June 15, 1941
ANALYTICAL EDITION
The amounts of sulfuric acid and buffer are adjusted so that the solution is certain to be acid during oxidation with bromine and the potassium acetate in the buffer will suffice to convert the excess of sulfuric acid to acetic acid. r cent potassium iodide solution is freshly prepared ea?fti:t dissolving 1 gram of the salt in 3.7 cc. of iodine-free water. The buffer is prepared by one-twelfth neutralization of glacial acetic acid with a saturated solution of otassium carbonate that has been washed with iodine-free aiohol. Let us suppose that the glacial acetic acid is 17.4 N , and the washed potassium carbonate solution 10.8 N . To 10 cc. of glacial acetic lo 17'4 or 1.34 cc. of the carbonate soluacid one would add 12 X 10.8 tion.
Summary The reaction between minute concentrations of chromate or dichromate and iodide is influenced b y many factors, notably acidity, iodide concentration, catalysts such as iron, and ions which form complexes with chromium, such as oxalate and tartrate.
443
Conditions were found under which the reaction between iodate and iodide goes t o completion long before liberation of iodine b y chromate can be detected. This made it possible to titrate iodate, and, after addition of acid and oxalate, t o titrate chromate present in the same solution. The procedure is of value in testing apparatus and methods for determination of iodine in blood, and in preventing returning end points when these are due to small amounts of chromate.
Literature Cited (1) dllott, E. N.,Dauphinee, J. A., and Hurtley, W. H., Biochem. J., 26, 1665-71 (1932). (2) Matthews, N. L., Curtis, G. M., and Brode, W. R., IND.ENG. CHEM.,Anal. Ed., 10, 612-15 (1938). (3) Shaffer,P. A., and Hartmann, A. F., J . Bid. Chem., 45. 365-90. especially 366 (1921). (4) Stevens, C. D., J. Lab. Clin. M e d . , 22, 1074-9 (1937). (5) Trevorrow, V., and Fashena, G. J., J . BioZ. Chem., 110, 29-38 (1935); 114,351-5 (1936).
A Method of Measuring Contact Angles J . J . BIKERRIAN Glass Fibres, Ltd., Firhill, Glasgow,
W
HEN a liquid drop lies on a horizontal solid plate, the
area of contact between liquid and solid depends on the contact angle, 8, between air, liquid, and solid, and on the shape and volume of the drop and t h e surface tension and density of the Liquid. The relation between these factors and the area of contact is generally too complicated to allow a calculation of 6'from the size of the area. If, however, the drop is very small and very nearly circular, the diameter, l a , of the basis of the drop is a function only of its volume, u , and
of
e:
ba = u
21 sin3@
,+ - 3 ~ 0 +~ 0COS^^)
(1)
This equation is arrived a t in the following manner. The surface i;ension tends to give the drop the shape of a part of a sphere (since a sphere has the smallest relative surface). Gravitation on the other hand tends to flatten the drop. The actual shape of the drop is determined by the simultaneous action of both these forces. The effect of the surface tension increases when u decreases, since the ratio of surface to volume increases as well; and the effect of gravitation decreases with v since the level differences in small drops are small. Therefore very small drops are pracxically spherical segments, and Equation 1 is the geometrical relation between the diameter, A", of the base of the segment, its volume, Y, and the angle, 8 , between the base and the curved surface. If the drops are larger than, say, 0.0001 ml., the diameter, A, of the circle of contact is appreciably larger than the value, 10, which it would have had in the absence of gravitation. The ratio a3 has, therefore, to be measured for several values of u and then extrapolated to Y = 0. A curve for the relation between A and u is given by Wark (4). I n order to make the extrapolation to u = 0 feasible, small drops-e. g., 0.001, 0.002, 0.004, and 0.008 m1.-have to be used. They are driven out from a microsyringe. The Agla micrometer syringe of Burroughs Wellcome & Co. , London, was found very convenient for determining the volume of such small drops. It is supplied with a steel needle which is not wetted by water and, therefore, delivers drops more readily than B micropipet of glass. The diameter, A, of the contact circle is measured after the evaporation of a drop. After evaporation drops leave a mark on
Iv. W., Scotland
the solid surface which may be due to corrosion of the surface by the liquid, deposition of the solutes present in the liquid, or settling of particles suspended in it. If the marks are not clear enough, this third effect may be artificially increased by dusting ignited talc onto the drop surface; it slides down the slopes of the drop and forms a white ring round its boundary. If the volumes recommended above are used, A has values between 0.05 and 0.5 cm. The diameter of the circular marks can be determined using any method suitable for such lengths. A microscope provided with eyepiece micrometer scale (for small values of A) and a mechanical stage (for large values of A) is most convenient. A total magnification of 20 diameters is sufficient. The precision of the value obtained for
AS
f is
limited chiefly
by the deviation of the mark from a n ideal circle.
TABLEI. RATIO, AOJlu,
9
Des. 30 32.5 35 37.5 40 42.5 45 47.5 50 52.5 55 57.5 60 62.5 65 67.5 70 72.5 75 77.5 80 82.5 85 87.5 90
4 U
18.54 17.00 15.64 14.45 13.40 12.46 11.62 10.87 10.18 9.55 8.97 8.44 7.94 7.475 7.04 6.635 6.25 5.89 5.55 5.22 4.92 4.62 4.34 4.075 3.82
FOR
VARIOUSVALUESOF
0.62 0.54 0.48 0.42 0.38 0.34 0.30 0.28 0.25 0.23 0.21 0.20 0.19 0.17 0.16 0.15 0.145 0.14 0.13 0.12 0.12 0.11 0.107 0.102 0.098
CONTACT
ANGLE@
Change of A:/V per 10
As no
e
a: -
Deg.
U
92.5 95 97.5 100 102.5 105 107.5 110 112.5 115 117.5 120 122.5 125 127.5 130 132.5 135 137.5 140 142.5 145 147.5 150
3.575 3.34 3.12 2.90 2.69 2.495 2.305 2.123 1.948 1.781 1.622 1.470 1.326 1.189 1.059 0.938 0.823 0.717 0.618 0.527 0.444 0.369 0.302 0.242
>hange of A:/D per 10 0 094 0 090 0 086 0 082 0 080 0 076 0 073 0 070 0 067 0 064 0 061 0 058 0 055 0 052 0 048 0 046 0 042 0 040 0 036 0 033 0 030 0 027 0 024
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INDUSTRIAL AND ENGINEERING CHEMISTRY
surface (under ordinary conditions) is rigorously uniform, the diameter of the contact mark varies from point to point. If the average diameter is wrong by 1 per cent the error of the A3 ratio ; is 3 per cent. When the value of
A3
is measured or extrapolated, 8 may
be calculated using Table I, which gives the values of the function *(2 - 324 for 49 values of 0. For nonCOS e coss e)
+
tabulated values of
$ a linear interpolation is sufficient. A3
Table I also shows that a 3 per cent error in f causes an error of about 1 per cent (or less) in the value of 8. If the solid surface is very good, like those of built-up multilayers (g), can often be determined within *0.6 per cent and 8 therefore calculated within * 0.2". In a recent paper Benedetti-Pichler and Rachele (1) estimated the size of minute droplets of water on cellulose nitrate by measuring the diameter of the circle of contact. They found that the ratio was about 9; Table I shows then
Vol. 13, No. 6
that the contact angle between air, water, and cellulose nitrate was about 55". The method here described was used for drops of water on built-up multilayers of soaps ( d ) , on lacquered tin plate (S), and on glass plates. U'hen compared with the usual method of a direct measurement of 8 this method is indicated when determination of volume and length is simpler than that of angle. It has also another advantage. When the profde of a drop is observed, the angle measured is that for one point only; the same drop viewed from another side may have a different shape and form a different contact angle. The area of contact between drop and solid is a measure of an average of all the contact angles present along the circumference of the drop, and the shape of the mark shows how constant (or otherwise) is the contact angle along this circumference.
Literature Cited (1) Benedetti-Pichler, A. A., and Rachele, J. R., IND.ENQ.CHEM., Anal. Ed., 12, 233 (1940). (2) Bikerman, J. J., Tram. Paraday SOC.,36, 412 (1940). (3) Sumner, C . G., The Metal Box Go., Ltd.. London, W. 3., private communications. (4) Wark, I. W., J . Phys. Chem., 37, 623 (1933).
A Wet-Combustion Micromethod for Determination of Carbon and Hydrogen Iodic Acid as an Oxidant for Wet Combustion BERT E. CHRISTENSEN AND ROBERT WONG Oregon State College, Corvallis, Ore.
C
HRISTEKSEN and Facer (1) have recently published a wetcombustion method using iodate as the oxidant for the determination of carbon and hydrogen. Previous experience has convinced the writers that the use of iodate in this connection is best adapted to micro work. Since a method which would handle small samples (2 to 5 mg.) is highly desirable in many cases, the possibility of devising such a procedure was studied. In connection with these studies the method of Christensen and Facer was modified. Changes in the procedures for determining the evolved carbon dioxide and the method of treating the unreduced iodate, and a number of other modifications resulted in a considerable saving of time. Although these wet-combustion methods are simple, permit intermittent operation, and employ the minimum of apparatus, their success depends upon the use of iodic acid as an oxidant for the organic material. This reagent has given excellent results for many organic compounds, yet some compounds under the conditions specified for analysis cannot be quantitatively oxidized by iodic acid. It was the opinion of the authors that much more information must be obtained regarding the behavior of this reagent toward all kinds of organic compounds (volatile, insoluble, etc.) before a wet-combustion method involving iodic acid could be established. It was possible that further study of the limitations of iodic acid as an oxidant in wet combustions might afford a criterion for predicting the behavior of a given compound when treated with iodic acid. For these reasons the following investigation was undertaken.
Reagents Sulfuric acid, 96 per cent. It is important that the sulfuric acid be made as free as possible from organic matter. This was accomplished by addin 50 ml. of Superoxol dropwise t o 200 ml. of hot concentrated sulfwic acid. This mixture was just brought to boiling and the heating continued until the sulfuric acid attained approximately its original volume. Barium hydroxide, approximately 0.1 N , was prepared and protected by a soda-lime tube. Hydrochloric acid, approximately 0.05 N , was prepared and standardized with potassium iodate. Thymol blue indicator, 0.2 gram, was dissolved in 43 ml. of 0.01 N sodium hydroxide and diluted t o 500 ml. with distilled water. Potassium iodidebarium chloride solution, approximately 0.5 N with respect to each reagent. Potassium iodate, c. P. grade. Potassium permanganate, approximately 0.1 N . Acetone, acid-free.
Apparatus The ap aratus is similar in design to one described in a previous article (8r Two changes were made in the reaction vessel: The bottom was enlarged to form a bulb and the cup on the side was constructed from a male standard taper joint. The bulb made it easier t o dissolve many substances, while the joint afforded a better means of connectin the carbon dioxide-free air system to the apparatus and provifed better protection a ainst organic contamination. Since no carbon monoxide was formed during iodate oxidation, the slow combustion unit was deleted from the train. The modified apparatus is illustrated diagrammatically in Figure 1. For the protection and accurate measurement of the standard barium hydroxide solution the automatic pipet described by West ( 4 ) was employed.
