sulfate and to phosphate are additive, and since the addition of 1OOOpg.of phosphorus (as phosphate) does not change the absorbance of a solution containing 25 pg. of sulfur (as sulfate). The ammonium form of Dowex 50 was used for the elimination of interfering cations be'cause the ammonium ions correspondingly liberated do not interfere in the reaction of sulfate with barium chloranilate in the presence of an ammonium formate buffer. The rapid and simple handling of a large number of samples was made possible by using the resin as a bulk and not as a column. This step is obviously omitted in the absence of interfering cations. Unsatisfactory results were obtained a t
first in the analysis of sulfur in samples containing calcium because of the very poor solubility of dry calcium sulfate in 50% ethanol. This difficulty is readily eliminated by first adding 0.6 ml. of water to the dry residue which is then easily brought into solution by shaking for 2 minutes. Aftervc-ards, 0.6. ml. of ethanol and 7 drops of Dowex 50 (ammonium) suspension are added and the ion-exchange step is continued as described in the procedure. ACKNOWLEDGMENT
The authors thank J. Folch-Pi for his interest in this work.
LITERATURE CITED
(1) Bertolanici, R. J., Barney, J. E., ANAL.CHEW29, 281 (1957). ( 2 ) Lees, M. B., Folch, J., Biochim. Bl'ophys. Acta 31, 272 (1959). ( 3 ) Lloyd, A. G., Biochem. J . 72, 133 (1959).. ( 4 ) LJJSSJ, I., Zarembo, J. E., ANAL. CHEM.3 0 , 4 2 8 (1958). ( 5 ) Schoniger, W., Microchem. Acta. 123 11955). (6) Spencer,' B., Biochem. J . 75, 435 (1960).
RECEIVEDfor review April 15, 1963. Accepted October 21, 1963. This investigation was done during the tenure of an Established Investigatorship of the American Heart Association. I t was supported by Research Grant SB-02840, Xational Institutes of Health, Feurological Diseases and Blindness.
(Eth yle nedi nit riIo)te t raacetic Acid Titration of Barium in Presence of Other Alkaline Earth Elements H.
F.
COMBS and E. L. GROVE
/ I T Research Institute, TO West 35th
Sf., Chicago J6, 111.
b The disodium salt of (ethylenedinitri1o)tetraacetic acid (EDTA) forms complexes with the other alkaline earth elements prior to complexing with barium. By double titrations using Eriochrome Black T and methylthymol blue (METAB) as indicators, barium and the number of milliequivalents of the other alkaline earth elements present can b e quantitatively determined. Magnesium controls the shape of the elementErio-EDTA titration curve so that it interferes with the procedure. Calcium and strontium can b e determined in the presence of each other.
M
(4) described a titrat'ion procedure for barium using disodium E D T h [disodium, dihydrogen salt of (ethylenedinitri1o)tetraacetic acid] and Eriochrome Iilack T as t'he indicator. Calcium and magnesium salts of EDT.1 were included in the titrant to improve the end point. This method was modified by Rowley et al. ( 6 ) who described a spectrophotometric titration procedure for milligram quantities of barium. Gordon ( 3 ) determined barium in the presence of small quantities of strontium by titrat'ing both with tetrasodium EDT-1. Then the strontium was determined by activation count, and the quantity of barium calculated by difference. Cohen and Gordon ( I ) also used Phthalein Purpur as the indicator in a spectrophotometric titration for small quantit,iee of barium. I n these articles there was no discussion roncerning the possible interference of the other alkaline earth elements. 400
ANNS
ANALYTICAL CHEMISTRY
At pH 10 to 11, EDT;I complexes with the alkaline earth elements in the order calcium, magnesium, strontium, and barium. Thus, it would appear that in an EDT.1 titration procedure for barium the other alkaline earth elements must be removed. This brief study resulted from the analysis of supposedly pure barium solutions found to contain appreciable quantities of other alkaline earth elements. EXPERIMENTAL
Materials and Equipment. EDTA Solution: 1.4890 grams of disodium (ethylenedinitri1o)tetraacetic acid per 2 liters of solution (0.002;M). This solution was standardized against a standard calcium solution and found to be 0.001994M. Buffer Solution: 1.66 grams of ammonium chloride and 166 ml. of concentrated ammonium hydroxide diluted to 1 liter. Eriochrome Black T : 50 mg. of the indicator dissolved in 50 ml. of triet hanolamine. JTethylthymol Blue: the sodium salt of 3,3' - [bis - -V,N' - di(carboxymethyl) aminomethyl] thymolsulfonphthalein in the form of M E T h B indicator tablets obtained from Fisher Scientific Co. Phthalein purple and methylthymol blue were briefly compared. Spectrophotometer recordings made with a Cary 11odel 14 indicated the bariumphthalein purple complex absorbance maximum to be at 572 mp and the barium methylthymol blue complex maximum to be a t 612 mp. The absorbance values a t the maxima were
equal when equivalent quantities of the indicators were complexed with equal and slightly in excess quantities of barium. However, the difference in the absorbance of the titrated samples and the maxima for the barium-complexes was found to be less for the phthalein purple. This was also observed in the actual titration curves for these barium complexes titrated with EDTA using the Fisher 590 mp band-pass filter. The observed difference in absorbance is not great but it definitely does exist. However, the shapes of the titration curves were the same. Methylthymol blue, obtained in tablet and in water soluble form, is convenient to use. There appears to be very little variation from tablet to tablet and no problem of stability has been observed. Standard barium, strontium, and magnesium stock solutions were 2.50, 2.50, and 1.00 mg. per ml., respectively. These solutions were prepared from Johnson Mathey Specpure chemicals. The standard calcium solution, 0.08016 mg. per ml., was prepared from XBS primary standard calcium carbonate. Working Solutions: diluted from the above stock solutions were 0.5 mg. of barium, 0.1 mg. of strontium, and 0.1 mg. of magnesium per ml. of solution. The barium was standardized gravimetrically and found to be 1 ml. = 0.4999 mg. -111 stock and working solutions were kept under argon. Beckman Model G pH meter. Fisher electrophotometer modified for photometric titrations. The cell compartment was modified to hold a 150-ml. borosilicate beaker in a fixed position. The beaker mas marked a t the 100-mi. level. The filters 6 5 0 - 1 n ~ and 590-mp were used for the Erio-
Chrome Black T and METAB indicators, respectively. The larger vo1umc:s were measured with 5-ml, semimicrciburets with platinum tips. The readings were taken with a buret-magnifier reader. Microsyringes were used for measuring small quantities. Procedure. Standardization with M E T A B indicator: Add 2 to 5 ml. of standard calcium solution from a semimicro buret into the titration beaker, and dilute to 50 ml. Add 0 . 5 5 sodium hydroxide until p H 12 is reached, then add 10 ml. of buffer solution, 25 ml. of 95% ethanol, and dilute t,o 100 ml. Add a M E T A B
tablet and keep beaker flooded with argon. Use the 590-mp filter. After the initial reading, add E D T A in small increments (0.1-0.2 ml.), stir for 30 seconds, stop stirrer, and take reading. Smaller increments should be taken near the end point. Plot absorbance as a function of milliliters of titrant to obtain the end point. For samples requiring only the METAB indicator, the procedure is the same. The procedure using Eriochrome Black T indicator is essentially the same. The pH is adjusted t o 11.1 f 0.1 with 1 t o 1 ammonium hydroxide. Then 10 ml. of buffer solution and 10 drops of Eriochrome Black T indicator are
Table I.
Added, Ba 300 0 500 0 300 0 500 0
Sr
added. Use the 650-mp filter. The remaining steps are the same as for the MET-IB indicator. Alkaline-Earth Elements Present. When other alkaline earth elements are present, except magnesium, titrate one aliquot sample using the M E T A B indicator and another with Eriochrome Black T. The M E T A U indicator will give a sharp break a t the total equivalence end point; the &iochrome Black T will give a break at the equivalence end point for calcium or calcium and strontium. The principle is illustrated in Figure 1 using barium and calcium. The difference between the first break in the Erio-
Summary of Titrations
pg.
Ca
? r lg
300.2 497.1
303.4 498.7
No. e.p.
200.0
198.2 105.1
241 . O 163.2 240,5 160.3
300 300 300 300 100
80 2 80 2 80 2 40 1 80 2 inn o 320 6 6'00873 meq. of Ba tnd Ca 200 0 200 0 0 00374 meq. of Ba tnd Sr 500 0 100 0 0 00478 mea. of Ba m d Sr 500 0 0 00775 meq. of Ba and Mg 500 0 0.00569 meq. of Ba :tnd Mg
159 9
Erio and METAB 305 8 78 9 79.4 289 5 83.4 298.3 298 4 41 2 94 2 83.5 With Erio; no e.p. to separate With Erio; no e.p. With Erio; no e.p. 100.0
50.0
100.0 500 0 0 00775 meq. of Ba and Mg 50.0 500 0 0 00569 meq. of 150 0 150 0 0 00471 meq. of 150 0 150 0 0 00471 meq. of 100 0 150 0 0 00314 meq. of 100 0 150 0 0 00314 meq. of 150 0 100 0 0 01044 meq. of 100,o 100 0 150 0 0 00834 meq. of Ba, Sr, Ca, and 0 80.2 100 0 150 0 0 00834 meq. of Ba, Sr, Ca, and Mg
160.3 50.0 160.3 50 0 160 3 100.0 80 2 0.00314 meq. of Sr and Ca
98.5 95.2
100.0 100.0 100.0 100.0
0 0 0 0 0
100.0
236 6
101.4 97.8 METAB only
0.00871 meq. of Ba and Ca 0,00378 meq. of Ba and Sr 0,00501 meq. of Ba and Sr
With Erio; 0 00744 meq. of Ba and Mg With Erio; 0 00562 meq. of Ba and Mg Titration curves for last two are very similar to that for Mg 0 00768 meq. of Ba and Mg
151 9 0 00467 meq. of Ba, Sr, and Ca 146 3 0 00469 meq. of Ba, Sr, and Ca 151 3 0 00316 meq. of Ba, Sr, and Ca 156 5 0 00308 meq. of Ba, Sr, and Ca With Erio; no e.p. to separate Titration curve similar to that for Ca With Erio; 0 00816 meq. of Ba, Sr, Ca, and Mg. Titration curve similar to that for Mg. 102 6 158 7 56 5 158 4 46 8 163 5 With Erio; e.p. very high
0 00573 meq. of Ba and Mg
0 01028 meq. of Ba, Sr, and Ca 0 00820 meq of Ba, Sr, Ca, and M g
0.00311 meq. of Sr and Ca
VOL. 36, NO. 2, FEBRUARY 1964
401
1
I
0
10
1 ml EDTA
I 20
1
30
Figure 1. Titration for barium with calcium present
chrome Black T curve and the end point represented by the lXETAB indicator represents the milliliters of titrant for barium. The first or definite end point in the Eriochrome Black T curve represents the milliliters of titrant equivalent to calcium or calcium and strontium. RESULTS A N D D I S C U S S I O N ,
The results of the different titrations using the two indicators on single or on combinations of the alkaline earth elements are summarized in Table I. When only calcium is present with barium, the amounts of both barium and calcium can be determined, except when calcium is the major constituent. The number of milliequivalents of calcium and strontium and the amounts of barium can be determined, except when the calcium and strontium are in large excess. The amounts of calcium and strontium in the presence of each other can also be determined except when strontium is in excess. KO end point is observed when strontium is titrated using Eriochrome 13lack T as the indicator. For this reason no titration curve for strontium appears in Figure 2. I n addition, when strontium is present with barium, the titration using Eriochrome Black T indicator has a very low slope value and does not yield a detectable end point. When magnesium is preqent with barium or with any other combination and Eriochrome Black T indicator is used, it is not possible to make any separate determination. The titration curve becomes essentially that for magnesium, Figure 2, even though it may be present only in small quantities. The value of the slope for each of the Table II.
EDTA
Magnesium Calcium Strontium Barium 402
108.89 (4]7 ) 101C.59 (7) 108.63
107.76
(7) (7)
ANALYTICAL CHEMISTRY
titration curves using Eriochrome Black T indicator, Figure 2, is related to the stability constant for the metal-indicator comple\. The high slope value for the magnesium-EDTA titration curve results from the high stability constant for the magnesium-Erio comples, Table 11, while the relatively low slope value for the barium-EDTA titration curve is primarily the result of the lorn stability of the barium-Erio complex The stability constant for strontium-Erio is apparently very small since there was not enough change in slope to detect an end point. .1ssuming the stability constant values by Diehl and Lindstrom ( 2 ) to be correct, the difference in stability constants for the two calcium complexes is lo6.'', while the difference between the two barium complexes is 105.76.I n the titration of a solution containing both comple\es, calcium-Erio and bariumErio, the calcium-Erio is titrated first; but because of the small difference in the above value< the calcium-EDTA does not control the slope of the total titration curve. For this reason a change in slope is also observed for the titration of the barium, although due to the influence of the calcium-EDTA complex this end point for barium is not stoichiometric. The less the amount of calcium present, the less the influence on the barium curve; the greater the amount of calcium present, the greater its influence until only one change in slope for the calcium-barium titration is observed. When both calcium and strontium are present with barium, the calcium still is the major influence on the shape of the EDTA%-Erio-titration curve.
1
L
-
1
I
I
I
I
ERIOCHROME BLACK T
I
a
,
I IO
I
ml.EDTA
I
20
I
l
30
Figure- 3. METAB-alkaline-earthEDTA titration curves
When calcium and strontium are determined in the presence of each other, the first break in the titration curve is the stoichiometric end point for calcium. The poor break that follows is not stoichiometric for strontium. K i t h the Eriochrome Black T indicator, barium cannot be determined in the presence of strontium because there is no change of slope corresponding to an end point for strontium or barium or for the sum of the two elements. The titration curve for each alkaline earth element-lIET.413 complex titrated with EDT.1, Figure 3, is similar to the titration curve for each of the alkaline earth element-Erio complexes. The EDTA titration of the magnesiumRIETAB comples produced the greatest change of slope, while the titration of the barium-METAB complex produced the least change. However, these slopes indicate less difference between the stability constant values of the alkaline earth-MET.113 complexes than between tlie stability constant values for the alkaline earth-Erio complexes. For this reaSon any two or more of these elements will yield only one end point, that of the total milliequivalents of the elements present. LITERATURE CITED
.'i I
0
I
IO
I ml EDTA
1
20
1
I 30
Figure 2. Erio-alka line-ea rfh-EDTA titration curves
Stability Constant Values
( I ) Cohen, A. I., Gordon, Louis, ANAL. CHEM.28, 1445 (1956). ( 2 ) Iliehl, Harvey, Lindstrom, Frederick, I b i d . , 31, 414 (1959). (3) Gordon, Louis, Reimer, C. C., Burtt, B. P., Ibid., 26, 842 (19.54). ( 4 ) Nanns, T. J., Reschovsky, M. U., Certa, A. J., I b i d . , 24, 908 (1952). ( 5 ) Rowley, Keith, Stoenner, R. W., Gordon, Louis, Ibid., 28, 136 (1966). (6) Schwarzenback, G., Biederman, W., Helv. C h i m . Acta 31, 6?! (1948).
( 7 ) U'elcher, Frank J., The Analytical Uses of Ethylenediarninetetraacetic Acid," p. 6, I-an Sost,rand, S e w York, 1958.
Eriochrome Black T RECEIVEDfor review Alap 23, 1963. Accepted October 30, 1963. Presented at Pittsburgh Conference on Analytiral Chemistry and Applied Spectroscopy, March 1963.
