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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 9, NO. 12
the final results shown in Table VIII. These results, with the exception of No. 2, agree well with the established percentage. It is likely that mechanical loss caused the slightly low results on the second sample.
washing, it is inconvenient to determine more than about 25 mg. by this procedure. EXPERIMENTAL RESULTS.I n Table IX are shown results from a series of determinations by this volumetric procedure. In determinations 7 and 8, where some slight mechanical loss TABLEVIII. DETERMINATION OB MAGNESIUM IN STANDARD of precipitate was suspected, the decantations and washings DOLOMITE SAMPLE were filtered for gravimetric determination of the suspended MgO Found matter by ignition to magnesium oxide. In both cases the Analy- Sample MgO Found on MgO Found on after Final weights of oxide corresponded exactly to the deficiencies in 81s Taken First Precipitation Seoond Preoipitation Correction Gram Gram % Gram % Gram % the volumetric results. Hence, by avoidance of mechanical 1 0.3395 0.0747 22.00 0.0737 21.71 0.0732 21.56 loss during washing, exact results should be obtained by this 2 0.5741 0.1268 21.91 0.1229 21.42 0.1222 21.29 procedure. It becomes rapid if a multiple-head centrifuge is 3 0.6137 0.1354 22.06 0.1323 21.56 0.1321 21.53 4 0 8057 0.1775 22.03 0.1739 21.58 0.1736 21.54 used for making a number of determinations simultaneously. ’ Av. 22.00 21.57 21.48 Stated MgO percentage, U. S. Bureau of Standards
21.49
TABLEIX. VOLUMETRIC DETERMINATION OF MAGNESIUM From these experiments on actual samples it was found, therefore, that this method for the separation and determination of magnesium could be fitted into the ordinary analytical ,scheme with little difficulty. Other experiments indicated that it also could be used in systematic qualitative analysis.
Indirect Volumetric Determination of Magnesium Instead of converting the precipitated magnesium oxalate to oxide for weighing, the determination may be completed by dissolving the precipitate in dilute sulfuric acid and titrating the liberated oxalic acid with standard permanganate. However, if this is done a modification in procedure is desirable, chiefly because the complete removal of the reagent from filter paper by washing with 85 per cent acetic acid is too time-consuming a process. It was found much more satisfactory to separate and wash the precipitate by means of the centrifuge according to the following method: Transfer the 5 ml. of magnesium VOLUMETRIC PROCEDURE. sample solution to a conical centrifuge vessel of about 126-ml. capacity, using glacial acetic acid for rinsing out the original vessel. Add glacial acetic acid to the contents of the centrifuge vessel until the total volume is 90 ml. Precipitate the magnesium by the slow addition of 10 ml. of saturated ammonium oxalate solution and coagulate the precipitate by placing the centrifuge vessel on a steam bath for about an hour. Then centrifuge at high speed until the precipitate packs at the bottom of the vessel. This should require less than 10 minutes. Carefully decant or siphon off as much supernatant liquid as possible without disturbing the precipitate and add 15 to 25 ml. of hot 85 per cent acetic acid. After shaking to distribute the precipitate in the wash liquid, and washing down any precipitate that has collected on the sides of the vessel above the level of the li uid centrifuge again. Repeat the washing process until a sampse o i the supernatant liquid fails to produce a precipitate when tested with calcium chloride solution. Ordinarily three washings will be sufficient. Dissolve the washed precipitate, together with the remaining few milliliters of wash liquid that cannot be safely removed, in 5 per cent sulfuric acid. After washing out this solution into the titrating vessel with additional 5 per cent sulfuric acid, titrate the oxalic acid with standard permanganate solution in the usual way. One milliliter of 0.1 N permanganate solution is equivalent to 0.001216 gram of magnesium. A stoppered centrifuge vessel with a stem into which the precipitate can be centrifuged is the most satisfactory type. In order to prevent troublesome adherence of the precipitate to the walls of the vessel it is necessary to clean it thoroughly with dichromate cleaning solution before use. The principal source of error is mechanical loss of fine particles of precipitate in the process of removing supernatant liquid. This usually occurs if an attempt is made to drain off the last few milliliters of liquid. The residual acetic acid left in and above the precipitate after the final washing does not react with the permanganate solution under the conditions of the titration. By reason of the volume of the precipitate, especially after
Analysis No.
Mg Taken Gram
0.0515 N XMnOa
Mg Found Uram
1 2 3 4 5 6 7 8 9 10
0.0010 0.0010 0.0025 0.0025 0.0102 0.0102 0 I0102 0.0102 0,0252 0.0252
1.60 1.53 3.91 3.93 15.92 15.93 15.90 15.88 40.21 40.13
0.0010
Required MZ.
0.0010
-
0.0024 0.0025 0.0100 0.0100 0.0100 0,0099 0.0252 0,0251
Difference, Error Gram
*o. 0000
*o, 0000 -0.0001
~0.0000
-0.0002 -0.0002 -0.0002 -0.0003
*o. 0000 -0.0001
Attempts to determine magnesium by measuring the volume of precipitate in a graduated centrifuge tube were not successful because reproducible results could not be obtained.
General Remarks Because of the bulk of magnesium oxalate precipitated in acetic acid solution and the attendant difficulties in filtering and washing, both the gravimetric and volumetric procedures are most satisfactory in practice when applied to the separation and determination of amounts of magnesium not exceeding 25 mg., though as much as 100 mg. can be handled by the gravimetric procedure. From the standpoint of speed, this method is superior to the phosphate method and compares favorably with the 8-hydroxyquinoline method. When magnesium must be separated prior to the determination of the alkalies and platinum ware is not available, this method has the peculiar advantage that the separation is carried out in acid solution so that introduction of alkalies from the glass or porcelain becomes a negligible source of error. Another distinct advantage is the ease with which impurities in the ignited precipitate, whether oxide or sulfate, can be tested for and removed. The chief inconvenience of the method, working with hot concentrated acetic acid solutions, is not serious in practice if a good hood is available.
Literature Cited (1) Classen, A.,2. anal. Chem., 18,373 (1879). (2) Nass, G., 2. angew. Chem., 7 , 501 (1894). (3) Souchay, A., and Lenssen, E., Ann., 99,31 (1856). RECEIVEDSeptember 9, 1937. Constructed from a portion of a dmsertatlorl submitted by Philip J. Elving in partial fulfillment of the requirements for the degree of doctor of philosophy, Princeton University, 1937.
Arsenate Method for Determination of Zirconium The article entitled “The Arsenate Method for the Determination of Zirconium,” by W. C. Schumb and E. J. Nolan [IND. ENQ.CHEM.,Anal. Ed., 9, 371 (1937)]was based largely on work supported by a J. T. Baker Chemical Company Fellowship awarded t o E. J. Nolan for the year 1934-35.
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