151
ANALYTICAL EDITION
February 15, 1942
fortunate that this method is used so little and that i t is not mentioned in the textbooks of analytical chemistry, particularly because it is the quickest and easiest of all. The method of Popoff and Neuman stands second in accuracy. Four determinations are not enough to establish the precision of a method, but they give a valuable clue regarding the reproducibility of results. I n this respect, the original method of Fales and Thompson is by far the best, with Hintz and Weber’s procedure second. It is interesting to consider the causes of the errors. When barium chloride is added slowly to sodium sulfate, the crystals of the precipitate grow in an excess of sulfate ions. Thus they acquire a negative charge and attract the sodium ions, which are then buried beneath the surface of the crystals by deposition of barium sulfate. This coprecipitation of sodium accounts for the low results in the method of Kolthoff and Sandell. This negative error is partly compensated by incomplete drying in the method of Fales and Thompson. Nitrate ion is extensively coprecipitated by barium sulfate and introduces a positive error. This accounts for the increasing positive errors with addition of sodium nitrate in the method of Fales and Thompson. The procedure of Popoff and Neuman decreases the co-
precipitation of sodium by the reverse precipitation. This method of precipitation, however, increases the usually small coprecipitation of chloride ion until it becomes the major error. This introduces a positive error. When the precipitant is added all a t once, as recommended by Hintz and Weber, the growth of the crystals occurs chiefly after the addition of the precipitant when there is a comparatively small excess of barium ion. Thus neither the positive error due to the coprecipitation of chloride ion nor the negative error due to the coprecipitation of sodium ion is great, and the method yields good results. It might be expected that the rapid precipitation would produce very fine-grained barium sulfate, but no difficulty was experienced in this filtration. Perhaps the rather high acidity of the method accounts for the filterability by increasing the solubility slightly. Literature Cited (1) Fales and Thompson, IND.EAG. CHEM.,ANAL. ED., 11, 206 (1939). (2) Hagen, George, master’s thesis, Rutgers University, 1941. (3) Hintz and Weber, 2.anal. Chem., 31, 714 (1906). (4) Kolthoff and Sandell, “Textbook of Quantitative Inorganic Analysis”, p. 319, New York, Macmillan Co., 1936. ( 5 ) h’jegovan and Marjanovic, 2. anal. Chem., 73, 271 (1928). (6) Popoff and Neuman, ISD. ENG.C H m f . , ANAL. ED., 2, 46 (1930).
Conductometric Assay of Inorganic Salt in the Presence of Wetting Agents J
J. H. PERCY
AND
C. J. ARROWSMITH, Colgate-Palmolive-Peet Company, Jersey City, N. J.
A method is described for the rapid assay of active ingredient in commercial wetting agents by a conductometric method. It is not necessary for the chemical nature of the material to be known so long as a small amount of material free of inorganic salt can be isolated for the calibration curve. The method may be applied wherever the components of a mixture have materially different specific conductivities in solution.
D
U
AMXOK (6) suggested that electrical conductivity of
aqueous solutions might be used as a physical constant for evaluation of mixtures m-ith the help of a calibration curve, just as specific gravity, refractive index, or viscosity is used. Relatively few applications of this principle have been made, most analyses by conductometric means having been carried out by titration methods (3, 6). References have been made to the use of conductivity for controlling the purity of a liquid such as distilled water (1, 7), evaluating the concentration of sulfuric acid ( 2 ) ,or determining the con_centration of a single electrolyte in aqueous solution (4). Sandera (8) determined the composition of a mixture of organic liquids by determining the conductivity of a saturated solution of a suitable electrolyte in the mixture. However, no use has apparently been made of the method in the rather frequently occurring problem of determining the composition of a mixture of two water-soluble materials which have different specific conductivities in aqueous solution. It is known that a t concentrations below the point where
appreciable errors are introduced by concentration effects, electrical conductivity is an additive property, so that when a mixture of two water-soluble solids is dissolved in mater and diluted to a fixed concentration, the conductivity is a straightline function of composition by weight in the original mixture. It is possible to apply this relationship very generally. For example, it may be employed in connection with a second constant, such as refractive index or analytical composition with respect to a single element, for the evaluation of threecomponent systems. This principle has here been applied to the rapid assay of sulfonated and sulfated surface-active agents for inorganic salt and active ingredient content. I n this case the surfaceactive compounds are organic sulfonates or sulfates of high molecular %eight which have specific conductivity in water solution approximately one seventh to one tenth that of sodium sulfate; in general, commercial products contain mixtures of several or many related compounds as active ingredients. The method gives results which compare favorably with those obtained by solvent means or by precipitation of the usual analytical precipitates in the presence of the surfaceactive material, and can be carried out in very much shorter time.
Experimental The measuring bridge used was Model RC-1 of Industrial Instruments, Inc., Jersey City, N. J., a 60-cycle alternating current bridge using a cathode ray tube as null-point indicator. The cell was of the dip type, with a constant of 1.108. Variations in pH within reasonable limits made little or no difference in the results obtained. The materials examined had normal pH values in dilute solution of 7.0, 6.5, and 5.5, respectively. In all three cases a variation of one full pH unit from the normal in the very dilute solution used gave no detectable error.
Vol. 14, No. 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
152
added from the buret. For each goint the temperature waa read with an accuracy of 0.2 and the reading was corrected to 25' by the formula Rta
=
Rt
+ 0.02 ( t - 25) Rt
Rt and Rsa being the observed and corrected resistances, respectively. When several conductivity cells are used for the work, the values should also be corrected for cell constant before the calibration curve is drawn.
0
25
50
75
Calibration curves for three typical materials are shown. Curves are plotted in Figure 1 for specific resistance us. composition and in Figure 2 for specific conductivity us. composition. Since the latter is an additive property, the curves for conductivity are straight lines. Each point represents a single determination, and the deviations from the straight line in the conductivity curves are a measure of the error of the method. Points are reproducible to 1 per cent salt content or better, and the maximum error is probably less than 2 per cent salt content. If another concentration than 0.040 per cent is used, it should be so chosen that the range of composition most often dealt with gives specific resistances of 2000 to 5000 ohm-cm. TIhese aie low enough t o eliminate errors due to variations in the distilled
too
PERCENT SALT
US. PER CENTSODIUM SULFATE IN FIQURE 1. SPECIFIC RESISTANCE DRYSOLID
(Curves A and D were determined by A. C. Bell.) Sodium sulfate and sodium salt of sulfated coconut oil monoglyceride, 0.20% solution B. Sodium sulfate and sodium salt of commercial sodium lauryl sulfate, 0 04% solution C. Sodium sulfate and sodium salt of mixed alkyl aromatic sulfonic acids. 0.04 solution D. Sodium sulfate and sodium s a g of sulfated coconut oil monoglyceride, 0.03% solution A.
SALT-FREE MATERIALS.In reparing the calibration curve, inorganic salt-free active ingre&ent was used. Three methods of preparation were employed. When the inorganic material present was insoluble in hot alcohol--e. g., sodium sulfite or sodium sulfate-two extraction methods were used. In the f i s t method, the oven-dried material was extracted in a Soxhlet extractor with absolute ethanol and the extract dried in a vacuum oven at 100' C. In the second method, a concentrated a ueous solution of a small amount of the material was extractel t+ce with butanol. The extracts were combined and diluted with several volumes of butanol; then the solvent was fractionally distilled off to remove water until the boiling point was constant (116') and the splution was filtered hot to remove the salt which crystallized during the distillation. Evaporation of the filtrate gave material free of inorganic sal!. The material was not considered salt-free unless a qualitative test for sulfate ion was negative. In cases not reported here, when the inorganic salts were soluble in alcohol, dialysis has been used for isolation of the active ingredient. With all methods care was taken that no fractionation of the .organic material occurred during the separation; more soluble fractions will in general have higher s ecific conductivity. CALIBRATION CURVE. With salt$ee .material obtained by the methods described above, the calibration curve was made by the following method: Exactly 200 ml. of 0.040 per cent solution of salt-free material in distilled water were prepared and the conductivity was read. (In all determinations distilled water having a specific resistance of more than 300,000 ohm-cm. waa used.) Without removing the electrodes, 10.53 ml. of a 0.040 per cent solution of sodium sulfate were added from a buret and the mixture was gently agitated with the electrodes until a constant reading was obtained on the bridge. This gave the point of the calibration curve for 5 per cent (10.53/210.53) salt content. More salt solution wm added and the procedure was continued down to the 50 per cent point. For the other half of the calibration curve tt- pro :' pe was reversed, with salt solution in the beaker and salt-Lor ~ ~ . ~ingredient lve solution
0
0
25
25
50 PERCENT
n
100
75
100
SALT
50 PERCENT SALT
FIGURE 2. SPECIFIC CONDUCTIVITY us. PERCENTSODIUM SULFATE IN DRYSOLID (Curves A and D were determined by A. C. Bell.) Sodium sulfate and sodium salt of sulfated coconut oil monoglyceride, 0.207 solution of commercial sodium lauryl sulfate, B. Sodium sulfate and sodium .%aft 0.04% solution C. Sodium sulfate and sodium salt of mixed alkyl aromatic sulfonic acids, 0 . 0 4 9 solution D . Sodium sulfate and sodium 8a?t of sulfated coconut oil monoglyceride, 0.03% aolution A.
ANALYTICAL EDITION
February 15, 1942
water and high enough to avoid errors due to polarization of electrodes.
Analytical Procedure In operation, a sample of the material was dried in a vacuum oven, and a solution of 0.040 per cent concentration made up in distilled water. The solution was allowed to stand 15 minutes to reach thermal equilibrium, and was then transferred t o a short specific gravity cylinder large enough to hold the dipping electrode. The solution was agitated gently wjth the electrode, which was inserted to a fixed mark, and the bndge was balanced. The temperature of the solution was read and the specific resisb ance corrected to 25" by the formula given above, and for cell constant when necessary. To ensure clean electrodes, it was found better to make another readin on a fresh sample of the solution and accept the values only ifthe two readings agreed within 1 per cent. The salt content was read directly from the calibration curve.
Any nonvolatile water-insoluble material that is present will affect the result by decreasing the apparent amount of salt present. This may be corrected for by determining the amount of water-insoluble material by a suitable method fether extraction for fatty material, filtration of aqueous solution for solids), and correcting for this in making up the solu-
153
tion for conductivity reading, so that the concentration is 0.040 per cent with respect to soluble inorganic salt plus soluble active ingredient. The result is then obtained as per cent salt on a moisture-free and water-insoluble-free basis. It is undoubtedly possible to apply this procedure when more than one inorganic salt is present. I n the particular case of sodium sulfite-sodium sulfate mixtures, when the sodium sulfite is present in variable small percentages, little error is introduced by considering the sulfite present as sodium sulfate, since the specific conductivities of the pure salts are almost equal. I n other cases it would probably be necessary to determine the concentration ratios of the inorganic salts present by some other means and use a calibration curve for the mixture actually worked with.
Literature Cited Archbutt, Analyst, 37,538-43 (1912). Bishop, to General Chemical Co., U. S. Patent 933,015(1909). Callan and Horrobin, J. SOC.Chem. I n d . , 47, 329T (1928). Coster, IND. ENG.CHEM.,25, 980 (1933). Dawson, J. SOC.Dyers Colourists, 35,123 (1919). Grant, I n d . Chemist, 10,350-4,391-5 (1934). Kendall, J . Am. Chem. Soc., 38,2460 (1916). &andera,K.,Chintie & industrie, Special No., 231-4 (1933).
An Improved Sublimation Apparatus 0. A. NELSON Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Washington, D. C.
P
ROBABLY the most common type of sublimation apparatus to be found in chemical laboratories consists of two test tubes, one within the other. The substance to be sublimed is placed in the larger test tube, which is then heated in a bath or with a free flame, while the inner tube, which functions as a condenser for the sublimate, is kept cold with running water. The capacity of such an apparatus is very limited, as only a small quantity of substance will adhere to the surface of the condenser, or will drop off when the inner tube is removed. The apparatus shown in Figure 1, A , has been found to give excellent results with a number of compounds. The condenser may be a straight tube or made with a bell-shaped lower end, as shown. Heating the end of this tube with a soft brushy flame to the softening point of the glass and applying a gentle suction to the other end produce an even convex or cuplike surface. The most important feature of this apparatus is a screen to catch any crystals of the sublimate that may break away from the condenser. This screen is supported by three glass rods fused into the edge of the condenser cup and bent inward a t right angles ( B ) . If a bath is used for heating, the surface of the liquid should be a t about the level of the screen or a trifle above, to prevent sublimation on the screen and clogging of the meshes. The screen must be resistant to action by the sublimate, and of such mesh as to retain all crystals that may fall on it. A 40-mesh German-silver screen was used in this laboratory. For sublimation under vacuum, a tube may be sealed into the side of the outer jacket, as shown in Figure 1, or run through the rubber stopper supporting the condenser. Cooling water may be run in and out of the condenser through a two-hole stopper a t the top, instead of through the one-hole stopper and side tube as shown in the diagram.
B A FIGURE1. SUBLIMATION APPARATUS
