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A,VBLYTICAL EDITIOS
Vol. 2, No. 4
Determination of Calcium and Magnesium in Dolomitic Limestones b y Means of Saccharate Solutions' Alkalimetric Method A. C. Shead and B. J. Heinrich UNIVERSITY OF OKLAHOMA, NORMAN, OKLA.
A new, rapid, accurate, alkalimetric method for the favor the formation of the UCROSE solut,ions have determination of calcium and magnesium in low-iron soluble m o n o s a c c h a r a t e . been used for some time dolomitic limestones is presented. Extension of the \Then further shaking proin roughly determining process to high-iron samples, to obviate the difficulties duces no further solution, the the amount of a v a i l a b l e attending the classical gravimetric determination of insoluble magnesia is allowed quicklime in the commercial these constituents in such materials, is tentatively to settle and the monocalcium article. Shead and Carson suggested. Further investigations in progress for the saccharate in solution is fil( 5 ) , in 1926, made use of the application of the method to materials other than tered off by means of the solubility of CaO as monocarbonates, and for determining the different states a n n n r a t u s shown in the saccharate in an attempt, of combination of calcium, are announced. accompanying figure. This to produce a primary standsetup is d e s i g n e d for the a r d alkali of -predetermined normality b y igniting a weighed sample of chemically protection of the solutions from carbon dioxide while underpure calcium carbonate to constant weight, with sub- going filtration. The preparation of the filter-paper pulp and its seating sequent slaking, and solution of the resulting oxide in a 30 per cent sucrose solution. Later, in 1928, Shead are vital features of this process. The pulp is prepared by ( 2 ) dissolved weighed metallic calcium in a 30 per cent shredding some best quality quantitative filter paper and cane-sugar solution with the same object in view. During heating it at the boiling temperature for a minimum time in the course of this research it was observed that metallic distilled water, every effort being made to disintegrate it magnesium, a n impurity in, the calcium, was apparentlv mechanically by whipping with a glass stirring rod, as conundissolved in the sugar solution, and subsequent investi- tinued boiling renders the paper slimy and worthless as a gation disclosed the difficult solubility of magnesia and filtering medium. The seating in the assembly, G, is carried magnesium hydroxide in the same medium. These ob- out as follows: A disk of quantitative filter paper is placed on the beveled side of the Gooch perforated disk, D, and the servations suggested the applications set forth in this papernamely, the ignition of the mixed carbonates of calcium and latter placed in the filter funnel, F , beveled side down with magnesium to their respective oxides, the treatment of the the paper disk underneath. The filter pump is started slowly, latter with water and sucrose to dissolve the calcium, the and prepared pulp in sufficient quantity to make about separation of this from the insoluble oxide or hydroxide a l/s-inch (3-mm.) mat is poured on the disk. The moisture of magnesium b y filtration, and the subsequent alkalimetric from the pulp in suspension softens the paper disk, which titration of the bases thus separated by means of standard breaks after first having given the pulp opportunity to settle somewhat. This procedure gives a filtering mat of optimum acid. porosity. On no account should the mat be packed by strong Procedure action of the pump or by tamping with a rod. The apparatus A weighed sample of approximately 0.5 gram is placed in a is now connected u p as indicated, the force of the pump porcelain crucible having a platinum foil in the bottom to gradually increased to maximum capacity, and the filtration prevent corrosion b y the alkaline oxides resulting from completed without further attention. A pinchcock placed mbsequent ignition. This sample is ignited a t 900-1000°C. between pump and safety flask will insure continued filtration to constant weight, which insures complete conversion of the without further wastage of water. Washing with dilute carbonates to the corresponding oxides. The latter, after sucrose solution or C02-free distilled water is carried out cooling somewhat, are transferred to a stoppered Erlenmeyer in a way that readily suggests itself. I n breaking connecflask containing about 25 cc. of C02-free distilled water, tion with flask I , care should be taken not to disarrange the in which the CaO is slaked to Ca(OH)2for about 5 minutes filter disk by too sudden stopping of the pump or abrupt a t the boiling temperature with vigorous shaking and stirring opening of the system, as this would cause cloudy filtrates. After washing several times to remove all soluble calcium, to break u p any lumps. After thorough disintegration has been accomplished, the mixture in the flask is cooled the insoluble magnesia, mat and all, is transferred to the flask to room temperature; otherwise the undesirable, insoluble I, and all connections including the filter funnel are rinsed tricalcium saccharate may be formed in subsequent opera- into this flask with large excess of undiluted standard acid tions. While cooling, the flask is kept well stoppered to (0.2 N ) , the mixture digested near the boiling point for a t prevent access of carbon dioxide which will precipitate calcium least 5 minutes or until no white magnesia can be observed. carbonate. When the mixture has reached room tempera- -411 connections are then rinsed into the same flask to recover ture, a COrfree 30 per cent cane-sugar solution measuring the standard acid b y flushing with distilled water. The about 100 cc. is added all at once, the flask restoppered, and excess acid is titrated back with standard alkali, using the contents vigorously shaken at intervals for about 5 phenolphthalein as indicator, and the percentage of magminutes to break up lumps and bring fresh sugar solution nesia calculated. Since sucrose locks up small quantities of calcium in a in contact with the solids. During this operation the solution must be kept a t room temperature, as low temperatures manner unknown to the writers, the calcium cannot be determined accurately by direct titration of the monosaccharate. 1 Received May 12, 1930.
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I
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I S D CSTRIAL A-VD ENG1.1’EERI~VG CHEMISTRY
October 15, 1930
This point is now under investigation in this laboratory, especially with reference to the possibility of substituting glucose for the sucrose. However, a very satisfactory indirect method for determining the calcium has been devised. This consists of digesting a weighed quantity of the magnesian limestone with excess of 0.2 N acid, at a temperature near the boiling point, until effervescence ceases and carbon dioxide is expelled. The excess acid is titrated back with standard base, using phenolphthalein as indicator. With these data and knowing the amount of magnesia as determined above, the amount of calcium oxide is easily calculated, inasmuch as limestones rarely contain substances interfering with this process. The time required for making these determinations is from 3 to 4 hours, a large part of which is consumed in the ignition and filtration, which proceed largely without the attention of the-operator.
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two methods is explained by the fact that part of the calcium in this sample is present as the double phosphate-fluoride which does not burn down t o oxide, indicating the usefulness of the method for distinguishing between different combinations of calcium.
A
Data
Normality of reagents used in experiments: HCl, 0.2678; C02-free KaOH, 0.1822. MAGNESIADETERMINATIONS W T . O F SAMPLE Gram
HCI ADDED
cc.
i i a O H REQUIRED CC.
hIgO
Per cent
SAMPLE I
21.47 21.52 21.78 21.86 21,55 21.56 21,72 .4v.. . . . . 21.64 Av. in U. S. Bur. Standards Dolomite h-0.88, which is sample used 21.48 0.5791 0.5000 0.5713 0.5070 0,5550 0,5130 0.6695
50 00 50.00 50.00 50.00 50.00 50.00 50.00
0.5651 0 5235 0.5814 0.5142 0,5387 0.5073 0.5854
50.00 50 :OO 50.10 50.00 50.00 50.00 50.00
39.65 44.20 39.60 43.30 40.60 43.40 39.80
SAMPLE 2
6.73 6.08 5.76
64.68 64.80 64.50 65.70 64,60 65,60 63.80
Av. by gravimetric method ( 4 ) .. . . . .
5,57
. . . . . . . ..
Av..
....
. . . .. .
6.04 5 71 6 08 5.8,i 5.86
SAMPLE 3
CALCIUM OXIDEDETERMINAT:ONS WT.OF SAMPLE Gram
HCI ADDED
cc.
lUaOH REQUIRED
SAMPLE 0.5247 0,5344 0,5099 0.5015 0.5430
7 5 , 10
75.00 75.00 75.00 75,00
cc.
con
Per cent
1
48.40 4i.15 49.85 50.70 46.10
U. S. Bur. Standards Dolomite No. 88. . . . . . . .
47.35 47.32 47.47 47.57 47.34 Av . . . . . 47.41 . . . . . . . . . 47.25
Per cenf MgO... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.64 CaO, calcd. from MgO and Cog values.. . . . . . . . . . . . . . . . . 30.32 CaO in U. S. Bur. Standards Dolomite S o . 88. . . . . . . . . . . . 30.4’8 S.431PLE 2 MgO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.85 coz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.99 CaO, calcd. from volumetric magnesia and COz determinations.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49.20 CaO by direct gravimetric determination ( 4 ) . . . . 49.19 SAMPLE 3 MgO.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.05 coz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.72 CaO, calcd. from volumetric magnesia a n d COP determinations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30.38 CaO, by direct gravimetric determination ( 3 ) ,. . . . . . . . . . . . 31.26
The values for carbon dioxide given for the last two determinations were obtained gravimetrically by the senior author. These values have been repeatedly checked by students in elementary quantitative analysis a t the University of Oklahoma, using the volumetric method outlined above for the indirect determination of CaO. Such students have also determined MgO by the process given for this determination, with excellent results. The discrepancy in the last determination for CaO by the
A p p a r a t u s for D e t e r m i n a t i o n of Calcium and Magnesium in D o l o m i t e A --Filt er pump F-Gooch filter funnel 8-Safety flask G-Filter assembly t o remove inC-Flask t o receive calcium sacsoluble magnesia charate filtrate H -Flexible connection D-Beveled Gooch filtrate plate I--Flask containing mixture being E-Filter-paper pulp filtered J-Soda-lime guard tube
Discussion
Theoretical considerations as well as concordant experimental results indicate the accuracy of the method presented. Magnesium hydroxide, which has a slightly greater solubility than the magnesium oxide probably obtained in this process, has a n ion-product constant, according to Kohlrausch and Rose ( I ) , of 3.4 X lo-” at 18’ C. This satisfactory value combined with the common ion effect of the excess (OH)- from the hydrolyzed saccharate of calcium, which is practically a calcium hydroxide solution and which serves to diminish further the solubility of magnesium hydroxide, tends to minimize errors due to the effect of the solvent. Furthermore, washing is discontinued when the common ion is removed; consequently no time is given for equilibrium to be established with the aqueous solvent alone. Experience with this method shows marked discrepancies between it and the classical gravimetric procedure, if iron is present in quantity. Preliminary experiments indicate that the error lies with the latter process, as usually carried out, in that insufficient ammonium salts are generally employed in the attempt to prevent the inclusion of Mg(OH)* in the R(OH)s precipitate and that calcium oxalate is not reprecipitated an adequate number of times to free it from the magnesium compound, that coprecipitates as double oxalate. The Bureau of Standards ( 6 ) indicates that four precipitations are necessary to effect the latter separation. Preliminary calculations by the authors on high-iron dolomitic limestones show that the summation of percentages is generally satisfactory in such instances but that the distribution of the constituents is an improper one. Further research on this point is being carried out. Literature Cited (1) Kohlrausch and Rose, Z. p h y s i k . Chem., 12, 24 (1893). (2) Shead, J. Am. Chem. Soc., 50, 415 (1928). (3) Shead, “Chemical Analyses of Oklahoma Mineral R a w Materials,” University of Oklahoma Studies No. 33, New Series 423 (1928). rlnalysis No. 64. (4) Shead, I b i d . , Analysis No. 261. ( 5 ) Shead and Carson, “Studies on Monocalcium Saccharates in Acidimet r y , ” unpublished thesis, University of Oklahoma, 1926. (6) U. S. Bur. Standards, Certificate on Dolomite, No. 88.
