Accurate Determination of Calcium without Reprecipitation In the Presence of Aluminum, Iron, Magnesium, Manganese, Phosphorus, Sodium, and Titanium WILBUR H. hICCOMAS, J R . , AND WILLIA31 RIERIAN 111 Rutgers Uni\ersity, Yew Brunswick, N. J.
I
T IS well known that calcium can be precipitated as the oxalate without interference by moderate amounts of iron or aluminum, if the p H is sufficiently low (1, 3, 7 ) . A search of the literature, however, fails to reveal an adequate study of the effect of variations in the quantity of the accompanying elements on the accuracy of the calcium determination. The literature also lacks a satisfactory report on t'he minimum p H at which calcium oxalate can be quantitatively precipitated. Perciabosco's ( 8 ) statement that complete precipihtion is obtained a t a p H of 1 seems hardly credible. Shohl's (9) limit of 4.0 is based on insufficient data; he failed to study any p H between 4.0 and 2.8. The minimum allom-able p H will vary slightly with the accuracy required and with the conditions of precipitation, such as the quantity of calcium and of accompanying elements, the excess of oxalate added, the volume, etc. From the data of the authors, presented elsewhere (6, 6 ) , it can be computed that a negligible amount of calcium (0.0005 millimole) will remain in 200 ml. of solution containing 8.0 millimoles of excess oxalate at a n ionic strength of 0.36 and a p H of 3.70. These conditions were chosen for the determination of calcium, and a formate buffer was selected. A major source of error in the determination of calcium in a limestone is the postprecipitation of magnesium. This contamination may be diminished by decreasing the digestion. On the other hand, too short a digestion may cause an incomplete yield of a nonfilterable precipitate. These difficulties were avoided by slow precipitation of calcium oxalate and digestion for 5 minutes in a hot, strongly acid solution, rapid cooling of this solution, adjusting the p H to 3.70, and a final digestion of 30 minutes a t 25' C.
Reagents A standard solution of calcium was obtained by dissolving a weighed quantity of Merck's reagent calcium carbonate (low in alkali and chloride) previously dried for 3 hours at 110' C. in a slight excess of hydrochloric acid and diluting in a volumetric flask. Solutions of aluminum, ferric, ferrous, magnesium, manganese, and sodium chlorides, and ammonium monohydrogen phosphate were pre ared from reagent grade salts. The first five solutions were male 0.01 N with hydrochloric acid to prevent hydrolysis. The concentrations of manganese and sodium chlorides were calculated from the weights of salts taken. All the other solutions were standardized by appropriate methods. The solution of ferrous chloride was passed through a silver reductor ( I O ) just before use. A solution of titanic chloride was prepared by slowly pouring distilled titanium tetrachloride into 0.4 M hydrochloric acid, and was standardized by a gravimetric determination of titanium. A wash solution 0.020 M with ammonium hydrogen oxalate and 0.010 M with ammonium oxalate was prepared by dissolving 0.020 mole of ammonium oxalate and 0.010 mole of oxalic acid in a liter of water. A solution of 2.0 31 formic acid was prepared by dilution of the chemically pure concentrated acid. Similarly 0.50 M oxalic acid and 2.0 M ammonium formate were prepared from xeighed quantities of the chemically pure solids. Since these three solutions are used to control the pH of the precipitation, they should have the recommended concentrations within 10 per cent. Or the concentrations may be determined and appropriate changes in the volumes applied.
Procedures DETERMIXATIOX OF C'ALCIGMIN A SOLUTION. The solution should contain preferably about 2 millimoles of calcium, If elements such as iron or phosphorus are present, the solution should contain enough hydrochloric acid to prevent the formation of a precipitate, but a large quantity of acid should be avoided. Add 12 ml. of 2.0 AI formic acid, dilute to 140 ml., and heat to 95" C. Add with stirring 20 ml. of 0.50 X oxalic acid through a funnel with the stem constricted so that about 85 seconds are required for the addition, Rinse the funnel with 2 ml. of water. Digest the precipitate 5 minutes a t 85' to 90" C. Set the beaker in a larger vessel of cold water, so that it cools to 25' C. in 15 minutes, then add with stirring 34 ml. of 2.0 h ' ammonium formate through the same funnel. Rinse the funnel with 5 ml. of water, and digest the precipitate 30 minutes at 25" C. Filter the solution through a mat of asbestos supported on a perforated porcelain disk in a funnel. Wash the precipitate three times by decantation with 20-ml. portions of the wish solution, then transfer the precipitate to the filter, using a total of about 150 ml. of the wash solution. Finally wash the precipitate and funnel with three 8-ml. portions of water. Transfer the precipitate along with the asbestos and porcelain disk to a 400-ml. beaker, and titrate with standard permanganate according to the method of Fowler and Bright ( 2 ) . Stir the solution occasionally for 5 to 10 minutes after the addition of sulfuric acid before starting the titration. After the solutions have been prepared, duplicate determinations of calcium by this method require less than 2 hours. In some cases, instead of titrating the precipitate with permanganate, one of the following gravimetric methods was applied: (1) The precipitate was dried at 110" C. for 2 hours and weighed as calcium oxalate monohydrate. (2) The precipitate was ignited for 1.5 hours a t 485" C. and weighed as calcium carbonate ( I I ) . (3) The precipitate was ignited in a covered platinum crucible to constant weight over a Meker blast burner, cooled over sodium hydroxide, and weighed quickly. DETERMINATION OF CALCIUMIS A LIMESTONE.Ignite a weighed sample of about 300 mg. in a platinum crucible. To the cool residue add cautiously 2 ml. of water, then 2 ml. of 6 M hydrochloric acid. Stir the mixture, crush any lumps, and transfer to a 400-ml. beaker. Repeat the addition of water and acid to the crucible, warm the crucible gently, and transfer the contents to the beaker, rinsing thoroughly. Heat the solution in the beaker until the sample is completely dissolved except for a residue of silica. Dilute the solution to about 100 ml. Add a slight excess of bromine (as judged from the color) and 10 drops of 0.1 per cent bromophenol blue. Now add 6 M ammonia dropwise with stirring until the solution turns blue or until the precipitation of ferric hydroxide indicates that the solution is nearly neutral. Add 12 ml. 2.0 M formic acid, dilute to 140 ml., and continue as in the foregoing procedure, using the volumetric method. In the case of argillaceous limestones, sodium carbonate must be added before the ignition.
Results COJIPARIsON OF METHODS FOR TRE.4TMENT O F P R E C I P I TATED CALCIUM OXALATEMONOHYDRATE. I n a series of
experiments, known quantities of about 2 millimoles of calcium were taken, and calcium oxalate was precipitated as described above. The precipit'ates \vere treated by one of the foregoing procedures. The results are shown in Table I. EFFECT OF THE QUANTITYOF C A L c I u h s . I n another series of experiments, varying known quantities of calcium were taken and determined as above. The results are shown in 929
INDUSTRIAL AND ENGINEERING CHEMISTRY
930
OF METHODS FOR THE TREATMENT OF TABLE I. COMPARISON CALCIUM OXALATE
Method
No. of Determinations
KMnO4 CaCIOd. Hz0 CaCOa CaO
6
Relative Mean Relative Mean Deviation Error Parts p e r thousand
*0.6
6
4 2
10.5
-1.8 -0.8
10.3 *3
+5
-1.0
OF QUANTITY OF CALCIUM TABLE 11. EFFECT
Ca Taken Millimoles
Method
C a Found
Millimolea 0.1944 0.1930 0.5036 0.5020 0.997 1.005 0.997 3.989 3.984
0.2004 '3.4996 0.999 3.998
PH 3.74 3.72 3.73 3.67
TABLE 111. EFFECTOF pH HCOOH Taken
HCOONHd Taken
pH
Miriimoica 67 69 70 71 72
Millimoles 4.4 59 123 438 1430
3.97 3.47 3.17 2.54 1.65
.Mean Error Millimoles X 10' -3
Table I1 indicates that the quantity of calcium may vary from 0.5 to 4.0 millimoles. Table I11 indicates that minor variations of the pH from the recommended value of 3.70 have no effect. The minimum p H for quantitative precipitation of calcium as the oxalate is about 2.5. The effect of phosphate on the calcium determination depends on the method of treating the precipitate. When the sample contains about equimolar quantities of calcium and phosphate, sufficient calcium phosphate is coprecipitated to give an appreciable negative error by the volumetric procedure (Table IV). This error becomes negligible, however, if a gravimetric method is used because the calcium phosphate contributes to the weight of the precipitate. I n the analysis of an argillaceous limestone (sample 1 contained 18.15 per cent silica), the sodium introduced as carbonate for the ignition is later coprecipitated as sodium oxalate and causes a slight positive error. This error is insignscant if only 25 mg. of sodium carbonate are used. Since potassium is less coprecipitated by calcium oxalate than sodium (4), it is probable that potassium carbonate would be a better flux for this purpose.
Summary
2:.
-2 - 17
Table 11. The values in column 4 were obtained by measuring the p H of the filtrate with a Beckman p H meter. EFFECTOF pH. I n another series of experiments, the quantities of ammonium formate and formic acid were altered so as to vary the pH. Two millimoles of calcium were taken in all cases, and the precipitates were weighed as calcium oxalate monohydrate. The results are shown in Table 111, where each entry represents the mean of two determinations which differed from each other by 0.001 millimole or less. EFFECTOF ACCOMPANYING IONS. A known quantity of calcium, approximately 2 millimoles, was taken in each case, mixed with the indicated quantity of other ion, and determined as above by titration with permanganate. The results are shown in Table IV. DETERMINATION O F CALCIUM IP; LIMESTONE. Since the quantities of other elements that are found in a limestone sample sufficient to yield 2 millimoles of calcium are far below the amounts that introduce serious errors as indicated in Table IV, good results should be obtained when the foregoing method is applied to a limestone. The results of such analyses of two limestones from the National Bureau of Standards are shown in Table V. When the calcium was determined as calcium oxide or calcium carbonate, the silica was removed by the standard procedure before the determination of calcium. The filtrate in these determinations had pH values between 3.69 and 3.80. The quantity of sodium carbonate shown in column 3 was mixed with the sample before the ignition.
Vol. 14. No. 12
A rapid and accurate method for the determination of calcium is described. The effect of seven elements commonly associated with calcium on the accuracy of this method has been investigated. This method is recommended for limestone and similar material but not for sand or other substances very low in calcium.
TABLE IV. EFFECTOF ACCOMPANYING IONS Ion AI+++
Fe'++
Fe++ Mgft
Quantity No. of of Ion Determinations ~i11im01~
2
0.500 1.00
1
2.00 0.050 0.100 0.200 0.300 0.400 0.500 1 .oo 1.50 2.00 0.100 0.500 2.00 5.00
2
PO,---
Na
2.OOb 0.200 0.500
Ti+++t
1.00 5.00 0.010 0.030 0.050
0
++I110 +11 +11 + 9 - 1
+ll
0 + I
1 1
2 2
+ 1
+11
- 1 + 4 + 5 4-8
1
2 1 1
- 6 - 4 -13 + 1 - 4 + 2 + 6 10 +17 + 1
1 1
2 2 2 1 1 1 1 1
Weighed as CaCpOn.Hz0.
b
1 3
3 1 1
3.72 3.67 3.66
3.72 3.73 3.71 3.66 3.74 3.70 3.68
1 8 1
3.70 3.70 3.70 3.70
1
1 1
+
1 1
PH
X for-
+ 6
2 2
1
Mean Deviation
+11
1 1
0 . ozo a
+ 2
3
0.005 0.010 0.020 0.030 0.500 1.00 2.00 2.00'
Error -Millimolea - 2 + 1
1 1 1 1 1
5.00°
hh++
Mean
3.70 3.69 3.72 3.69 3.67 3.60
+ 5
+11 +21 Weighed as CsCOa
Discussion From Table I it appears that the method gives excellent results when the precipitate is treated volumetrically or weighed aa calcium oxalate monohydrate or calcium carbonate. High and less precise results are obtained by weighing as calcium oxide. However, from Table 1V it is clear that the weighing as calcium oxalate monohydrate also gives high and erratic results when magnesium is present in the sample. Probably traces of occluded magnesium oxalate hinder the drying process.
TABLE V. ANALYSISOF LIMESTONES Sample Correct NazCOs No. CsO Taken Method
% 30.49
Mo.
88 1
37.65
0 25 150
0 0 0
KMnOd CaO CaCOa KMnOd KMnOd KMnO4
No: of Determinations 5 2 1
2 6 4
Mean CaO Found
% 30.30 30.37 30.30 37.15 37.74 37.86
Mean Deviation
% 0.06
0.01
0.24 0.04 0.10
December 15, 1942
93 1
ANALYTICAL EDITION
Literature Cited (1) Chapman, H. D., Soil Sci., 26,479 (1928). (2) Fowler, R. W.,and Bright, H. A,, J . Research N d . Bur. Stmdurds, 15, 493 (1935). (3) Hillebrand, W.F.,and Lundell, G. E. F., “Applied Inorganic Analysis”, p. 501,New York, John Wiley & Sons, 1929. (4) Kolthoff, I. M., and Sandell, E. B.,J. phys. Chsm., 37, 443 (1933). (6) McComas, W.H., Jr., doctor’s thesis, Rutgers University, 1942.
(6) McComas, W. H., Jr., and Rieman, W., 111, accepted for publication in J . Am. Chem. Sac. (7) McCrudden, F., J . Biol. C h a . , 7 , 83 (1909); 10, 187 (1911). (8) Perciabosco, E., Ann. d i m . applicata, 30,362 (1940). (9) Shohl, A. T., J . B i d . Chem., 50, 527 (1922). (10) Walden, G. H., Jr., Hammett, L. P., and Edmonds, S. M., J . Am. Chem. Soc., 56, 350 (1934). (11) Willard, H. E., and Boldyreff, A. W., Zbid., 52,1888 (1930). PB~SENTED before the Division of Analytical and Micro Chemistry at the 104th Meeting of the AUERICAN CHIMICALSOCIETY, Buffalo, N. Y .
Color Reactions of 1,lO-Phenanthroline Derivatives M. L. MOSS WITH M. G. MELLON, Purdue University, Lafayette, Ind., AND G. FREDERICK SMITH, University of Illinois, Urbana, Ill.
M
Determination of Iron
ANY examples of the influence of slight changes of
structure on the behavior of organic reagents may be found among the benzene arsonic and arsinic acids, a score of which have been prepared and studied. Differences between 8-hydroxyquinoline and its derivatives, such as ferron, have also been reported. The oxidation potential of the ferrous o-phenanthroline complex is 1.14 volts compared with 1.25 for the corresponding nitro derivative, nitro-ferroin, and the range of high-potential, oxidimetric indicators is thus extended (3). Dimethylglyoxime, after 35 years of useful service as the nickel reagent, may soon be rendered obsolete by 1,%cyclohexanedionedioxime. Since reactions involving formation of the metal complexes appear to be susceptible to slight changes in the reagent used, it is of interest to study the effect of various substituents whenever possible. These effects are usually unpredictable. Accordingly, a comparison of 1,lO-phenanthroline and five of its derivatives was made with respect to their reactions with iron, copper, and molybdenum. (The various phenanthroline derivatives were prepared by Frederic Richter a t the University of Illinois. The methods will be described in a paper by him and G. Frederick Smith.)
COLORREACTION. Substitution of various groups in 1,lOphenanthroline does not materially affect the hue of the ferrous complexes except in the case of the nitro derivative which has a more purplish color than the others (Figures 1 and 2). A weak absorption band at 480 mfi characteristic of the other compounds is absent. An extra band a t 440 mp is exhibited by the &methyl but not by the 5-nitro-6-methyl derivative. The latter compound gives a slightly lower transmission for 2 p. p. m. of iron than the others, although the difference is too small to be significant. I n more highly concentrated solutions-e. g., 0.025 M in iron-the nitro derivative appears visually to be the most sensitive. For solutions of 50 ml. containing 2 p. p. m. of iron, 2 to 3 ml. of reagent solution provide an excess for development of the maximum color and larger amounts do not affect the intensity, provided the acidity is not less than the lower limits specified in Table I.
Apparatus and Solutions Transmittancy measurements were made with a General Electric recording spectrophotometer set for a spectral band width 10 mp. pH values were measured with a glass electrode meter calibrated with Clark and Lubs buffers. One-tenth er cent solutions of the following com ounds were prepared by &solution in 95 per cent ethanol and Zlution with redistilled water: 1,lO-phenanthroline and its 5-bromo, 5-chloro, 5-methyl, nitro (position in ring uncertain), and 5-nitro-&methyl derivatives. The last two are practically insoluble in water and require about 80 per cent alcohol for complete dissolution. The final alcohol content was 5 per cent by volume for the four other reagent solutions. A standard ferric nitrate solution prepared from iron wire of reagent grade contained 0.01 mg. iron per ml. and sufficient nitric acid to prevent hydrolysis. Measured quantities of this solution were reduced with a 10 per cent solution of hydroxylamine hydrochloride to obtain known concentrations of ferrous iron. The merits of this reducing agent have been established (1). Twenty per cent ammonium acetate solution was used as a buffer and pH adjustments were made with 6 N solutions of ammonium hydroxide and hydrochloric acid. Copper sulfate solution containing 0.05 mg. copper per ml. was pre ared by dissolving cop er wire in nitric acid, evaporating wit{ sulfuric acid, and suitagly diluting with redistilled water. A standard molybdate solution, made by dissolving 1.503 grams of molybdic oxide in 10 ml. of 10 per cent sodium hydroxide solution and diluting to 1 liter, contained 1 mg. of molybdenum per ml. A solution of chlorostannous acid contained 110 grams of c. P. “stsnnous chloride” and 170 ml. of concentrated hydrochloric acid per liter.
1
1
L ‘CO
420
140
‘$0
480
500
1
520
540
560
580
690
Wavdengfh, mp
620
610
660
680
