metal over which the film is applied except that it be nonmagnetic. I n this respect i t complements the magnetic instruments ( I , 5) so widely used throughout the coatings industry. Accuracy is limited more by the nature of the film surface on which the pickup coil must rest during operation than by the sensitivity of the instrument itself. Films applied to curved surfaces cannot be measured accurately and a flat surface as much as 1 inch in diameter is necessary. If the edge of the pickup coil is placed nearer than 0.5 inch to the edge of the panel the inductance of the coil is affected. Finally. the zero point must be adjusted by contact with a sample of the unpainted metal. LITERATURE CITED (1)
Brenner, Abner, J. Research Natl. Bur. Standards, 20,
357
(1938).
(2) Dinger, J. E., U. S. Patent Application Serial No. 495,776. (3) Rusher, M. A., BUZZ.Am. Ceram. Soc., 14, 365-7 (1935). PRESENTED before the Division of Paint, Varnish, and Plastics Chemistry, CHEMICAL Paint and Varnish Group, at the 108th Meeting of the AMERICAH SOCIETY, New York, N. Y.
Determination of Iodine in Thyroid by Cerate Oxidimetry D. T. ENGLIS
AND
A U G U S T A A. KNOEPFELMACHER, Noyes Chemical Laboratory, University of Illinois, Urbana, I l l .
D
U R I S G the examination of a number of methods for estimating iodine in thyroid and other organic materials, attention was given to a method proposed by Hilty and Wilson ( I ) which involved a n oxidation of iodide ion by ceric sulfate. It became evident during the examination of the method that the reaction postulated by the authors for the oxidation Sa1
+ 6Ce(S04)2
7
HC1 3HlO
-t
SaIOa
+ 3Cez(S04)3+ 3H2SOd
(1)
was incorrect. Under the conditionb employed it could be a5sumed ( 2 ) that the reaction xvould proceed as follows: 21-
+ 4C‘e---- + 2C1-
HC1 -f
+ 2IC1
4Ce++&
checked againb tthe one using the o-phenanthroline indicator with satisfactory agreement. Upon this basis the equivalence of each atom of iodine is 2 instead of 6. Hilty and Wilson (1) state that “each cubic centimeter of 0.005 N ceric sulfate is equivalent to 0.0003178 gram of iodine in thyroid combination”. This is correct upon the basis of Equation 2, but not of 1. Hence, the results reported are properly evaluated and the conclusions drawn from them are not invalidated, even if they are not in accordance with the assumed reaction. The formulation of the expression for the calculation of the normality of the ceric sulfate is misleading. It should be represented simply as MI. of Ce(S0,)s required X its X = ml. of FeSOl X its A’
2)
This was confirmed experimentally by the addition of chloroform and observance of the appearance of the iodine color in this layer in the initial stages of the reaction and later change to the light brown color as the iodine was oxidized to iodine monochloride. This method of detection of the end point was
LITERATURE CITED (1)
Hilty, W. W., and Wilson, D. T., IND.ENG.CHEM.,ASAL. ED., 11, 637 (1939).
(2) Willard, H. H., and Furman, N. H., “Elementary Quantitative Analysis”, 3rd ed., p. 197, New York, D. Van Nostrand Co., 1941.
