The combination of a rapid hydrolysis of the nitrate ester with the Devarda reaction and a simple titration of the distilling ammonia has resulted in a rapid method, which is less sensitive to environmental influence and "operator experience" than the gas-volumetric techniques such as the Lunge method and the Schlosing method. It is pointed out that the application of an automatic titration of the distilling ammonia could further reduce the possibility of human errors.
ACKNOWLEDGMENT The technical assistance of W. R. Urban is gratefully acknowledged. Thanks are also due to the department of analytical chemistry of the Technical University Twente for performing the Dumas analyses. Received for review April 6, 1973. Accepted September 4, 1973.
Consecutive Titration of Calcium and Magnesium in Ethanol-Water Mixture Bo Wallen Department of Analytical Chemistry, University of Uppsala, S-751 21 Uppsala 1, Sweden
Recently Rorabacher et al. ( 1 ) suggested that the result of an analytical procedure normally carried out in water might be improved if performed in a mixed solvent. This suggestion was based on the results of a determination of the stability constants of some metal ions with polyamines and polyaminopolycarboxylic acids in water-methano1 mixtures. The values of the stability constants generally increased with the methanol content of the medium. End-point determinations in complexometric titrations may hence be improved in mixed solvents [See also ( 2 ) and ( 3 ) ] .It was also observed that the changes in the stabilities of the complexes varied from one metal ion to another, a fact that might be used to increase the selectivity ofa titration. Some years ago a method based on these effects was developed in this laboratory for the simultaneous determination of calcium and magnesium. It will now be briefly presented as an example of what can be achieved by changing the medium. With a mercury indicator electrode, two potential breaks are obtained when a mixture of calcium and magnesium is titrated with ethylene glycol bis-(Pamhoethylether)-N,N-tetraacetic acid (EGTA) a t pH 10 in an aqueous medium containing 7040% (v/v) ethanol or methanol. In water, only the break corresponding to the titration of calcium is observed. The magnesium endpoint break is obtained, however, a t the expense of the quality of the calcium end-point break. To avoid this negative effect, calcium is titrated as usual in water a t pH 8.5-9.0. When the end-point break for the calcium titration has been obtained, the titration is interrupted and alcohol added to make the solution 70-8070 (v/v) with respect to this component. The pH is raised a t the same time to about 10 and the titration continued until the magnesium end-point break is obtained.
EXPERIMENTAL Apparatus. A fiber-tip. mercury(1) sulfate reference electrode and an amalgamared silver wire indicator electrode ( 4 ) were used. (1)
D. 8. Rorabacher. B.
J.
Blencoe, and D. W . Parker, Anal. Chem.,
44, 2339 (1972).
(2) G. Schwarzenbach and H . Ackermann, Helv. Chim. Acta, 31, 1029 (1948). (3) T. A. Kiss, F. F. Gaal, T. M. Suranyi, and I . J . Zsigrai, Anal. Chim. Acta, 43, 340 (1968). ( 4 ) C. N. Reilley. R. W. Schmid, and D. W. Lsmson, Anal. Chem.. 30, 953 (1958)
304
The titration curves were recorded with a Metrohm Potentiograph E 336. A Metrohm pH meter E 396 equipped with a Metrohm 109 UX glass electrode and a mercury(1) sulfate reference electrode were used in the adjustment of pH. The instrument was calibrated against NBS standard buffers. Solutions. Standard solutions (0.01M) of calcium and magnesium were prepared by dissolving weighed amounts of primary standard calcium carbonate and magnesium metal in nitric acid and sulfuric acid, respectively. The calcium carbonate (Merck) was dried for 24 hours a t 150 'C before use. The magnesium metal (Johnson, Matthey & Co., Ltd.) contained not more than 0.01% by weight of total metallic impurities. The metal was washed successively in 5M hydrochloric acid, water, ethanol, and acetone and was allowed to dry a t room temperature. A 0.01M solution of EGTA (Eastman) was prepared by dissolving the acid in the appropriate amount of sodium hydroxide. The solution was standardized against calcium at pH 9.0. A solution (0.002M) of EGTA-mercurate(I1) was prepared by mixing equivalent amounts of mercury(I1)acetate (Mallinckrodt p.a) and EGTA (Eastman). Ammonium nitrate (1M) and concentrated ammonia solution were used for buffering. End Point. The end point was determined either from the inflection point of the curve or in the case of asymmetrical titration curves from the point of steepest potential break. The uncertainty in the end-point volume was estimated to +ti.til cm3 or less. Procedure. Add 2 cm3 of 1M ammonium nitrate to the sample, which contains 0.005-0.1 mmole of each of calcium and magnesium, and introduce concentrated ammonia dropwise until the pH is 8.8-9.0. Add a few drops of 0.002M mercury(I1)-EGTA and titrate with 0.01M EGTA using the amalgamated silver wiremercury(1) sulfate electrode combination. Stop the titration shortly after the first end-point break, which corresponds to the amount of calcium present, and make the solution 80% (v/v) in ethanol. Add a few drops of concentrated ammonia solution, so that the p H meter reading is 10. Continue the titration until a potential break is obtained. The EGTA consumed between the first and second potential break corresponds to the amount of magnesium present.
RESULTS AND DISCUSSION
A representative selection of results is shown in Table I. The errors in the calcium titrations are probably due to the uncertainty in the location of the end point, since the theoretically calculated titration errors (5, 6) are less than 0.1% assuming ApM = 0.1. The differences, which appear to be systematic in nature, correspond to an overtitration (5) L. G . Sillen in "Treatise on Analytical Chemistry," I . M . Kolthoff and
P. J. Elving, E d . , Part I, Vol. 1. The Interscience Encyclopedia, Inc., New York, N.Y.. 1959. (6) A. Johansson and E. Wanninen, Sw. Kern. Tidsltr., 77, 492 (1965)
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c.3
Ca Mg 9 1
Mg 1 9
TPH90
Ca Hg 1 1
pHloo
50mV
I
L.
8
9 C M aoiu ~ EGTA
Figure 1. Titration of
b
IO
I
C M 3 M I M EGTA
..
8
9
C M 3 QOlM EGTA
mixtures of calcium and magnesium with 0.0100M EGTA
Table I. Recovery of Calcium and Magnesium f r o m Standard Solutions Calcium, mg
Magnesium, rng Ca:Mg
Taken
Found
Diffemnce, %
Taken
Found
Difference, %
mole ratio
0.201 0.402 0.402 1.207 2.013 3.622 2.013 2.013 2.013
0.202 0.405 0.403 1.212 2.017 3.630 2.015 2.020 2.015
+O .5
1.235 2.223 1.235 1.235 0.123 0.247 0.247 1.235 0.741
1.229 2.216 1.220 1.230 0.120 0.244 0.243 1.225 0.725
-0.5 -0.3 -1 . 2 -0.4 -2.2 -1.2 -1.5 -0.8 -2.2
1:lO 1:9 1:5 6:lO 1O:l 9:l 5:l 1:l 10:6
+O .7 +0.2 +O . 4 +0.2 +0.2 +0.1 +O . 3 f0.1
by 0.01 cm3. A similar error is present in the magnesium determinations. Titration Curves. Titration curves for different ratios of calcium and magnesium in the sample are reproduced in Figure 1. They show that the potential of the indicator electrode changes sharply at the equivalence point corresponding to the titration of calcium, whereas the endpoint break of magnesium is less sharp. Attempts have been made to use a Gran function to determine the equivalence point of magnesium. Gran functions have proved useful, when one is dealing with less sharp end-point breaks (7-10). The data obtained in the titrations of aqueous solutions of calcium with EGTA in ammonia buffer a t pH 9.0 did not, however, give straight lines when the values of the appropriate Gran function (10) were plotted us. the volume of added EGTA after the equivalence point. Further investigations of the be(7) G. Gran, Analysf (London). 77, 661 (1952). (8) F. lngrnan and E. Still, Talanta, 13, 1431 (1966). (9) D. Dyrssen. D. Jagner. and F. Wengelin, "Calculation of Ionic Equilibria and Titration Procedures," Alrnquist 8 Wiksell, Stockholm, Sweden, 1968; Wiley, New York. N.Y.. 1968. p 204. (10) A. Johansson. Talanfa, 20, 89 (1973).
havior of the mercury electrode [J-type ( 4 ) ] in different buffers, which presently are performed in this laboratory. seem to confirm that this electrode does not follow Nernst's equation at higher pH and especially not in ammonia buffer. Potential-pH ,titrations according to Reilley and Schmid ( 1 1 ) of calcium and magnesium with EGTA have been made in different ethanol-water mixtures. Our preliminary experiments have shown that the potentials in the pH-independent regions are shifted toward higher potentials with increasing concentrations of ethanol. Furthermore the change of the magnesium level is consistently greater than that of calcium. This seems to be in agreement with the results reported by Rorabacher et al. (I) and also explains why the stepwise titration of calcium and magnesium is possible. ACKNOWLEDGMENT
I wish to thank F. Nydahl for suggesting this problem and A. Olin for helpful discussions. Received for review June 25, 1973. Accepted August 28, 1973. (11) C. N . Reilleyand R. W. Schmid.AnaL Chern., 30, 947 (1958)
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