Spectrophotometric Technique for Calcium LEONARD ACONSKY' and MOTOKO MOR1 Department
o f Chemistry,
4 0 6 t h M e d i c a l General Laboratory, Tokyo, Japan
A method for the determination of calcium has been developedwhich utilizes theMurexide-ethylenediaminetetraacetic acid principle of colored complexes. The procedure requires the use of only 10 ml. of sample and can be run in about 3 minutes. With the aid of a spectrophotometer to detect the end point, the results are readily reproducible even with colored samples.
a t 610 mp and 100% transmittance with distilled water. The end point is reached when there is no further decrease in percentage transmittance and is detected by approaching it with a dropat-a-time addition of the EDT.4 when close to the lavender color. Standardization of EDTA. To a spectrophotometer tube containing 1.0 ml. of calcium standard solution add 9 ml. of distilled water, 1 ml. of sodium hydroxide, and 0.3 gram of Murexide indicator. Titrate n-ith EDTA as described above. (The 0.3 gram of Murexide indicator may be obtained by using a spoon marked to contain the required amount.)
T
HE calcium concentration in a water sample is significant in the determination of calcium hardness and also may be used to determine magnesium hardness by the difference between the llurexide and Eriochrome black titrations (1). .2 rapid, reproducible, and accurate method for the quantitative detection of calcium is based upon the titration of the sample with the disodium salt of ethylenediaminetetraacetic acid (EDTA, Versene) using Murexide (ammonium purpurate) as the indicator (4-6) a t a p H above 12.4 ( 2 ) . The color change during the titration is from pink to lavender and the end point is attained when there is no further darkening upon the addition of ethylenediaminetetraacetic acid. By the use of visual techniques the end point is not sharp ( 1 , 2 ) but with the aid of a spectrophotometer, as suggested by Fales ( S ) , a sharp end point can be detected and is easily duplicated even with colored samples. One calcium titration can be completed in about 3 minutes utilizing a 10-ml. sample. Table I. EDTA Buret Reading, 511. Start 0.038
End
Typical Run Data Spectrophotometer Reading,
%
Time, Min.
7 3 , O Pink
0,918 0.973 0,991
4 9 , O Lavender
1.030
46.5
0
Table 11. Comparative Calcium Determinations Ca Solution, P.P.M. 400
~~~
365 370 bo 0 50.20 19.8 19.6 10.0 10.4 5.4' 5.8 1.2 2.0 0.6= 0.6 0.1 0 2 Samples were colored and contained 10 p.p.111. of iron(II1).
50.0
20.0 10.0 5.0 1.0 0.5 0.1
I11 397 338 50 8 19.8 10.2 5.6 1.3 0.7 0.2
EYPERIMENT4L
The end point is detected by running the EDTA into the tube until there is a definite lavender tinge. Thereafter with a onedrop-at-a-time addition of EDTA (readings taken in the spectrophotometer after each drop), the reading prior to the drop which yields no further decrease in transmittance is the end point, For a typical run a 10.0-ml. sample, containing 20 p.p.m. of calcium, was used; 19.8 p.p.m. of calcium was detected using an EDTA standard of 0.208 mg. of calcium per milliliter of EDTA (Tables I and 11). DISCUSSION
4 6 , O End point
3
REAGENTS
The Murexide is unstable in aqueous solutions (1) and is, therefore, used in the solid state dispersed in sodium chloride, which is then stable for long periods of time. The EDTA in aqueous or solution has been found to be stable for a t least one month. The presence of nickel(I1) and cobalt(I1) can inhibit the determination. Iron(II1) and aluminum(TI1) within 15 p.p.m., copper(I1) within 0.3 p.p.m., and manganese(II), zinc(II), magnesium(I1). and sulfate and chloride ions do not affect the results. The effect of interfering ions can be eliminated by adding small amounts of sodium sulfide after the addition of sodium hydroxide. As can be seen from the data, the results are good over large ranges of calcium concentrations. This method is simpler and more rapid than the standard oxalate procedures.
SPECIAL EQUIPMENT
Coleman Junior spectrophotometer, Model 6A. Photometric test tubes, 19 X 150 mm. Microburet. PROCEDURE
Place 10.0 ml. of the water sample to be tested in a spectrophotometer tube. Then add 1 ml. of 1N sodium hydroxide and 0.3 gram of Murexide indicator to the tube and shake to dissolve the dye. (At this point the solution is pink.) Titrate with EDTA, using a microburet, to a lavender end point. Measure the end point precisely with the aid of a spectrophotometer set Present address, 25-53 38th St., Long Island City 3, N. Y .
308
364
47.3
Calcium Standard. Dissolve a 1-gram sample of calcium carbonate in dilute hydrochloric acid and dilute to 1 liter nith distilled water; 1.000 gram of calcium carbonate = 0.400 gram of calcium. Titrating Solution. Dissolve a sample, approximately 2 grams, of disodium ethylenediamine tetraacetate (Versene) in 1 liter of distilled water. Sodium Hydroxide, 1N. Dissolve 40 grams of sodium hydroxide in distilled water and dilute to 1 liter. Murexide Indicator. hIix and pulverize 0.20 gram OP Murexide (ammonium purpurate) n-ith 100 grams of sodium chloride
1
Runs I1 401
I
REFERENCES
(1) Bete, J. D., a n d Noll, C. A., J. Am. W a t e r Works Assoc., 42, 49 (1950).
(2) Buckley, E. S., Jr., Gibson, J. G., 11, a n d Bortolotti, T. R., J . Lab. Clin. 'Wed., 38, 761 (1951). (3) Fales, F. W., J . Biol. Chem., 204, 577 (1053). (4) Schwarzenbach, G., a n d Biederman, W., Chimia, 2, 56 (1948). (5) Schwarzenbach, G., B i e d e r m a n , W., a n d B a n g e r t e r , F., Helv. Chim. Acta, 29, 811 (1946). (6) Schwaraenbach, G., a n d Gysling, H., Ibid., 32, 1108, 1314, 1484 (1949).
( 7 ) Ibid., 32, 839, 1108, 1314, 1484, 1682 (1949). R E C E I V Efor D review September 21, 1954.
1001
Accepted January 28, 1955.
