aoo
ANALYTICAL CHEMISTRY
99.3%) shows that a more complete recovery of tot.al iodine is obtained by summation of the bound and free iodine determined separately.
Table IV. Recovery of Total Iodine after Separation and Fusion Sample No.
DISCUSSION
The experimental evidence found by the procedures described in this paper indicate that losses occurring during the alkaline fusion procedure were negligible. The experimental evidence indicates that acidification of smaller amounts of ashed serum per unit volume of solution resulted in a smaller loss. This was proved by the fact that the total iodine determined in 1 nil. of serum was of the order of 82.3% of the original serum, or of the total iodine calculated as the sum of the protein-bound iodine contained in 1 ml. of serum and the inorganic iodine in the same 1 i d . of serum. Hence, a dilution of 100% gave a 99.3% recovery. Thus it is necessary onlj- to determine protein-bound and free iodine, oniitt.ing the step of determining t,otal iodine ae such. From 100 cases t,he total iodine deterniined on 0.5 ml. of serum chemically varied in the order of 1 0 . 2 microgram from the total iodine calculated by the t x o sepnrat>efractions.
Protein-Bound Iodine and Iodide Counts
Standard Counts
.4v. recovery
7%
Recovery
%
Error
99.3
fraction by separation before digestion (Table IV average recovery, 99.3%). There is a large and variable loss upon acidification of the total iodine sample following digestion (Table I11 average recovery, 83.2%). This may be avoided by the modification of the method presented in this paper. LITERATURE CITED
Barkey, S. B., J . Bid. Chem., 173, 715 (1948). (2) Chaney, A. L., INDENQ.CHEM.,ANAL.ED., 12, 179-81 (1940). (3) Salter, W.T., and Johnston, RlacA. W., J . Clin. Endocrinol., 8, 911-33 (November 1948). (4) Saltei, W.T., Johnston, Macii. W., and Gemmel, J., Symposium on Radioiodine, Brookhaven National Laboratories, pp. 24-34, July 1948. ( 5 ) Salter, W T., Karandikar, G., and Block, P., J . Clin.Endocrinol., 9, 1080-98 (Sovember 1949). (6) Salter, W. T., and hfcKay. C. A., J . Biol. Chem., 114, 495 (1936). (7) Sandell, E. B , and Kolthoff, I. SI.,Mik~ochim.Acta, 1, 9 (1937). (8) Seaborg, G. T., Jaffey, A. H., Kohman, T. P., and Crawford, J. A.. “Manual on the Measurement of Radioactivity,” U. S. Atomic Energy Commission, MDDC-388 (1944). (1)
COSCLUSIONS
Salter’s method for blood Serum iodine gives both precision and accuracy, if total iodine IS calculated as the sum of two fractions, inorganic and orga:iic. The biologically important proteinbound iodine is shown to be rletelmined accurately. The precipitation of protein-bound iodine is adequate as noted in Table I1 (average recovery 100.1%). The alkaline digestion procedure gives an average recovery of 99.8% (see Table I). An adequate recovery of total iodine present is obtained by combining the results of the protein-bound fraction and iodide
RECEIVED April 14,19.50.
Conversion of Pfund Gage Reading to Dry Film Thickness M. H. SWITZER Continental Can Co., Inc., Chicago, I l l . H E Pfund gage ( 2 ) has for many years been employed for c s of applied organic coating films. Tmeasuring the wet thickness Briefly, this instrument is a plano-convex lens of known radius of curvature, mounted so that its conves surface can be pressed reproducibly int,o the \vet coating film down to the substrate; the diameter of the paint spot thus produced on the convex surface of the lens is employed as a nieaiure of the thickness of the film. The author has frequentl?. needed to determine the thickness of dried coating film. where direct measurement is inconvenient. In many instances, direct nieawrement of the thickness of the dried film is either very difficult or impossible. because of the nature of the article t.o which the coating is applied. On the other hand, very few instances have been found in which the Pfund gage could not be employed to obtain an estimate of wet film thickness; later touching up of the emall marks left in the finish is ordinarily a simple procedure. .\ccordingly, an equation has been developed relating the Pfund gage reading to the resulting dry film thickness and x nomogram ronstructed for the solution of this equation throughout the useful arguments of its factors. The equation is developed as follows: Let 7’ = thickness of wet film, millinieters L = diameter of spot on Pfund film thickness gage, millimeters r = radius of curvature of lens of Pfund gage = 250 rnm. Then
Let TV = weight in milligrams of wet coating film on any selected area, A A = selected area, square inches G = weight per gallon of coating material, pounds as prepared for application M = mm. per inch = 25.40005 n = cubic inches per gallon = 231 q = mm. per pound = 453592.4277 Then (Thickness of wet paint in inches) = (weight of wet film per sq. inch) (weight of 1 cu. inch of coating) Hence,
Combining Equations 1and 2, _La= -
W AG
7
LZAG X
9 E
16r
mn
and
W Let S
=
(3)
= per cent solids of the coating material divided by
10O-i.e., weight of a sample of the coating material after drying in accordance with a schedule
V O L U M E 2 3 , NO. 5, M A Y 1 9 5 1
W d
801
ordinarily employed to reduce a film of t'he coating to a state suitable for its ultimate intended use divided by t,he weight of the corresponding wet sample of the coating m:ttei.ixl :is prrpared for application = weight in milligranis of the dried coating film on tht, selected area, -4-i.?., dried in accordance with :I schedule t,hnt will reduce the coating film to ;I state suitable for its ultimate intended USP
Let D
=
Then Ii-d
= T1S '
(1)
:inti substituting Equatioii 3 into Equation 1,
-2
L' 7'd
= =
density (or specific gravity referred to water at 4' C.) of the dried coating film. The density of the film may be determined by flotation of a detached specimen of the dried film in a salt solution, the specific gravity of which is adjusted so that the film specimen neither rises nor falls in the liquid, and subsequent measurement of the specific gravity of the salt solution ( 1 ) cubic centimeters per cubic inch = 16.38716 thickness in mils (0.001 inch) of dried film
0
-I5
-IO L 9 , 9..--
_.--
7
6
-
-9 2 -
L
0:
t
4
g
3
U'
0a. a. U
67
6
n. w K .
0.01
a w.
K .
ln.
lo.
0
a
-7
E.
W
I-
-I
W
4 .
1
LL-
0 .
%:
5-2 f
0
30 -
la
-z
0
0 4 W
-6
E.
0.001
K.
c.
W
0-
I
la 3 Q
w.
s
u
I
a 3 2
L"
iL
U
0 I
A
-
xP
A A -
lo
c' 10.000,
0.001179 LzQS/O
*u I
v
-
t
t
ANALYTICAL CHEMISTRY
802 Then Wd ?Id = -
(6,
DvA
(The dimensions of Equation 6 might appear to be in terms of inches; multiplication of both sides of the equation by 1000 might seem to be required in order to express Td in mils. However, because Wd is expressed in milligrams and D is expressed in grams, a factor of 1000 must appear in the denominator of the right-hand side of Equation 6. Hence the factors of 1000 on the right-hand side cancel and the 1000 times inches on the left-hand side become mils.) Substituting Equation 5 in Equation 0
and
L2GS
Td
=
7
9
l6rmnv
(7)
Finally, subetituting the values of the constants in Equation 7 , T,j
=
E I1
X 0.001179
The practical validity of Equation 8 depends upon the physical and chemical properties of the coating material associated with two of the equation factors, D and G. Considering the first of these tivo, ?!le method for determination of the dried film densit) will provide accurate results, if proper attention is accorded the details of the experimental procedure. Hoiiever, the method is tedious to perform. For practical purposes, therefore, it is satisfactory to determine the density of a particular coating material on a specimen of its dried film which is known to have been cured carefully in accordance with the recommended practice. This density value is thereafter employed as if it were a constant associated with the coating material, including subsequent hatches of the same material. In using this practical expedient, it must be recognized that in so far as the density of the dried film is sensitive to curing schedule variations, the use of the constant value of D will tend to invalidate Equation 8. I t is assumed that the curing practice in subsequent coating application schedules will be substantially in accord with that used for establishing the constant. Physical and chemical tests on the successive batches of the coating material minimize the chance that changes in formulation have occurred which would significantly alter thc dried film density. Equation 8 has so far been applied mainly to industrial organic coatings, for which the cure is effected by forced drying a t controlled elevated temperatures. These precautionary measures, the materials employed, and the conditions under which they are employed are conducive to satisfactory stability of the value of D. With regard to the value of factor G, it is assumed that the gallon weight of the coating material as prepared for application
(at which time the value of G may be determined quickly and conveniently by means of a gallon weight cup or a hydrometer) is the same as that value which it will be supposed might be determined for the applied coating a t the time of measurement with the Pfund gage. Theorrtically, evaporation of solvent from the coating material during the application process and subsequently during any delay time prior to obtaining the Pfund gage reading will generally tend to result in a gallon weight effectively higher a t the time of the reading than prior to coating application. The more highly volatile the solvent with which the coating material is reduced for application, and the longer the delay between the time of coating application and the time of making a Pfund gage reading, the more important in respect to invalidating Equation 8 does this theoretical consideration become. An example of one of the most severe conditions in this regard would be encountered in sprev finishing with a nitrocellulose lacquer; the solvent has high volatility, the volatility is assisted by the spraying operation, and the reading with the Pfund gage must be delayed until the end of the spraying operation. Spray applieation methods, in general, h a w been found to invalidate Equation 8 even when solvents of low volatility have been employed. On the other hand, results with rollcr-coated industrial finishes subsequently cured a t elevated temperatures have been highly satisfactory. Here, the cheaper solvents of loa volatility are ordinarily employed and a Pfund gage reading may he made in a matter of seconds after the coated article leaves the coating machine. Under these conditions, the errors associatrd xith the value of G are well within the uncertainty of the Pfund gage reading itself. The nomogram is constructed to provide a rapid solution to Equation 8. Assume that an organic coating as prepared for application has a weight of 12 pounds per gallon and 40% solids, and that the density of the dried coating film is 1.1 grams per cubic centimeter. This coating is to be applied a t a wet thickness which will provide a Pfund gage spot diameter of 8 mm. What will be the resulting dry film thickness in mils? The solution to this problem is drawn as a key on the nomogram. Draw a straight line connecting 8 and 12 on the L and G axes, respectively. With the intersection of the L to G line and the q axis as a starting point, draw a straight line to 40 on the S axis. With the intersection of the z1 to S line and the x2 axis as a starting point, draw a straight line to 1.1 on the D axis. The intersection of the z2 to D line with the T d axis indicates a dry film thickness of 0.33 mil. LITERATURE CITED (1)
Clark, G. L., and Tschentke, H. L., Ind. Eng. C/mm.. 21, 621 (1929).
(2) Gardner, H. A,, and Sward, G. G., “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors,” 10th ed., p. 146, Bethesda, Md., Institute of Paint and Varnish Re-
search, 1946. RECEIVED May 29, 1960.
Oxygen Removal in the Polarography of Biological Solutions CH4RLES TANFOHD A N D JACK EPSTEIN State University of Iowa, Iowa City, Iowa S COXSECTION with work in this laboratory on the polarog-
raphl- of metal solutions in the presence of proteinss it was necessary to develop a new method for the removal of oxygen, which should find application in any polarographic analysis of solutions containing proteins or other large molecules of biological origin-for example, analyses based on the catalytic sulfhydryl wave (3). Gases cannot be bubbled through such solutions because of the formation of very stable foams, and the usual method of oxygen removal is therefore not possible.
This same problem is encountered in pH measurement hy means of the h>-drogrn electrode' Solutions are customarily saturated with hydrogen I>\- bubbling the gas through the solution, but this cannot he done for biological solutions. A special cell has therefore been devised for such solutions by Clark ( I ) , in which continuous rocking causes continuous breaking and renewal of the solution surface, and, therefore, fairly rapid equilibration with the surrounding atmospheie, in this case hydrogen. The author8 have applied Clark’s technique to the removal of
