Vol. 3, No. 1
ANALYTICAL EDITION
32
by the last addition has about ceased. This precipitation properly carried out should take about 20 minutes during which time the solution should always be saturated with free chloric acid in order to precipitate the manganese completely. If the solution is overheated a t this point, the sodium chlorate decomposes too rapidly and the manganese may not be entirely precipitated. Remove the solution from the hot plate before the effervescence from the last addition of sodium chlorate has entirely ceased, filter on an asbestos pad with suction, and wash thoroughly with water. It is not necessary to allow the solution to cool and settle before filtering although it does no harm. Transfer the pad to the beaker in which the precipitation was made and wash any adhering manganese dioxide from the funnel into the beaker with 100 cc. of water. Dissolve the manganese dioxide in 20 cc. of 0.1 N ferrous sulfate containing 100 cc. of sulfuric acid (sp. gr. 1.84) per liter, and titrate back to a permanent pink end point with 0.1 N potassium permanganate. Calculate the manganese by means of the following formula:
A B C D
[ ( A X B ) - C] X 0.2747 X D Per cent Mn = Weight of sample = cc. of 0.1 N ferrous sulfate = permanganate solution equivalent of the 0.1 N sulfate = cc. of 0.1 N potassium permanganate = normality factor of 0.1 N potassium permanganate
I n order to estimate the accuracy of this method, the manganese in two stainless steels containing approximately 1.5 per cent silicon, 8 per cent chromium, and 22 per cent nickel, was carefully determined by both the persulfate and bismuthate methods, after separating the chromium with
zinc oxide. The average percentage obtained from four determinations on each steel was assumed to be the correct manganese content of these two samples which are designated in the table as 1 and 2. Determination of Manganese in Eight Samples MANGANESE OBTAINED 2 3 4 Av.
SAMPLE
1
2 12c 72 32a 30b 1oc
7a
%
%
%
%
%
0.85 0.49 0.41 0.64 0.25 0.49 1.12 0.42
0.81 0.50 0.39 0.64 0.24 0.48 1.15 0.41
0.81 0.49 0.38
0.83 0.51 0.43 0.62 0.24 0.50 1.13 0.42
0.83 0.50 0.40 0.64 0.25 0.49 1.14 0.42
0.65
0.25 0.48 1.14 0.42
MANGANESE PRESENT
% 0.83 0 61 0.409 0.651 0.244 0.499 1.13 0 446
The manganese in five Bureau of Standards standard steel samples and one iron (7a) was also determined and included in this table in order to compare further the accuracy of the above method with the commonly used procedures. The results on iron (?.) were obtained without separating the silicon or graphitic carbon. Four determinations on each sample were run at the same time without employing any more care or refined apparatus than is ordinarily used to obtain reasonably accurate results in routine steel work. The theoretical titer was used as shown in the preceding formula to calculate the results thus obtained, which were tabulated without rejecting the percentages obviously inaccurate. These values, however, considered as a whole would be closer to the theoretical percentages if multiplied by 1.02 as an empirical factor.
Investigation of Ammonium Acetate Separation of Sulfates of Lead, Barium, and Calcium' Wilfred W. Scott and Samuel M. Alldredge UNIVERSITY OF SOUTHERN CALIFORNIA, LOSANGELES,CALIF.
METHOD commonly given by standard texts upon quantitative chemical analyses for separating lead from barium and calcium depends upon the solubility of lead sulfate and the insolubility of barium and calcium sulfates in ammonium acetate solution. It is the intention of this piece of work to test out that method as regards its completeness of separation.
A
Previous Investigations
In speaking of the precipitation of lead as a sulfate in the determination of lead in ores and metallurgical products, Treadwell and Hall (7) claim that the precipitate of lead sulfate containing silica and barium sulfate (also strontium and sonietimes calcium sulfate) can be purified by redissolving the lead in hot ammonium acetate solution. They suggest that the lead extraction be made with 20 cc. of hot 2 N ammonium acetate. Low ($1 suggests the use of ammonium or sodium acetate solution in dissolving lead sulfate. He would titrate the lead solution with standard ammonium molybdate solution while the calcium remains out of solution. I n the analysis of fluorspar Sisco (6) claims that lead and barium sulfates are separated by the acetate extraction using a 20 per cent solution of ammonium acetate. Scott (5) considers the bariua? sulfate slightly soluble in 1 Received
August 12, 1930.
ammonium acetate and hence a possible contaminant of the extracted lead. The problem has two main parts-namely, (1) is barium extracted with the lead; and (2) is calcium extracted with the lead when ammonium acetate, hot and concentrated, is used upon a precipitated mixture of the sulfates of lead, barium, and calcium? A subdivision of the problem, however, of minor importance was added by the claim of Majdel (3) that the separation of lead from barium in the presence of barium, by precipitating both components with sulfuric acid and dissolving lead sulfate in ammonium acetate, is not possible as the double salt of lead and barium sulfate is formed which is insoluble in ammonium acetate, and, therefore, a part of the lead remains with barium. The error is greater as the ratio of barium to lead increases. I n proportion of Pb:Ba as 1:0.6,9 per cent lead remains in the undissolved barium sulfate; as 1:1,45 per cent; as 1:2, %per cent; as 1:7, 100per cent; and is thereby lost. The separation must be accomplished, therefore, with hydrogen sulfide. The ratio of 1 lead to 100 barium was taken. The precipitated mixture of their sulfates was extracted ten times with boiling 50 per cent ammonium acetate as given below. The filter and its residue then were boiled with 100 cc. of 50 per cent ammonium acetate until spattering commenced. Fifty cubic centimeters more of ammonium acetate were added, the mixture heated to boiling and allowed to settle.
January 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
33
E x p e r i m e n t a l Data
DETERMINATION Solution 01 ammonium ac‘etate, per cent Number of extractions made Volume used in each extraction, cc. Lead in sample, gram Lead in extract, gram Lead extracted per cent Barium in samile, gram Barium extracted Calcium in sample, gram Calcium extracted, per cent
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
50
50
50
50
50
50
50
50
25
25
6 6 6 6 8 8 25 25 25 25 25 25 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.01990.0200 0.0199 0.0199 0.0199 0.0198 99.5 100 99.5 99.5 99.5 99.0 0.0100 0.0100 0.0200 0.0200 0.0800 0.0800 None None None None None None 0.0100 0.0100 0.0200 0.0200 0.0800 0.0800 3.37 3.62
Method of Procedure
Known mixtures of lead nitrate, barium nitrate, and calcium carbonate are dissolved in water acidulated with nitric acid. An excess of sulfuric acid is added to the mixture and taken to sulfuric acid fumes. This is cooled, diluted, filtered on Swedish filter paper, and washed well with water containing 10 per cent sulfuric acid. The filter paper with its residue is placed in a casserole and extracted with 30 cc. Qf hot, concentrated ammonium acetate, and the solution decanted through filter paper. This extraction is repeated until the filtrate indicates no lead when a drop of the filtrate is put in some potassium chromate solution. It requires about six to eight extractiom to get negative tests for lead on the filtrate. Filtrate I . This contains the lead and possibly some of t h e barium and calcium. The filtrate is made faintly acid with hydrochIoric acid and hydrogen sulfide is passed through it for 30 minutes. I t is then heated on a steam bath for same period of time and filtered. Filtrate II. Contains barium and calcium. This filtrate is evaporated nearly t o dryness and the sulfides dissolved in the least amount of hydrochloric acid, and the excess acid expelled by evaporation. T h e residue is taken up in 300 cc. of water containing 5 drops of acetic acid. Sufficient ammonium acetate solution is added to make the solution neutral. An excess of potassium chromate solution is added after heating t h e so!ution t o boiling. The solution is allowed to settle until cold. It is filtered through a Gooch crucib!e.
Fillrate I I I . Contains calcium. T o the neutral solution after heating t o boiling, 10 cc. of acetic acid are added and 15 CC. of a saturated solution of oxalic acid, and after 5 minutes, a slight excess of ammonia. I t is allowed t o cool a n hour, then decanted through a filter. The precipitate is ignited wet in a weighed crucible and strongly heated t o constant weight. It is weighed as calcium oxide.
Residue I .
Contains most of the barium and calcium. This was discarded in this problem.
Residue II. Contains the lead a s a sulfide.
This residue is washed well with water saturated with hydrogen sulfide gas. It is then treated with hot hydrochloric acid and then nitric acid and heated t o boiling. Ten cubic centimeters of concentrated sulfuric acid are added and the solution evaporated t o strong sulfuric acid fumes. I t is cooled, diluted with water, and filtered on a n asbestos mat in a weighed Gooch crucible. The residue is washed with dilute sulfuric acid and with alcohol, and dried at a dull red heat. I t is weighed a s lead sulfate. Residue 111. Contains barium a s a chromate. I t is washed with a dilute potassium chromate solution until free of calcium and with water until free of potassium chromate. It is then washed once with dilute alcohol, dried a t 110’ C., and weighed as barium chromate.
50 Hot Hot 50 60 water water 8 12 8 8 8 8 25 30 25 25 25 25 0.02000.0100 .... .... .... 0.0198 0.0095 .... 99.0 95.0 .... .... .... 0.0600 1.000 .... . . . . ..,. .... None .... . . . . . . . . 0.0600 None 0.0600 0.0600 0.0600 0.0600 5.00 .... 26.3 35.3 3.16 19.5
....
....
.... .... .... ....
It was then filtered, the filtrate being passed into the filtrate of the first ten extractions. The results of this extraction may be found under column 11 of the data, I n all extractions the filter paper with its precipitated sulfates was transferred to a casserole, 25 cc. of concentrated ammonium acetate added, and heated to boiling. The acetate solution was filtered through a Swedish filter paper. This was repeated six or eight times until the paper in the casserole and funnel did not darken when touched with a rod containing a drop of hydrogen sulfide solution. The filter paper in the funnel was washed four times with hot water and squeezed well to remove all liquid.
(11)
.... ....
.... ....
Results
The data of the first ten columns (of the table of experimental data) are the result of twenty-two determinations on lead, If the lead did not check with the amount in the original sample, the sample was discarded and another one begun. I n case the amount of lead sulfate ran high, further heating sometimes brought the amount of lead down to the amount in the sample. The general tendency, however, was for the lead to run low. The work was done with extreme care. Each sample was weighed separately and carefully. Considering the slight solubility of lead sulfate and the small amount of lead in the sample, a slight loss of lead is to be expected, as is shown in the table. Outstanding Findings of the Work
(1) In a ratio of calcium to barium of 1to 1: (a) all lead is extracted, (b) no barium is extracted, (c) as much as 8.5 per cent of calcium is extracted. (2) I n a ratio of calcium to barium of 1to 9.3, no barium is extracted. (3) I n a ratio of lead to barium of 1 to 100, 95 per cent of lead is extracted. Conclusions (1) Lead may be separated from barium by ammonium acetate extraction of their sulfates, the barium not passing into the lead solution in appreciable amounts. (2) Lead cannot be separated entirely from calcium by ammonium acetate extraction, the calcium passing into the lead solution in considerable quantity. (3) The amount of calcium passing into the lead solution seems to depend upon certain physical factors not in control of the operator. The writers suggest these as possible factors: the time required in filtering each extract, the amount of cooling during filtration, small changes in the density of the acetate solution during boiling. (4) As the amount of barium is increased there seems to be an increasing difficulty in extracting all the lead. This may be due to physical occlusion of the lead sulfate by the barium sulfate or to the formation of a definite lead-barium salt which is insoluble. The writers are inclined to believe from the data in column 11 of the table that this noticeable difficulty may be due to occlusion. (5) Less calcium is extracted in the presence of lead and barium than when it is the only constituent of the sulfate precipitate. Literature Cited Hillebrand and Lundell, “Quantitative Analysis,” Wiley, New York,
1929. Low, “Technical Methods of Ore Analysis,” Wiley, New York, 1827. Majdel, Art. hem. farm., 4, 76-8 (1930). Olsen, “Quantitative Chemical Analysis,’’ Van Nostrand, New York,
1926. Scott, “Standard Methods of Chemical Analysis,” Vol. I, Van Nostrand, New York, 1925. Sisco, “Technical Analysis of Steel and Steel Works Material,” McGrawHill, New York, 1923. Treadwell and Hall, “Quantitative Analysis,” Vol. 11, Wiley, New York, 1919.
