Factors Influencing the Quantitative Deterrnination of Sulfate as Barium Sulfate HAROLD A. FALES
AND
WILL S. THOMPSON,' Columbia University, New York, N. Y.
A
LMOST the entire history of barium sulfate precipitates is marked by references to annoying, unexplained, and often uninvestigated interference by common substances, the presence of which might be expected from theoretical considerations to produce little or no interference. Of these one of the commonest and most troublesome has been the nitrate ion, usually present with the alkali elements (BO, BY). The phenomenon has been variously blamed on adsorption, coprecipitation, occlusion (,%I), and the formation of various types of unisolated, insoluble complex-compounds (4). Quantitative results based on the weight of precipitate range from slightly low to several per cent greater than the amount required by theory. These variations depend upon conditions observed during the formation and treatment of the precipitated material. It seemed desirable, as a basis for further study as well as for the purpose of limiting analytical conditions, to study in more detail ,the relationship between these errors and the conditions that influence them. Equal amounts of sulfates were precipitated with an excess of barium chloride solution in the presence of various amounts of salts and acids. Potassium salts were used for the most part, and the results compared under certain conditions with other salts. A standard procedure for the treatment of the precipitate was adopted, and the effect of varying nearly all possible variable factors in this procedure was tested a t certain regular intervals. The salts and acids used were of analyzed reagent grade, which were tested for freedom from interfering substances and found to conform in general to A. C. S. standards for reagent grade chemicals.
variation on nearly all conditions was studied a t certain points, most conditions have remained fixed throughout the greater part of this investigation. To simplify comparison of the numerical results given, and to reduce the number of conditions that must be listed in connection with each set of figures, the following values apply, except where different values are specifically stated: The precipitates were not ignited, but were dried to constant weight at 110" to 120" C. The concentration of sulfate at the start of precipitation was 0.01 molar and the volume was 350 cc. The concentration given for various salts and acids present is for the initial volume. Barium chloride solution 0.05 molar at 25" to 30" C. was added to 5 per cent in excess. Cold precipitation was at 25" t o 30" C for hot precipitation the initial temperature of the sulfate solution was 95' to 100' C. The final volume with 5 per cent excess barium chloride was 425 t o 430 cc.; with 50 per cent excess, 460 t o 465 cc.; and with 100 per cent excess, 495 t o 500 cc. Stirring was always by a motor-driven stirrer at uniform rate of speed. Hot digestion was at the temperature maintained by a steamheated plate, at 80" to 85" C. Wash water was 200 to 300 cc. of distilled water at 25" t o 30" C. The sulfate used for recipitation was in each case theoretical1 equivalent to 0.8166 &0.0002) gram of barium sulfate. Eacg result quoted in the following tables should theoretically have been 0.8166 gram. The difference in each case represents the error.
Experimental Investigation of Factors CONCENTRATION OF SULFATE IONIN SOLUTION.The figures given in Table I show the weight of barium sulfate obtained from equal weights of potassium sulfate a t different concentrations, under the conditions indicated. Variations in the
General Technique Where precipitation was conducted above room temperature. the sulfate solution was maintained at that temperature on an electric hot plate. Solutions were stirred mechanically while barium chloride solution 0 825 was added from a buret a t a rate determined GRAM by the attachment of a calibrated delivery tip. After the measured volume of precipitant had been added, the stirrer and cover glass were cleaned, and the solution was allowed to stand, 0815 either on a steam plate or at room temperature, for a definite length of time. The solution was d then decanted through a porcelain filtering m crucible, and the filtrate tested for excess of n barium ion. The precipitate, after having Ee 0805 been washed by decantation, was transferred g 0825 to the crucible, washed further, and finally E dried to constant weight. 8 + This investigation has considered the effect y of some twenty different factors that are comW monly encountered in the course of an ordinary OB15 sulfate determination.
100
DIGESTED 12 H O M S 80'- 85' C
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Present address, Adelphi College, Garden City, N. Y .
I
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99
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100
1
DIGESTED 24 HOURS 25'C
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080
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IO1
General Conditions The behavior of potassium sulfate in the presence of potassium nitrate under different conditions has been studied extensively, and the results have been compared with other sulfates and nitrates. Although the effect of
1 101
OI(PHIJ
001(~H21 MOLAR
I
0 0001( ~ " 4 1
0001(pH3) CONCENTRATION
OF
000001l~H51
HCI
FIGURE 1. EFFECTOF HOTAND COLDDIGESTION Unignited precipitate obtained by addin 0.05 molar BaCL t o 5 per cent in excess in 5 minutea t o hot (95' t o looo C.) 0.01 molar &Or solution of varying aoncentration with respect t o " 2 1 and KNO&
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APRIL 15, 1939
ANALYTICAL EDITION
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207
of potassium nitrate is less between 0.01 and 0,001 molar hydrochloric acid, but all results then carry a negative error of 2 to 3 parts per 1000.
KINDOF BARIUMSALTUSED AS PRECIPIBarium chloride was used throughout and always a t room temperature. No other salt or acid was added to the barium chloride solution except in a few cases of reverse precipitation (Table IV). The results in these cases are abnormally high.
TANT. c w
B
CONCENTRATION OF 100
.001
002
004 006 MOLAR
01 02 04 CONCENTRATION OF KNO)
06
10
OF REVERSE AND REGULAR PRECIPITATION METHODS FIGURE 2. COMPARISON
BARIUM SOLUTION.
Barium chloride 0.05 molar was used in this investigation, and was compared under the conditions shown in Table I1 with 0.1 and 0.5 molar solutions. The latter solution gave slightly higher results in the presence of nitrates.
TEMPERATURE OF SOLUTION DURING PRE-
Temperatures were taken a t the start of, rather than during, precipitation. The beaker was heated over a gas burner, then transferred to a small electric hot plate during precipitation. This plate furnished sufficient heat to keep the solution a t the boiling temperature with slow ebullition. CIPITATION.
Unignited precipitate obtained by adding 0.05 molar BaClz to 5 per cent in excess t o hot (95" t o 100' C.) 0.01 molar KzSOa st different rates, where sulfate solution is of varying conoentration with respeot to KNOs (solid lines), compared with results (broken lines) obtained by adding same weight of KzSOr t o hot BaCll solutio?. No aoid added except in the Popoff and Neuman determination.
concentration of potassium sulfate between 0.005 and 0.02 molar produced differences of less than *0.5 mg. in 0.815 gram of the precipitate if no potassium nitrate was present. CONCENTRATION OF POTASSIUM NITRATEPRESENT.The effect of nitrates upon the weight of barium sulfate precipitates has been shown to be a function of the concentration of the a t least up to a certain point (1). nitrate (4,
TABLE 11. CONCENTRATION OF BARIUM CHLORIDE SOLUTION (Bas04 preoipitated hot in 5 minutes by BaClz to 5 per cent exoess. Digested 12 hours hot) Barium Sulfate Precipitated KNOs HCI 0.05 molar BaCh 0.1 molar BaClz 0.5 molar BaClp Molarity Cram Gram Cram None 0.01 0.8142 0.8148 0.8146 0.01 0.01 0.8153 0.8161 0.8194 0.1 0.01 0.8211 0.8211 0.8239
TABLE I. EFF~CT OF VARYINGCONCENTRATION OF SULFATE [Same weight of KzS04. 0.01 molar KNOs, 0.01 molar HCI, Bas04 preoipitated hot (95O C.) in 5 minutes by BaClz t o 5 per cent exoess. Digested 12 hours hot] 0.01 Molar 0.02 Molar 0.005 Molar &so4 &so4 KzSor Gram Gram Gram 0.8141 0.8153 0.8161
The effect of potassium nitrate has been studied where the concentrations a t the start of the precipitation were 0.005, 0.01, 0.05, 0.1, 0.5, and 1.0 molar. The effect of these concentrations upon the weight of the precipitate of barium sulfate is shown in Figures 1 to 4. (For the sake of brevity, only graphic data are given in some instances. For the numerical data the reader is referred to the original thesis.) CONCENTRATION OF HYDROGEN ION IN SOLUTION. Sufficient hydrochloric acid was used to render the solution at the start of precipitation 0.1, 0.01, 0.001, 0.0001, or 0,00001 molar with reRpect to hydrochloric acid. These concentrations are equivalent to pH values of approximately 1, 2, 3, 4, and 5, respectively, for the initial volume of 350 cc. of water in which the potassium sulfate and potassium nitrate are dissolved and to which the barium chloride solution is added during the precipitation. The curves (Figure 1) for hot and cold digestion differ in some cases, but are nearly parallel to the base line between 0.01 and 0.001 molar hydrochloric acid for concentrations of 0 to 0.01 molar potassium nitrate. I n the case of hot digestion, all values lie within 3 parts per 1000 of the theoretical value required. The solution, then, should not be more than 0.01 molar with respect to nitrate and should be made from 0.01 to 0.001 molar with respect to hydrochloric acid (pH 2 to 3), preferably nearer 0.01 molar. The sulfate is precipitated from hot solution in about 5 minutes and digested hot for 12 hours. If the solution is digested cold for 24 hours, the effect of larger concentrations
Since the barium chloride was always used at room temperature, the heater did not entirely counteract the cooling effect of the added barium chloride in the more rapid additions (10 seconds, 1 minute, 2 minutes). Here the drop in temperature was less than 15" C. for boiling hot solutions. For additions in 5 minutes' time, the cooling effect was less than 5" C., and for slower additions (12 and 30 minutes) it was practically canceled. I n precipitations made at room temperature, this effect is absent. For precipitations a t intermediate temperatures (&io, 60°,and 75" C.), heat from the hot plate was supplied during precipitation, and the temperature was kept within * 5 O of the value given. Where 50 or 100 per cent excess barium chloride was added at the end of precipitation time stated, the solution was cooled further as a result. TABLE111. EFFECTOF TEMPERATURE OF SULFATE SOLUTION DURING PRECIPITATION (BaSOr precipitated by 0.05 molar BaCh to 5 Der cent 25' to 550 to KNOs HC1 3OOC. 40'C. 65OC. Molarity Cram Uram Gram Digested 12 Hours Hot 0.01 0.01 0.8277 0.8197 Digested 24 Hours Cold 0.1 0.01 . , . 0.8456 .. 0.01 None 0.8402 0,8329 0.8261 0.1 None 0.8710 0.8504 0.8360
....
.
. .
excess, in 5 minutes) 950 to 75'C. 100' C. Gram Gram
. . ..
0.8153
0.8187 0.8223 0.8300
0.8118 0.8201 0.8203
The effect of temperature may be seen in Figures 3 and 4. Cold precipitation (25" C.) increases errors of contamination somewhat in all cases (2, 17, 16),but much more where the addition of barium chloride is rapid (12),where the excess of barium chloride added is large, or where the concentration of nitrate is high. It is necessary to precipitate barium sulfate near the boiling point if these errors are to be reduced to a minimum.
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TABLE IV. REGULAR AND REVERSE PRECIPITATION Conditions
Precipitation Min.
BaCln added t o KnSO4, KNOs in &SO4
Po off and Neuman reverse method: h e d a t 115’ C. Ignited 1 hour at 600-700’ C. KzSO, added to BaCh, KNOI in BaCL
5
5 5 10 seconds hot 5 minutes hot
0.01 Molar I
