Coprecipitation of Chromate with Barium Sulfate WILLIAM B. RIELDRUM, WILLIAM E. CADBURY, JR., AND CL.IRK E. BRICKER’ Haverford College, Haverford, Penna.
The coprecipitation of chromate with barium sulfate has been studied quantitatively under carefully controlled conditions. Acidity, rate of precipitation, ratio of chromate to sulfate, and time of digestion were varied in turn. The results obtained support the view that for the mas* part the chromate coprecipitated is probably in the form of barium chromate in mixed crystals with barium sulfate.
T
HE error caused by coprecipitation of dissolved sub-
stances with barium sulfate in gravimetric analysis has been widely recognized. Studies of coprecipitation of various anions have been reported recently (6, 6 ) , but chromate has been omitted from detailed consideration. Kolthoff (9) and hlanov and Kirk (3) have considered briefly the coprecipitation of chromate with barium sulfate during volumetric analysis for sulfate by the method originally devised by Andrew (1). Their methods were, however, not precise, since conditions were not rigidly controlled and the amount of chromate coprecipitated was estimated by analyzing the filtrate rather than the precipitate. The authors’ preliminary experiments showed that inconsistent results are obtained unless conditions are carefully controlled. I n neutral solution barium chromate is precipitated nearly quantitatively whether or not sulfate is present. Barium sulfate ordinarily is precipitated from acid solutions, however, in the presence of a slight excess of barium ion. Willard and Schneidewind (7) in presenting their widely accepted method of analysis for sulfate in chromium plating baths, state: “Barium chromate is so insoluble in dilute acids that it almost invariably contaminates the barium sulfate, and if
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FIG~R 2.E V.\RI.\TIOS O F COPRECIPITATIOS K I T H LOGARITHM OF TIMEIX SECOSDS OF ADDINGPRECIPITAST 0. Without stirring
0. With stirring
not isomorphous with barium sulfate, the extent of mixed -
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crystal formation is largely dependent on the acid concentration. Other factors which might be expected to affect the amount coprecipitated, whether from mixed crystal formation or from other causes, are rate of addition of the precipitant, concentration of chromate, and length of time of digestion of the precipitate. This paper reports the results of a study of the extent to which chromate is coprecipitated with barium sulfate, one condition at a time being altered while the others were held constant.
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Present address, Princeton Unirersity, Princeton, N. J
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sufficient acid is used to prevent this, incomplete precipitntion occurs.” This statement is an oversimplification, since barium chromate will not precipitate by itself, even in the rather dilute acid solutions from which it is customary to precipitate barium sulfate, unless the chromate concentration is high. Coprecipitation of chromate with barium sulfate from acid solution is presumably due for the most part to the formation of mixed crystals, since barium sulfate and barium chromate are isomorphous. Other causes may account for a small proportion of the coprecipitation of chromate, perhaps as sodium chromate or dichromate. The coprecipitation of barium chromate with barium sulfate in acid solution is unusual in that the concentration of chromate ion is very low, although there is a “reservoir” of potential chromate ions in the solution, since in the presence of acid, equilibrium exists among Cr04--, HCr04-, and Cr?O;-ions in aqueous solution:
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Procedure
The chromate in a given barium sulfate precipitate was determined by the method described by Meldrum, Cadbury, and
ANALYTICAL EDITION
September 15, 1943
561
Since the amount of acid necessary to convert chromate into dichromate (assuming completeness of reaction) increased as the concentration of sodium chromate was increased, “constant acid concentration” was defined in two different ways: constant total acid, and constant excess acid. E o determinations were made a t constant pH. The first set of determinations was carried on with a constant amount of acid regardless of the amount of chromate present: 25 ml. of 2.068 N acid were more than sufficient, according to the stoichiometric equation, to convert all the chromate into dichromate. In the second set of experiments, 25 ml. of 2.068 N acid were added in addition to the amount calculated for the conversion of chromate into dichromate. Other conditions were the same as when the acid concentration was varied. VARIATION OF TIMEOF DIGESTION.Equal volumes (25 ml.) of equimolar solutions of sodium sulfate and sodium chromate were mixed, 15 ml. of 2.068 1V acid and 60 ml. of water were added, and the precipitant was added slowly to the hot solution 0 1I I I I I I I I I as before. The period of digestion was considered to begin as 1:l 2:l 3:l 4:l 5:l 6:l 7:l 8:l 9:l 1 O : l soon ab the last drop of precipitant was added. At the end of Ratio of Sodium Chromate to Sodium Sulfate the time of digestion, the mixture was cooled in ice water as FIGURE3. VARIATION OF COPRECIPITATION WITH CONCENTRA- rapidly as possible to 20” C., and the precipitate was filtered off T I O S O F SODIUM CHROM4TE, SODIUM SULFATE HELDCONSTANT and analyzed. 0. Amount of acid constant ,I
C.
Excess acid constant
The reagents and solutions were prepared as described in the paper previously referred to (4). Sodium sulfate solution containing about 0.3 gram of sodium sulfate in 25 ml. was introduced from a 25-m1. pipet into a 200-ml. Erlenmeyer flask, and the desired amounts of dilute hydrochloric acid, water, and sodium chromate solution were added. The flask was fitted with a reflux condenser, in the top of which was placed a cork carrying a glass tube of 1-mm. bore. This “diffusion tube” was intended to prevent loss of vapor and consequent change of concentration. Its effectiveness wa6 demon3trated by refluxing an acid solution for 3 hours, cooling, and titrating a measured sample with standard base. Comparison with similar samples which had not been refluxed showed no detectable change in acid concentration during boiling. The flask with its contents was placed on a hot plate and heated to boiling, the diffusion tube was removed, and 25 ml. of barium chloride solution of such concentration as to supply a .light excess of barium ion were added from a dropping funnel through the condenser. The diffusion tube was replaced and the precipitate was digested by boiling gently on the hot plate. The mixture was then cooled and filtered, the paper burned o f f ,and the precipitate weighed and treated as described above to determine the amount of coprecipitated chromate. VARIATION OF ACID CONCENTRATION. Twenty-five-milliliter portions of equimolar solutions of sodium sulfate and sodium chromate were mixed, and the desired amount of hydrochloric acid was added from a buret, together with enough water to make the volume of acid plus water 75 ml. The total volume of the solution, after addition of 25 ml. of precipitant, was thus 150 ml. The precipitant was added dropwise through a fine capillary tube, so that its addition took from 15 to 19 minutes. With this slow rate of addition boiling, with its resultant stirring effect, was continuous. The mixture was digested for 3 hours, after which the hot plate was turned off. The flask was allowed to cool for 45 minutes, then removed from the hot plate, and cooled rapidly in ice water to 20” C. The precipitate was filtered off and treated as described above. -411 factors lvere thus kept con tant except the acid concentration, which could be varied at v ill. Tests showed that even in the most concentrated acid solutions used no oxidation of hydrochloric acid took place. VARIATIOS O F O F i l D D I T I O S O F PRECIPITAKT. Equal volumes (25 ml.) of equimolar solutions of sodium sulfate and sodium chromate were again used, together with 15 ml. of 2.068 A\r acid and 60 ml. of water. When the solution was boiling steadily, 25 ml. of precipitant mere added through the condenser as before, the time of addition being noted. TT-hen addition was slow, as in the experiments in which the acid concentration v a s varied, gentle boiling produced adequate stirring. When the cool precipitant was added rapidly, however, boiling stopped for a while. Two series of runs were made: one Tvithout auxiliary stirring, and one using a long stirring rod operated through the condenser, turned by a low-speed motor. The other factors, such as time of digestion, tvere kept constant as before. \-ARIATION OF CHROMATE COSCENTRATIOX. The quantity oi sodium sulfate was held constant as before, but the ratio of sodium chromate to sodium sulfate was varied over the range from 1 to 10 to 10 to 1 while other conditions were held const.ant.
Results and Discussion The results are plotted in Figures 1 to 4,in each of which the ordinates represent the number of milligrams of chromate, calculated as barium chromate, coprecipitated with about 0.5 gram of barium sulfate (0.502 gram in Figure 1, 0.546 gram in Figures 2, 3, and 4). Each point on Figures 1 and 4 represents the average of tn-o or more determinations. When the acid concentration is varied (Figure 1) the amount of chromate coprecipitated decreases regularly with increasing acid concentration. Coprecipitation drops off nearly to zero a t high acid concentrations, but from the analytical point of view this is no advantage, since barium sulfate becomes appreciably soluble when the acid concentration exceeds 0.5 -I-. In very dilute acid, the results are inconsistent. When rate of addition of precipitant is varied (Figure 2), the results are less regular, but the general trend is definite: the more rapidly the precipitant is added, the greater is the amount of chromate coprecipitated. Stirring reduces the amount only a little. V%en the chromate-sulfate ratio is varied (Figure 3), the results are what would be expected: the greater the ratio, the more chromate is coprecipitated; and the more acid there is present (lower curve) , the less chromate is coprecipitated. From Figure 4, showing variation in time of digestion, it is clear that in order that coprecipitation may not be too great, the mixture must be digested for an hour or more. Times longer than 3 hours have little additional effect. The most
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FIGURE 4. VARIATIOP; OF COPRECIPIT-\TIOX WITH TIME OF DIGESTION
INDUSTRIAL AND ENGINEERING CHEMISTRY
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likely explanation for this is that most, if not all, of the coprecipitation remaining after 3 hours is due to mixed crystal formation. However, without additional evidence, such as x-ray examination of the crystals themselves, this interpretation cannot be considered as proved. The excess above the minimum is probably due to one or more of the other causes of coprecipitation. The data from which Figure 4 is plotted are shown in columns 1 and 3 of Table I. (Similar tables for the other graphs are unnecessary.)
Since the errors are small when the precipitate is digested for 3 hours or more, it is apparent that the assumptions made are valid. K h e n the precipitate is not digested long enough, the errors are larger and negative in sign. This would be accounted for if the chromate, calculated as barium chromate, were actually coprecipitated in part as sodium chromate, sodium dichromate, or chromic acid. Similar analysis of the data from which the other graphs were plotted indicated that when the precipitate is formed under the best analytical conditions, with slow addition of the precipitant and sufficiently long digestion, chromate is coprecipitated as barium chromate. Appreciable negative errors a t high acid concentrations are in accord with the well-known fact that barium sulfate is somewhat soluble in strongly acid solutions. Since the investigations of Nichols and Smith (6) and of Schneider and Rieman (6) were made under Conditions different from the authors’, no direct comparison is possible beta-een the amount of chromate coprecipitated and the amounts of other anions as found by these workers. Their investigations were made in neutral solutions, for example, whereas this investigation mas of necessity in acid solution, since chromate would precipitate virtually completely in the presence of excess barium ion in neutral solution. Other factors making a direct comparison impossible include differences in concentration and in rate and temperature of precipitation.
Assuming that all the chromate in the precipitate is in the form of barium chromate, the weight of barium sulfate is found by subtracting the weight of barium chromate from the weight of the precipitate. I n column 3 is given the amount of chromate found from the titration, converted to the equivalent amount of barium chromate. This is subtracted from the total weight of the precipitate, column 2, to obtain the weight of barium sulfate, given in column 4. This in turn is multiplied by the gravimetric factor, 0.6086, to convert to sodium sulfate, given in column 5. The difference between this and 0.3327, the number of grams of sodium sulfate taken each time, is called “error”, listed in column 6.
(1) Andrews, L. W., .4m. Chem. J., 11, 567 (1889). (2) Kolthoff, I. M.,Rec. trav.chim., 40, 686 (1921). (3) Manov, G . G., and Kirk, P. L., IWD.ENG.CHEM.,~ ~ N AED., L 9, 198 (1937). (4) Meldrum, 13‘. B., Cadbury, IT. E., Jr., and Lucasse, U’.W., Ibid., 13, 456 (1941). ( 5 ) Nichols, M.L., and Smith, E. C., J . Phys. Chem., $5, 411 (1941). (6) Schneider, F., and Rieman, W,, 111, J . Am. Chem. Soc., 59, 354 (1937). (7) Willard, H. H., and Schneidemind, R., Trans. Am. Electrochm. Soc., 56, 333 (1929).
WITH V A R Y I N G TIMES O F DIGESTION TABLE I. RESULTS
Time ~-~~ .
of
Digeation Hours 0 0 ’/I
‘/a ‘/I ‘/t
‘/a ‘/a
3 3
10
10 24 24
Total Weight of Ppt. Gram
0,5734 0.5678 0.5666 0.5693 0.5672 0.5657 0.5592 0.5600 0.5640 0.5640 0.5637 0.5628 0.5607 0.5615
BaCrO4 Found Gram 0.0275 0.0245 0.0237 0.0233 0,0222 0.0224 0,0185 0,0200 0,0179 0.0171 0.0168 0.0159 0.0152 0.0153
Bas04 Found Gram
0.5459 0.5433 0.5429 0.5460 0.5450 0.5433 0.5407 0.5400 0.5461 0.5469 0.5469 0.5469 0.5455 0.5462
.
NazSO4 Found Gram 0.3323 0,3307 0.3304 0.3324 0.3318 0.3307 0.3291 0.3287 0,3324 0,3328 0,3328 0.3328 0.3321 0.3325
Error Gram -0.0004 -0.0020 - 0,0023 - 0.0003 - 0.0009 - 0.0020 - 0.0036 - 0.0040 - 0.0003 +o ,0001
+o ,0001 +0.0001 0.0006
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- 0.0002
Literature Cited
Moderately Large Extractor-Percolator Assembly FRANCIS A. GUNTHER, University of California Citrus Experiment Station, Riverside, Calif.
M
ANY large-scale Soxhlet-type extractors have been
reported in the literature, yet a simple modification of the original continuous Soxhlet extractor has proved to be the most useful in the author’s laboratory when a moderately large extraction is to be made. This modified extraCtor can also be used as a percolator of the automatic type, the choice of function depending only upon the method used in packing the extraction chamber.
A diagram of the apparatus is shown. The upper rim of a standard 2-gallon glass percolator is ground flat with 280-mesh Carborundum powder, so that a desiccator lid fits it snugly. If both the percolator rim and the desiccator lid are given a final inding with 800- or 1000-mesh Carborundum powder, no gbrication of the joint will be required even with petroleum ether or diethyl ether as extracting solvent. This final grinding, however, is time-consuming and, for most purposes, unnecessary. A piece of glass tubing, 15 mm. in diameter, is irregularly flanged a t one end, and bent as indicated in the diagram so as to avoid baqkdrip directly into the boiler. This tube, or “chimney”, carries the solvent vapors from the boiler to the condenser. A cotton plug is packed around this chimney where it contacts the neck of the percolator; for percolation, this plug is all that is required. If true extraction is desired, however, a 2.5-cm. (1-inch) layer of clean sea sand or other inert, finely divided material is placed on to of this cotton plug in order to retard the rate of downflow of sorution. Finally, on the neck of the percolator is placed a rubber stopper of the appropriate size to fit the boiler, and an efficient
condenser 15 attached to the upper opening of the desiccator lid by means of another rubber stopper. For an extraction, the packing should not extend above a level which is 5 cm. (2 inches) below the top of the chimney tube. Ordinarily, %/ I’ channeling around this tube does not occur if the material to be extracted is 40-mesh or finer, 50 that the packing operation requires no great care. The rate of boiling of the solvent is so regulated that . icAToR a layer of liquid about 1.25 cm. (0.5 inch) deep remains on top of the material being extracted. For a percolation procedure, CH TU6E the packing plug consists of cotton only, and the rate of boiling is immaterial, so long as the rate of percolation is not exceeded. Occasionally, the drip from . .... - - S E A S A N D the condenser tends to dig a .... - C O T T O Y hole in the top layers of the marc even in an extraction. This may be overcome by placing a m a l l watch , ‘ glass, convex side down, or a small piece of filter paper on the mare. After an initial warming-up I of 5 to 10 minutes, the solvent that rondenses in the chimney is remarkably small, because of the insulating effect of the marc packed around it. MNEV
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