Separation of Calcium from Magnesium Preliminary Precipitation of Magnesium Hydroxide J. -4.GREEAR AND E. R. WRIGHT Texas Division, The Dow Chemical Company, Freeport, Texas Jn the analysis of magnesites or other magnesium compounds containing little calcium. 95% of the magnesium can be precipitated as the hydroxide without appreciable error due to precipitation of calcium. Traces of carbonate do not interfere, nor does the addition of sugar or mannitol aid the separation under the conditions studied.
mixed thoroughly, and allowed to stand (usually 1 to 3 hours) until 250 ml. of nearly clear supernatant liquid can be siphoned off. A slight cloudiness a t this point can be disregarded. (-4 permissible alternative procedure is to filter the magnesium hydroxide slurry through a 24-cm. Whatman No. 12 paper. However, in this case a correction may be necessary for traces of calcium in the filter paper.) The 250-ml. aliquot portion is placed in a 400-ml. beaker and acidified with 5 ml. of 6 N hydrochloric acid. Ten milliliters of 5% ammonium oxalate solution are added, and the solution is heated nearly to boiling and is made alkaline to methyl red by dropwise addition of 6 S ammonium hydroxide. After digestion for about 2 minutes, 5 grams of solid ammonium oxalate are added and the solution is heated to boiling again. The precipitate is digested 1 hour, and is then allowed to stand for several hours or preferably overnight. The calcium oxalate is filtered on a fritted Pyrex filter and washed four times with small portions of cool 0.1% ammonium oxalate solution. The precipitate is then dissolved in 75 ml. of hot 2% hydrochloric acid, 1ml. of 5% ammonium oxalate solution is added, and the hot solution is made slightly alkaline with 6 N ammonium hydroxide. After digestion for 5 minutes, an additional 5-ml. portion of 5% ammonium oxalate solution is added and the precipitate is digested for 0.5 hour, then allowed to cool with occasional stirring for a t least 3 hours. The precipitate is finally filtered on a Pyrex filter, washed four times with 0.1% ammonium oxalate solution and then four times with distilled water, and after solution in 100 ml. of hot 2% sulfuric acid, the oxalate is titrated with 0.05 N potassium permanganate.
A
METHOD of analysis for calcium ( 1 , 5 ) used for many years in industrial laboratories depends upon the precipitation of most of the magnesium as magnesium hydroxide followed by filtration and precipitation of the calcium in the filtrate as calcium oxalate by standard procedures. This method was devised by Prutton ( 4 ) in the summer of 1925 for the analysis of magnesium chloride brines low in calcium. No systematic study of the method has been published, although a modification has been proposed by Hazel and Eglof ( 8 ) . The method is useful chiefly in two cases, ( 1 ) where the ratio of magnesium to calcium is so high that a separation cannot be made by direct oxalate precipitation, and (2) where a rapid method is desired involving only a single rather than a double oxalate precipitation. In the first classification fall most magnesites and magnesium chloride brines containing magnesium-calcium weight ratios of 20 to 1 or greater. Such materials are difficult or impossible to analyze by direct oxalate precipitation. An example of a magnesite difficult to analyze by direct precipitation is Bureau of Standards sample 104 which has a certificate value of 85.67% magnesium oxide and 3.35% calcium oxide, or a magnesium-calcium ratio of 21.6 by weight. This sample was analyzed by careful triple precipitation of calcium oxalate with results shown in Table I. The average value obtained for calcium (3.31% CaO) is reasonable, as no correction was made for error due to the solubility of calcium oxalate. The error from this source amounts to about 0.1 mg. of calcium per precipitation ( S ) , or 0.07% calcium oxide on the 0.6-gram sample taken for analysis.
RESULTS
Effect of Sugar. The method as originally devised involved the addition of sugar as an aid in preventing loss of calcium through inadvertent addition of too much sodium hydroxide. Mannitol has also been recommended (8). In order to show whether such additive agents are necessary, a series of known mixtures were analyzed (Table 11). I t is apparent from these results that neither sugar nor mannitol is required, so long as not more than 950/, of the magnesium is removed, but with removal of lOO%, or in the presence of excess sodium hydroxide, the results are invariably low. Effects of Carbonate. In order to find out what quantity of carbonate can be tolerated. a series of determinations was made
In the method herein described, most of the magnesium (usually 950j0) is removed as magnesium hydroxide in order to lower the magnesium-calcium ratio sufficiently to allow a direct oxalate precipitation to be made conveniently. REAGENTS
A 16.0% magnesium chloride solution and standard calcium
rhloride solutions were prepared by the methods used by Wright and Delaune ( 6 ) . Potassium permanganate solutions were standardized against Bureau of Standards sodium oxalate. Carbonate-free sodium hydroxide solution was made by filtration of a 50% solution made from C.P. material. Analysis showed less than 0.1% . _ sodium carbonate present in the 50% sodium hydroxide. All reagents were examined spectrographically, and were found to contain negligible amounts of calcium or strontium.
Table I.
Analysis of Bureau of Standards Sample 104 % CaO Found
By triple oxalate precipitation
PROCEDURE By recommended procedure without removal of iron
A sample is taken of sufficient size to contain 1.0 to 2.5 grams of magnesium, In the case of magnesites the sample is dissolved in hydrochloric acid, and the silica is removed by customary dehydration procedures. Iron and aluminum must also be removed if present to the extent of 30 mg. or more. The neutral solution from which ammonium salts have beeR removed is placed in a 500-ml. volumetric flask and diluted to about 200 ml. and an amount of 1N sodium hydroxide is added equivalent to 95% of the magnesium. The solution is then made to volume,
B y recommended procedure with removal of iron
.z
Analyst A.
b Analyst B.
596
3.29 3.28 3.34 3.20" 3.20n 3.20" 3.335 3,34,3 3.350 3. 3 6' ~. AY. 3.35"
3.31 3.32 Av. 3.31 3.09b 3.10b 3.326 3.346 3.306 3.30b 3.32b
597
V O L U M E 21, NO. 5, M A Y 1 9 4 9 Table 11.
Effect of S u g a r and M a n n i t o l w i t h Varying A m o u n t s of S o d i u m Hydroxide Calcium Found
Magnesium Removed
Calcium Present”
%
MQ.
Without sugar or mannitol
With sugar b
MQ.
With mannitol
Table 111. Effect of Carbonate Na%CO:* Present
70
.lf 0.
Calcium Presenta
Calcium Found
XQ.
MQ.
10.0
0
DISCUSSION
b
Mg.
MQ.
9.8 10.0 90 9:s 9.8 9:s 95 10.0 9.8 98 9.8 9.8 10.0 .~ 9.2 9.4 10 .o 9.2 100 8.6 8.6 8.0 102 10.0 50.1 50.1 50.2 50.0 80 50.0 49.7 49.7 50.0 90 49.9 49.7 50.0 49.9 95 49.4 49.2 49.2 50.0 100 46.0 45.6 43.4 50.0 105 4 Total weight of calcium taken. b 20 grams used per determination. -411 figures are average of duplicate determinations.
Magnesium Removed
when 5.0 and 50.0 were taken, respectively. Apparently the negative error to be expected from hot filtration and washing is more than offset by the positive error due to coprecipitation of magnesium oxalate and failure to make a blank correction.
9.7
Results by this procedure are consistently low by 0.1 to 0.3 mg. of calcium because of solubility errors which are inherent in d l oxalate methods for calcium. In the most accurate work, this error can be corrected by recovery of the traces of calcium precipitated along with the magnesium ammonium phosphate according to the method of Hillebrand and Lundell (5). HOTever in ordinary work, the magnitude of the error is small enough to be neglected, or a fixed correction may be applied. I t is better to make such corrections than to depend on a procedure that involves compensating errors (3). In Tables 11, 111, and IV, these solubility errors have been doubled, as the results are calculated to the original sample basis. In the presence of magnesium ion, hydrolysis of carbonate would be expected to take place, resulting in the formation of magnesium hydroxide and bicarbonate ion. Such hydrolysis is in part responsible for the fact that small amounts of carbonate can be tolerated without significant loss of calcium. It appears also that part of the carbonate present is precipitated with the magnesium hydroxide, and consequently the error caused by carbonate is smaller than expected.
Total weight of calcium taken.
with sodium hydroxide containing known amounts of sodium carbonate (Table 111). As could be expected, the results shoiv that the error due to the presence of carbonate increases with the amount of calcium present, and with the proportion of magnesium removed. In general, the small amount of carbonate normally left in 50y0 sodium hydroxide and the traces picked up during handling are not sufficient to cause appreciable error. Effect of Impurities. As shown in Table I, low results were consistently obtained on Bureau of Standards sample 104 unless iron and aluminum were removed by a double precipitation with ammonia. This is not unexpected (3). In order to show the magnitude of the effect, a few determinations were made with known amounts of impurities (Table IV). It is apparent that even comparatively small amounts of phosphate cause serious error, but that the error from iron is serious only when 50 mg. or more are present. Other Applications. Results with three other samples are shown in Table V. Because the spectrographic method may be in error by 10 to 15% of the quantity of calcium present, the results in the case of the magnesite sample agree somewhat fortuitously. However, the authors believe that the chemical method gives acceptable accuracy on such materials. * The published rapid method (1,6),which consists of removal of 95% of the magnesium, followed by a single hot precipitation of calcium oxalate, tends to give slightly high results. Thus by this procedure the authors found 5.2 and 51.7 mg. of calcium
T a b l e V.
O t h e r Applications
Description
Mg-Ca Ratio b.y Weight
Calcium Spectrographic Method
%
%
Crude 34% hfgC12 liquor
140:1
0.075
Synthetic magnesite A
110:1
0.48
0.064 0.063 0.063 0.48 0.48 0.48
Calcium Chemical Method
The elimination of iron by double precipitation with ammonia is time-consuming, as ammonium salts must also be removed before precipitation of magnesium hydroxide. A faster procedure would be to remove the iron by electrolysis with a mercury cathode or by extraction with ether. However, most of the materials with which the authors have dealt have not required removal of iron and aluminum. Many sources of sodium hydroxide contain traces of strontium compounds.. Possible error from this cause must be avoided by analysis of the sodium hydroxide or by use of a carefully determined blank correction. The authors know of no procedure by which the magnesium can be precipitated completely as hydroxide without some loss of calcium. It is possible that adsorption errors become significant the equivalence point is passed. LITERATURE CITED
Table IV.
Effect of I m p u r i t i e s
Calcium Calcium Impurity Found Present Presentn MQ. MU. MQ. 50.0 49.8 10 F e 50.0 49.2 50 F e 50.0 47.4 100 F e 50.0 49.0 10 POI 50.0 43.8 50 PO1 a Total weight of calcium taken. 95% of magnesium removed in every case.
(1) Dow Chemical Go., “Magnesium Laboratory Methods,” 1946. (2) Hazel, W.M.,and Eglof, W. K., IND. ENG.CHEM.,ANAL.E D , 18,759-60(1946). (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 495-6, New York, John Wiley & Sons, 1929. (4) Prutton, C.F., Dow Chemical Co., Rept. 365 (Aug. 13, 1925). (5) Scott, W.W., “Standard Methods of Chemical Analysis,” Vol. I, p. 551,New York, D. Van Nostrand Go., 1939. (6) Wright, E. R., and Delaune, R. H., IND.END. CHEM.,ANAL. ED., 18,426-9(1946). RECEIVED March 4,1948.
