INDUSTRIAL AND ENGINEERING CHEMISTRY
562
Vol. 15, No. 9
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 VARYING 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
Bas04 Found Gram
0.0275 0.0245 0.0237 0.0233 0,0222 0.0224
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
0,0185
0,0200 0,0179 0.0171 0.0168 0.0159 0.0152 0.0153
.
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
-
- 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
U”
