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
(7)
l6rmnv
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 S t a t e University of Iowa, Iowa C i t y , Iowa
S COXSECTION with work in this laboratory on the polarographl- 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 p H 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
V O L U M E 23, N O . 5, M A Y 1 9 5 1
803 is used as the inert gas. Catalytic hydrogen-oxygen combination should take place during the shaking process, and should cause complete oxygen removal to be accomplished somewhat sooner. This procedure is not necessary for ordinary polarographic work, which requires only that the oxygen content shall be reduced to :I small value.
E’iyiire I .
Diagram of Apparatus
oxygen froiii liioIo,~,ii~~~l wlutioii.. Figure 1.
‘1’111~:ilip:tratus is shon.ti
iir
i t end and is c c t u d to rock The board, ‘1, is pivoted a t i through an angle of aboLit 20 ducer, which rotates a t a s:)eetl clips on the board holtl fuur g can be placed. I*>achvess ,I is :I little iiiorc~than 10 cm. in overall length, and 1 cni. i n esrt:rii&l dianieter. I t i p designed to holtl about 3 to 4 ml. of solution. l‘he openings, C, of the veswls a w connected hy rul)t)er tubing to a tank of pure hydrogen or nitrogen (only one connection is shown in the figure), aiid the vessels are flushed out with hydrogen for a feiv niiriut8eclbefore the solutions are introduced through D. The board shou when the solutions are introduced, and the vessil more than half filled. Openings D are then tightlj the solutions are rocked for about 1 hour. A beaker, E , filled with a salt solution with about the same vapor pressure as t)hr solutions in the vessel$, acts as a safety valve t o prevent build-up of g:is pressure. The gas flow during the rocking operat so adjustcd ,that very slow bubbling occurs a t this safet . The rcrlang inn? be stopped after 30 minutes and the agaiii flwhrtl o u t to remove the small amount of oxygen accumulated i n thc, atmosphere a t that time. In any event equilibrium is established after about 1 hour, and only a negligibly small fraction of the osygt’n originally present in the solution remains.
Polarograms obtained with a 1% solution of bovine serum albumin in tartrate medium before and after treatment are shown in Figure 2. For solutions containing no substances which are reducible by hydrogen, an additional refinement is possible. A platinized platinum disk, sealed into glass tubing, is wedged tightly into opening D in place of the stopper described above, and hydrogen
Figure 3. Diagram of Polarographic Cell
Because it is generally not desirable to use a large volume of solution where biological materials are involved, the familiar Htype polarographic cell developed by Lingane and Laitinen ( 2 ) , which contains an internal reference electrod?, has been modified for use with 2 to 3 ml. of solution, and to take account of the fact that oxygen need not be removed in the cell. Provision has been made for the passage of a stream of inert gas over the solution during analysis, but none for passage of gas through the solution. -1 diagram of the cell is shown in Figure 3. Deoxygenated solutions can be poured directly into these cells for immediate analysis, No appreciable reosggenation ordinarily occurs. ACKNOW LEUGMENT
The authors wish to express their appreciation to Homer Hall for designing and constructing the shaking apparatus. LITERATURE CITED
(1) Clark, W. M., “Determination of Hydrogen Ions.” 3rd ed., Baltimore, Williams & Wilkins Co., 1928. (2) Kolthoff, I. hl., and Lingane, J. J., “Polarography,” p. 212, Xew
York, Interscience Publishers, 1941. (3) Ibid., p. 411. RECEIVED August 1.1, 1950.
1 s ; /I -
-*
Figure 2.
.-
-
/
___
Polarograms of 1% Solution of Bovine Serum Albumin Before ( u p p e r ) and after
( l o w o r ) ov-yqen removal
Converting Platinum Resistance to Temperature-Correction I n the article on “Converting Platinum Resistance to Temperature” [Eggenberger, - 1 5 ~CHEW, ~ . 22, 13336 (1960)l an error occurs in one of the formulas.
should read t , = .2
-
n 4s
+ 1011z
