1640
ANALYTICAL CHEMISTRY
rcl. The correct choice of r will bring A Zinto the range of lower relative analysis error. This will now become
d>/s dT
- 0.4343 TB log Ts
But dca is the error in measurement of this new solution caused by an error d T in measuring the transmittancy of it. But this same absolute error in c, calculated as relative error in ci, will be r times the relative error calculated on cp. Thus, whatever value the function 0.4343/T2 log T , reaches, the error function *d will be r times as great. d Tz
0.4343 r 10 --rA l.rAl
But, since T is always between 0 and 1, T‘ is less than unity for positive values of r. Hence this quantity is greater than the relative analysis error for solutions of concentration ci when measured directly. I t is to be concluded that for solutions of low absorbancy no improvement in the precision of analysis is achieved by enriching the solution with the constituent being determined to the point of obtaining a final absorbancy corresponding to a low analysis error. LITERATURE CITED
Then,
0.4343 10- I 1,~ A I
(1) Ayres, G. H., AXAL.CHEM.,21, 652 (1949). (2) LeRoy, G., Ann. faZs. et f r a u d e s , IO, 208 (1917). (3) Mellon, M. G., “.lnalytical Absorption Spectroscopy,” p 101, Nevi Y o r k , John TTiley B- Sons, 1950. ( 4 ) Ibid., p. 110. (5) R i n g b o m , A., Z. anal. Chein., 332 (1949). (6) Sandell, E. B., “Colorimetric D e t e r m i n a t i o n of Traces of Metals.” 2nd ed., p. 64, New T o r k , Interscience Publishers, 1950.
-
RECEIVED for review February 14, 1!432.
Accepted J u l y 2. 1952.
Determination of Calcium, Magnesium, and Iron in limestone By Titration with Versenate KUANG LU CHENGI, TOUBY KURTZ, ~ N ROGER D H. BR.AY University of Illinois, Urbana, Ill. YTANDARD methods ( 1 ) of analysis for the major cations in limestone involve time-consuming precipitation and separation of calcium, magnesium, and iron. The procedure described in this paper permits direct determination without separation of these constituents by titrations based on the complexing action of Versenate a t appropriate pH levels. This proposed procedure is rapid and accurate, and lends itself to mass analysis technique. A well-known complexing compound (7), the disodium salt of (ethylenedinitrilo) tetraacetic acid (Versenate), has been applied successfullj7 to m t e r , soil, and plant material analyses (S-5), and recently to limestone analyses ( 2 ) . The application of this reagent to the analysis of limestone provides a simple procedure which is subject to little interference. PROCEDURE
Digestion of Sample. Weigh 1.000 gram of properly prepared sample into a 250-ml. beaker and add, cautiously, 10 ml. of water and 10 ml. of perchloric acid. Heat over a gentle flame on the hot plate until the solution becomes colorless ( a few hours may be required for silicates), and then evaporate the solution to dryness. Cool, and take up the residue by adding 3 ml. of 1 to 1 hydrochloric acid and 10 ml. of water. Filter off the silica, if necessary, and make up to 250 ml. with water. DETERMINATION OF CALCIUM
Reagents. VERSEXATE SOLUTIOK.Dissolve 4 grams of the disodium salt of (ethylenedinitrilo) tetraacetic acid in 1 liter of water. Standardize this solution against a standard calcium solution. STAKDARD CALCIUM SOLUTIOS. Dissolve 2.500 grams of reagent grade calcium carbonate in about 5 m]. of l to l hydrochloric acid and dilute to 1 liter nith water. This solution contains 1 mg. of calcium per milliliter. PoTAssIuar HYDROXIDE, 20% aqueous solution. Mix thoroughly 40 grams of CALCIUM IKDICATOR POWDER. powdered potassium sulfate and 0.2 gram of murexide. Murexide can be obtained from Eastman Kodak Co , or prepared from uric acid (3). Titration. Pipet a IO-ml. aliquot of the solution to be analyzed into a 200-ml. porcelain dish, then add about 20 ml. of water, 1 ml. of potassium hydroxide, and a tiny scoop of calcium indicator powder (20 to 30 mg.). Stir, and titrate with the 1
Present address, Commercial Solvents Corp., Terre Haute, Ind.
standardized Versenate. The end point is reached when the color changes from pink to violet. If overtitrated, the end point may be located conveniently by backtitrating with a calcium solution of the same normality as the Versenate solution. DETERMINATION OF MAGh ESIUM
Reagents. VERSENATE SOLUTION, as described under determination of calcium. BUFFERSOLUTION.Dissolve 60 grams of ammonium chloride in about 200 ml. of water, add 570 ml. of concentrated ammonium hydroxide, and dilute to 1 liter with water. POTASSIUM CYAXIDE, 10% aqueous solution. F241 INDICATOR. Dissolve 0.15 gram of Eriochrome Black T(F241) (also obtainable from Eastman Kodak Co.) and 0.5 gram of sodium borate in 25 ml. of methanol. Titration. Pipet a 10-ml. aliquot of the solution to be analyzed into a 250-ml. beaker, then add about 25 ml. of water, 2 to 3 ml. of buffer solution, a few drops of potassium cyanide solution, and 8 drops of F241 indicator. Stir, and titrate with the Versenate solution. The end point is reached when the color changes from wine red to pure blue. The volume of this titration is for both calcium and magnesium; however, magnesium can be calculated by subtracting the calcium value from the value for both calcium and magnesium. The Versenate solution can be either standardized by a standard magnesium solution (use MgSOa. 7HzO) according to the described procedure or calculated from the molality of the Versenate solution as found with the standard calcium solution. \T7hen difficulty in the F241 end point is encountered because of low magnesium in the sample, 10 to 15 ml. of a 100 p.p.m. standard magnesium solution may be titrated to the end point before the aliquot of the unknown is added. The titration may then be carried out as described above. A back-titration may also be used if the end point is difficult to detect. I n this technique ( 4 ) a definite excess amount of the standard Versenate is added to the aliquot of unknown and is back-titrated by a standard magnesium solution of the same molality. DETERMINATION OF IRON
as described under deterReagents. VERSEKSTESOLCTIOX, mination of calcium. SODIUM ACETdTE, 3570 aqueous Solution. Dissolve 2 grams of salicylic acid in 100 nil. of INDICATOR. methanol. STANDARD IRON SOLUTIOX.Dissolve 1.0000 gram of electrolytic iron in 10 ml. of 1 to 1 hydrochloric acid, digest on the steam bath until all iron has been dissolved, and make up to 1 liter with water.
V O L U M E 2 4 , N O . 10, O C T O B E R 1 9 5 2
1641
Titration. Pipet a 100-ml. aliquot of the solution to be analyzed into a 250-ml. beaker. Add sodium acetate reagent to adjust the reaction of the solution to p H 2 to 3, as indicated by a pH meter or suitable p H paper ( 4 ) . Then add 1 ml. of salicylic acid solution and titrate with the Versenate solution. The color change a t the end point is from purple-red to colorless or light yellow. Titrate until any reddish tint disappears. Versenate solution can be standardized against a standard iron solution according to the described procedure, but it is necessary to add a few crystals of ammonium persulfate to the standard iron s o h tion before titration to assure the oxidation of all ferrous ions to ferric. The iron equivalence of the Versenate solution can also be based on the of the Versenate as found with the standard calcium solution.
by Versenate titration. However, limestone normally contains only slight or moderate amounts of iron and aluminum, and very amounts Of manganese and phosphate. If the contains more than 1% of iron, the solution may turn from blue to bromrn a few minutes after reaching the end point, but this does not affect the accuracy of the results. If a high amount of either iron O r manganese is Present in the sample, it can be rapidly eeparated from calcium and magnesium by the carbamate method ( 5 ) . The titration in which murexide is used as the indicator is for calcium only, while the titration with F241 as indicator is for both calcium and magnesium. Thus, if two equal aliquot3
____--
Table I. Determination of Calcium, %Iagnesium,and Iron in Standard Limestone Samples ~ _ Calcirini _ Oxide, _ % _ ~ Magnesium Oxide, % Ferric Oxide. % Sample S o . Hach Chemical C:o 1010 1011 1012 1013 1014 1015 1026 NBS (6) la 88 Illinois State Geologica,l Survey N F 419 R-2215 N F 443 R-2726 N F 449 R-5188 a
b
Reported value
T'ersenate valuea
Difference
49.11 48.77 48.47 48.15 47.82 47.50 41.99
48.90 48.79 48.61 47.72 46.83 47.72 41.79
-0.21 -0.02 +O. 14 -0.43 -0.99 -0.22 -0.20
41 32 30.49
41.49 30.28
30.59
Reported value
T'ersenate value"
Difference
Reported balueb
3.10 3.39 3.67 3.96 4.24 4.53 9.37
3.10 3.25 3.66 3.86 4.27 4.27 9.36
0.00 -0.14 -0.01 -0.10 -0 0 3 -0.26 -0.01
1.38 1.54 1.74 1.86 2.05 2.23 0 725
1.40 1.53 1.77 I . 87 2.12 2.23 0.728
+O 03 +o, 01 +0.07 0.00 + O . 003
+ O . 17 -0.26
2.19 21.48
2.14 21.35
-0.05 -0.13
1.63 0.084
1.63 0.09
TO. 006
30.23
-0.36
21.52
20.95
-0.57
0.21
0.202
-0.008
55.53
54.09
-0.56
0.70
0.61
-0.09
0.11
0.11
0.00
53.23
52.16
-1.07
0.69
0.81
+o.
0.33
0.35
0.00
12
J'ersenate valuea
Difference f0.02 -0.01
0.00
Averagps of triplicates. Fez03 foi samples 1010-18 and 1026 was determined colorimetrically with o-ehenanthroline ( 8 ) by the senior author.
CALCULATIOIVS
If 1 gram of sample is made up to a 250-ml. solution and a 10-ml. aliquot is taken for titration, calcium and magnesium can be calculated as follows: A
25
1000
25
'2 X
1'66
1000
(D
100 = % calcium oxide -
B , X 100 = 70 magnesium oxide
If a 100-ml. aliquot of the 250-ml. solution made from the I-gram sample is taken for iron titration, iron can be calculated as follows: E
2'5
10001'43
X 100 =
% ferric oxide
A = milligrams of calcium per milliliter of Versenate solution B = milliliters of Versenate solution used in titration, with murexide as indicator c = milligrams of magnesium per milliliter of Versenate solution D = milliliters of Versenate solution used in titration, with F241 as indicator E = milligrams of iron per milliliter of Versenate solution F = milliliters of Versenate solution used in titration, with salicylic acid as indicator DISCUSSION
Data in Table I show that the results obtained with Versenate titration when analyzing standard limestone samples for calcium, magnesium, and iron are in good agreement with the results obtained by conventional procedures. The proposed Versenate procedure has the advantages of simplicity, rapidity, and accuracy. Usually iron, aluminum, manganese, and phosphate are important interfering ions in the magnesium determination
of solution are taken and one is titratpd xith murrxide as the indicator, and the other with F241 as the indicator, the difference betn-een the two titrations is the volume of Versenate needed to titrate the magnesium. The titration using salicylic acid as the indicator is for ferric ions only, since calcium and magnesium are not complexed by Vervnate in arid solutions. Strontium and barium, if presmt, do not affect the titration of calcium to the murrxide end point. Both ions will, however, titrate before magnesium and cause a positive error in the amount of this element. This error can almost always be ignored, since strontium and barium are rarely present in limestones in appreciable amounts. Interference bl- barium and strontium can, however, be eliminated by the addition of potassium sulfate to the unknown solution before the titration Tvith F241 is begun. 4CKROWLEDGMENT
The authors express thrir appreciation to J. E. Lamar of t h e Illinois State Geological Survey for supplying limestone samples. LITERATURE CITED
(1) .4ssoc. Official Agr. C h e m i s t s , "Official a n d Tentative M e t h o d s of Analysis," 6th ed., pp. 42-50, 1945. (2) Banewicz, J. L., a n d K e n n e r , C. T., ANAL. CHEM.,24, 1186 (1952). (3) C h e n g , K. L., a n d B r a y , R. H., Soil Sci., 72,449-55 (1951). (4) C h e n g , K. L., B r a y , R. H., arid Kurtz, T o u b y ( s u b m i t t e d for publication).
( 5 ) C h e n g , K. L., Melsted, S . IT., a n d Bray, R. H., Soil Sci. (December 1952). (6) N a t i o n a l Bureau of S t a n d a r d s , Cirr. 398, 18 (1949). (7) Schn-arzenbach, G., a n d A c k e r m a n n , H., H e h . Chim. Acta., 30, 1798-1807 (1947). (8) Shell, H. R., ANAL.CHEM.,22,326-8 (1950). RECEIVEDfor review April 25, 1952.
Accepted August 1, 1952.
