c
Determination of Sulfur Accuracy and Precision of Several Methods WILLIAM RIERIAN I11 AND GEORGE HAGEN, Rutgers University, New Brunswick, N. J.
I
T IS well known that the gravimetric determination of sulfur as barium sulfate is subject to rather large errors, due chiefly to coprecipitation. Many methods for the precipitation, filtration, and drying of the barium sulfate have been recommended. The author of each method generally gives experimental data on the determination of sulfur in a pure compound, such as sodium or potassium sulfate, to indicate the accuracy of the method. Frequently data are also included for the analysis of a mixture of the alkali sulfate with about an equal quantity of Some other salt, such as sodium chloride or nitrate. But data for the determination of known amounts of sulfate in the presence of much larger quantities of other salts are exceedingly scarce in the literature.
Method
K and S
F and T F and T (modified) P and N H and W
chloride. The method of Njegovan and Marjanovic (5) was also tested and discarded because difficulty was experienced in filtering the very fine-grained precipitate.
Reagents Reagent grade sodium sulfa& was purified by recrystallization and dried at 110" C. No further loss was incurred by heating to a dull red heat. The sodium chloride was purified by treating the saturated solution with hydrogen chloride. In some cases, reagent grade sodium chloride was used without purification; then appropriate blank corrections were applied to the results. The other compounds were of reagent grade.
Procedure An appropriate quantity of pure sodium sulfate was taken, either by direct weighing or by weighing a standard solution with a weight buret, and t o it were added known quantities of pure sodium chloride lor nitrate). TABLEI. IMPORTANT FE.4TURES O F VARIOUS METHODS The mixture was then analyzed NarSOr HCI Total Excess Order of Time of Temperature for sulfur according t o the proceTaken Taken Volume BaClz Mixing Mixing Digestion of Drying dure under investigation. Blank Millimoles M1. Millimoles Min. Hours c. corrections were applied in all 2.1 12 350 21.5 .9 Usual 4 1 800 cases. 450 Usual 8 12 115 3.5 3 O
3.5 3.5 7.5
3 18 27
450 350 550
1.5 0.5 1.5
Usual Reverse Usual
8 4 Fast
This paper describes the results of determinations of sulfur (2) in known mixtures of sodium sulfate and sodium chloride by several well-known methods. Sodium chloride was chosen as the foreign salt because it is often present in solution when barium sulfate is precipitated for the determination of sulfur. The fusion of inorganic substances with sodium carbonate and subsequent acidification with hydrochloric acid introduce quantities of sodium chloride much larger than the sulfur that may be in the sample. The sodium peroxide bomb method for the determination of sulfur in organic substances also introduces large quantities of sodium chloride-for example, when 0.5 gram of sodium benzene sulfonate is oxidized by 14 grams of sodium peroxide, the resultant mixture after acidification contains 130 moles of sodium chloride for each mole of sodium sulfate. The following methods were investigated: The method of Kolthoff and Sandell (4) was tested because it is typical of those found in most textbooks of quantitative analysis. The procedure of Fales and Thompson (1) was investigated because of the unusual treatment of the precipitate-drying at 110" to 120" C. instead of the customary ignition. This method was found to be very inconvenient because of the digestion period of 12 hours; therefore, a modification with a digestion period of 21 hours was also investigated. Fales and Thompson found that nitrate ion introduced less error in their method than chloride ion. I n the determination of sulfur following a fusion with either sodium carbonate or sodium peroxide, the nitrate ion can be introduced as easily as the chloride by using nitric acid for the acidification. Therefore the modified method of Fales and Thompson was also tested in the presence of sodium nitrate instead of sodium chloride. The method of Popoff and Neuman (6) mas investigated because it has been found better than the usual methods. Hintz and Weber (5) claimed an unusual accuracy for their method in the presence of a large quantity of sodium
21
130
1
800 800
1
The important features of the various methods are listed in Table I.
Results Table I1 gives the relative errors of the methods tested. The values for no foreign salt and for the maximum quantity of foreign salt are the mean of four determinations. The other values generally represent single determinations. The precision of the methods is indicated by the relative mean deviation of the values with no foreign salt and with the maximum quantity of foreign salt (Table 111).
Discussion Table I1 reveals that the method of Hintz and Weber is by far the most accurate when the entire range of ratios of sodium chloride to sodium sulfate is considered. It is un-
TABLE11. RELATIVE ERRORS OF METHODS Sfethod
hIolar Ratio of NaCl (or NaSOs) 1 2 5 10 20 40 60 Paris per thousand -3 -8 - 8 -9 -11 9 - 1 2 -16 -2 ... . . . . . . -12 0
K and S F and T F and T (modified) FandT(NaNO8) P and N H and W
-
... ...
- 5 -6 -5 + 2 +3 +3 + 2 +2
-6 - 7 +1 + 2 +4 +4
+1
+4
t o NazSO4 110 190 309
- 8 -10 . , -13 -15 . . -15 +I3 +14 +++ 444 +++ 336 ++' 43 +12 ++ 47 +++ 949 +.4 ::
TABLE111. PRECISION OF METHODS Method
K and S F and T F and T (modified) F and T (NaNOs) P and N H and W
150
..
-17 -16 -13 . . -12
Relative Mean Deviation htaximum foreign salt Parts p e t thousand 1.5 1.3 0.1 0.2 0.5 1.4 0.5 0.2 0.3 1.1 0.3 0.6
N o foreign salt
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.
