Determination of Sodium in the Presence of
Molybdenum C. H. HALE, Esso Laboratories, Standard Oil Co. of Louisiana, Baton Rouge, La.
A
per cent tartaric or citric acid solution for each 0.1 gram of molybdic oxide present. Filter into the concentrated solution a tenfold excess of zinc uranyl acetate reagent, stirring the solution vigorously during the addition of the reagent to prevent crystallization. Allow the sample to stand, with occasional stirring, for one hour a t 20" C., and filter through a tared filtering crucible. Wash 5 to 10 times with 95 per cent ethyl alcohol saturated with sodium zinc uranyl acetate, and finally twice with ether or acetone. Dry in a desiccator for 10 minutes and weigh. Correct for a blank determination carried out in the same manner. Weight of NapO = weight of precipitate X 0.0202.
LTHOUGH several publications ( 2 , 3, 5, 7) have dealt with the interferences encountered in the determination of sodium by the zinc uranyl acetate method of Barber and Kolthoff ( 2 , 51, no reference is found to the effects of molybdenum. Molybdate interferes by the formation of a precipitate with the zinc uranyl acetate reagent. This precipitate, nhich is a yellow, amorphous material, is apparently the uranyl molybdate, U02MoOl, described by Lancien (6). The interference of the molybdate may be either positive or negative. If the precipitate is retained on the filter and weighed with the sodium, the results will be high. If the precipitate, which is usually very fine, passes completely through the filter, the correct value may be obtained unless the depletion of reagent by its reaction with the molybdate causes incomplete precipitation of the sodium, in which case the values will be low. These interferences are illustrated in Table I. The removal of the molybdenum by means of hydrogen sulfide is tedious and time-consuming, especially since repeated precipitations are often necessary (8). The use of complex ions to effect the removal of interferences is common in analytical chemistry. Of the complexes that may be formed TT ith molybdenum, phosphate precipitates the reagent; fluoride retards the formation of the sodium zinc uranyl acetate; and the oxalate complex is not sufficiently stable. The tartrate and citrate complexes, however, are both stable (1, 4) and do not interfere. Although, as Barber and Kolthoff stated (a, 6 ) , a n excess of such organic acids precipitates the reagent, the concentration necessary to effect the removal of the molybdate is not large enough to cause interference.
TABLE
DATA. Table I1 summarizes the results of the analyses of a number of solutions prepared by adding measured vol-
umes of a standard sodium sulfate solution to varying amounts of molybdenum. TABLE 11.
DETERZfIS.iTI0S
NarO Added
ht003
NazO Found
Mg.
Mg. 500 100 200 100 100 100 100 500 100
Mg. 0.4 2.1 2.1 2.5 4.0 8.2 10.1 9.9 50.0
0.4 2.0 2.0 2.5 4.0 8.0 10.0 10.0 50.0
SODIUM MOO3
Gram
Gram
NalO Found Gram
0 0020 0.0020 0 0000 0 0030 0 0020 0 0020
0.0 0.01 0.1 0.1 0.1 0.1
0.0020 0.0023 0.0046 0.0021 0.0002 0.0043
Error Mg. 0.0 1 1 0.0 0.0 $0.2 +0.1 -0.1 0.0
+o. +o.
To prove the applicability of the modification to the determination of sodium in aluminous materials containing molybdenum, known amounts of the sodium sulfate solution and molybdenum oxide were added to excesses of aluminum hydroxide. The results of the analyses of these samples are shown in Table 111.
Discussion
O F MOLYBDEXUhI ON THE DETERMISATION OF I. EFFECTS
S a l 0 Added
O F SODIUM IN THE PRESENCE O F
MOLYBDENUM
The results that were obtained showed the same degree of accuracy and precision as analyses of similar materials that contained no molybdenum. The size of sample used for a n analysis should be such that the amount of molybdenum does not exceed 0.5 gram of molybdic oxide because of the difficulty of concentrating the solution to 10 ml. or less without the precipitation of molybdic acid. I n the presence of potassium salts, citric acid is preferable to tartaric because of the limited solubility of potassium tartrate.
Experimental
Summary
The method used is a modification of that introduced by Barber and Kolthoff ( 2 ) . The solution, slightly acid with sulfuric acid, is oxidized with a drop of nitric acid and the molybdate is converted to the complex by the addition of citric or tartaric acid. The sodium is then precipitated by the addition of an excess of zinc uranyl acetate reagent. REAGENTS. Tartaric or citric acid solution. Dissolve 50 grams of the acid in 50 ml. of water. Zinc uranyl acetate solution. Dissolve separately ( A ) 308 grams of uranyl acetate dihydrate i n 1640 ml. of water and 54 ml. of glacial acetic acid, and ( B ) 924 grams of zinc acetate trihydrate in 1048 ml. of water and 27 m]. of glacial acetic acid. Mix solutions A and B a t 70" C., and allow to stand for 2-1 hours at 20" C. Filter immediately before use. PROCEDURE. The sample, in solution as sulfates with a slight excess of sulfuric acid, is concentrated t o as small a volume as possible without crystallization (10 ml. or less). Add one drop of concentrated nitric acid to oxidize any molybdenum that ha6 been reduced during the concentration, and 0.2 ml. of 50
Molybdenum interferes with the determination of sodium by the zinc uranyl acetate method by the formation of a precipitate with the reagent. This interference can be OF SODIUM IN THE PRESEWE OF TABLE 111. DETERMISATIOS MOLYBDESUM ASD ALUXISUM
NazO Added MQ.
516
.\Io03 Mg.
AlzOa Mg
.
S a l 0 Found Mg.
Error
MQ.
August 15, 1943
ANALYTICAL EDITION
overcome by the formation of a complex of molybdenum with citric or tartaric acid.
Literature Cited (1) Abegg, R., and Auerbach. F., “Handbuch der anorganischen Chemie”, Vol. IV, Part 1, 11, p. 1018, Leipaig, S. Hirzel. 1921 (2) Barber, H. H., and Kolthoff, I. M., J. Am. Chem. Soc., 50, 1625 (1928).
517
Ibid., 51, 3233 (1929). Feigl, F., Z . anal. Chem., 74, 389 (1928). Kolthoff, I . M..Ibid., 70, 397 (1927). Lancien, A., Compt. rend., 144, 1434 (1907). Petukhovrt, E. V., Zavodskaya Lab., 9, No. 1, 108 (1940). ( 8 ) Yagoda, H., and Fales, H. A., J . Am. Chem. SOC.,58, 1494 (1936).
(3) (4) (5) (6) (7)
P R E ~ E N T Bbefore D t h e Division of Analytical and Micro Chemistry a t t h e CHEMICAL SOCIETY, Detroit, Mich. 105th Meeting of t h e AMERICAN
Analvsis of Commercial Oil Emulsions and Wax Dispersions FRANK M. BIFFEN AND FOSTER DEE SNELL Foster D. Snell, Inc., 305 Washington St., Brooklyn, N. Y.
Oil emulsions, which may or may not contain suspended solids, are analyzed by first distilling off any water-immiscible solvent. Benzene is then added and distilled with continuous return, carrying with i t the water which does not return to the sample. This avoids formation of troublesome emulsions during the conventional extractions of the oil phase. This procedure is sufficiently longer than the conventional extraction to be inadvisable for simple oil emulsions if they are readily extractable with ether. The aqueous distillate contains any alcohol. By centrifuging if necessary and evapora-
ETHODS of analysis of emulsions and suspensions as previously published (2) have been revised with further experience to simplify the procedures and provide greater accuracy. One major change has been in methods applied to oil emulsions, which may or may not contain suspended solids, to give more complete separation, avoid troublesome extractions in a separatory funnel, and prevent oxidation of the oils present. The importance of water-base wax suspensions has increased steadily and the types of soap stabilizers used have been complicated by commercial introduction of numerous amines. For brevity many conventional methods of analysis are referred t o only briefly without details.
Oil Emulsions The products in this class may vary from a medicinal oil emulsion to automobile cleaner-polishes. Many of the products contain fractions, such as petroleum naphtha, \+hich are volatile mith steam. Glycerol is often present, alcohol occasionally. Some products contain from a trace to 2 per cent of waxes. The emulsifying agents encountered individually or as mixtures include 1 to 5 per cent of bentonite, varying with consistency and type of clay, usually less than 1 per cent of various gums, and surface-active agents, usually sulfated oils or soaps. The latter are apt to be amine salts of fatty acids, but occasionally the other members of the class are present. Persistence of soap and gum emulsions is a problem in conventional extractions which to a greater or lesser degree extends to other surface-active agents. Of all the emulsifying agents mentioned, bentonite is the worst offender. Ethern-ater emulsions stabilized with it will often not break even
tion of benzene, the oil is recovered without undue oxidation. The solid residue is then separated by conventional methods into alcohol-soluble, chlorohydrocarbon-soluble, w-ater-soluble, and mineral fractions for separate analysis. Amine emulsifiers are determined on another portion of the original sample but all other determinations are carried out on the single sample. Improved methods for analysis of water-base wax dispersions provide for precipitation by acid, followed by separation and approximate identification of resins, wax, amines, etc.
on prolonged centrifuging or on addition of minor amounts of alcohol ( I ) . The method provides for first distilling any solvent volatile with steam, then adding benzene and distilling water with it. The oil, emulsifying agent, and abrasive, if any, remain in the flask in excess benzene and receive more conventional treatment. Only a single sample is required unless a nitrogen determination is required for estimation of amines.
Outline of Separation of Commercial Oil Emulsions Original sample SteLrn distillation - Immiscible solvent, water, alcohol I
Reflux nith henzene - Water, sometimes alcohol I
Centrifuge cold
-
Oils and bulfated oils in benzene solution
i
Extract with hot alcohol - Glycerol. soap, fatty acid in alcohol I
Extkact nith hot,C?H,Cl, - Waxes in C2H3C11 Exiract with hot water - TI-ater-soluble gums RIikeral matter Generally speaking, it is not necessary to use this method if the emulsion is of oil in water without added mineral mat-
