556
ANALYTICAL CHEMISTRY
determination. Rieman and Helrich ( 7 ) separate the phosphate by a cation exchange resin and then perform a pH titration. The procedure described in this paper offers no new technique for separating phosphate but provides a new method for its determination. The flame photometric technique is rapid after the separation has been completed. The accuracy that can be expected is of the order of 1% when only a single determination is made. However, if several analyses are averaged, as shown in Table I, the accuracy may be as good as 3 to 5 parts per thousand. High concentrations of various anions mill interfere with the phosphate determination by this method. This makes the accurate determination of low percentages of phosphate in certain mixtures unattractive. Other Applications. It is obvious from the data presented that a small amount of phosphate constitutes a serious interference when calcium, magnesium, or potassium is being determined by the flame photometer. Thus, in order to determine any of these cations accurately in the presence of phosphate, the phosphate should be removed by an anion exchange resin prior to the determination. ..ilthough the determination of phosphate by the flame photom-
eter may have some limitations, it is significant that an inhibiting substance can be determined even though it does not emit in a Lundegardh flame. This principle may have some other useful applications. LITERATURE CITED
Barnes, R . B., Richardson, D., Berry, J. W., and Hood, R. L., IND. ENG.CHEM., ANAL. ED., 17, 605 (1945). Berry, J. W., Chappell, D. G., and Barnes, R. B., Ibid.,18, 19
(1946). Conrad, A. L., and Johnson, W.C., A s . 4 ~CHEX., . 22,1530 (1950). Gilbert, P. T., Hawes, R. C., and Beckman, A. O., Ibid., 22, 772 (1950). Leyton, L., Ann. Repts. Progr. Chem. (Chem. SOC. London), 45, 326 (1948). Parks, T. D., Johnson, H. O., and Lykken, L., ANAL.C m x , 20, 822 (1948). Rieman, W., and Helrich, K., IXD. ESG. CHEM.,ANAL.ED.,19, 651 (1947). \5'est, P. W., Folse, P., and llontgomery, D., ANAL.CHmi., 22, 667 (1950). RECEIVED for review July 25, 1953. Accepted October 24, 1953. T h i s research was supported b y Contract AT(30-1)-937, Scope I of the U. S. lltomic Energy Commission.
Amperometric Titration of Calcium With the Disodium Salt of Ethylenediaminetetraacetic Acid H. A. LAITINEN and R. F. SYMPSON Department o f Chemistry and Chemical Engineering, University o f Illinois, Urbana, HE use of an indicator in amperometric titrations was first described by Ringbom and Wilkman (IO),who added a substance yielding a polarographic current to the titration mixture. The indicator material would not react during the course of the titration, but would react with excess reagent in such a way that a sudden change in polarographic current at some appropriate applied potential occurred a t the end point. The indicator material was added in relatively small concentration, so that the observed change occurred in a narrow interval of the titration. In the present titration the indicator is present in a concentration comparable with that of the sample being determined. The end point is then determined by the traditional extrapolation method, which does not require the addition of small increments of reagent in the vicinity of the end point. The choice of an indicator ion is complicated by the fact that ethylenediaminetetraacetic acid (Versenate) forms more stable complexes with most of the common metallic ions than with calcium. It was desired to find an ion which could be complexed by some other agent to such a degree that the 1-ersenate would react with the calcium until it was titrated and then form the Versenate complex with the indicator ion, decreasing its diffusion current proportional to the decrease in its concentration. I n the ideal case the current would remain constant (except for dilution) until the end point, after which a linear decrease would occur. In selecting such an indicator ion and complexing agent, the equilibrium which exists a t the equivalence point and following the equivalence point must be considered. At the equivalence point of the calcium titration, the following equations describe the equilibria: MX;+p"
+ Cay--=
MYm-4
+ Ca++ + pXn
(1)
where AI" is the indicator metal ion, X" is the competing complexing agent, and Y - - - - is the Versenate ion. +
111.
The equilibrium constant, KA, may be evaluated from K,, K,, and K 8 , which are the dissociation constants of the following complexes:
cay-- $ cat++ y----
[ C a ~ + ] [ r ' - - - - ] = Kl
'
[Cay--]
The constant K A should be so small that Reaction 1 does not proceed to the right to any appreciable extent. Beyond the equivalence point, the following equilibrium must be considered: Jfx:+P"
+ Y----
MYm-4 f p-P
(2)
The equilibrium constant is given by
The constant K B must be large enough so that Reaction 2 proceeds quantitatively to the right as soon as an excess of Y----is present beyond the calcium end point. I t is not possible to make accurately qumtitative calculations using these equilibrium constants because of large and unpredictable ionic strength effects. In a qualitative way, however, it is possible to predict a combination of metal ion and complexing agent which might be satisfactory. According to Schwarzenbach and Ackerninnn (1%') the dissociation constant of the calcium Versenate complex is 2.6 X lo-". By an inspection of the magnitudes of the dissociation constants of various combinations of metal ions and complexing agents, it
V O L U M E 26, NO. 3, M A R C H 1 9 5 4 appeared that zincate ion should be satisfactory. For the reaction ZnOp--
+ 2Hp0 e Zn++ + 40H-
the dissociation constant is 3.6 X 10-le (8),while that of the zinc Versenate complex is 7.1 X 10-1' (13). Thus K4 is 1.3 X 1O-lo, indicating that practically all of the zinc would still be in the form of zincate in hydroxide solution a t the equivalence point. Kg is 5.1, indicating that zincate should be converted to the 1-ersenate complex whan a slight escess of T-ersenate is present. The half-wave potential of zincate ion in 1M sodium hydroxide is - 1.53 volts us. the saturated calomel electrode (6). The negative value is an obvious disadvantage from the viewpoint of interference from relatively high concentrations of polarographically reducible impurities. However, resdily reducible metal ions in general form more stable complexes with Versenate than does calcium, and would interfere with the titration reaction in any event. Other reducible materials which do not interfere with the F'ersenate reaction would yield an abnormally high but constant current before the end point. The removal of such reducible impurities to a concentration comparable to that of the cdcium does not appear to present insurmountable difficulties.
557
of zincate indicator solution, and titrating wih 0.05J1 sodium dihydrogen Versenate. In most titrations, the volume of reagent was less than 5 ml.; therefore no correction for dilution was necessary on the diffusion current readings. In 27 titrations, a standard deviation of 5.9 parts per thousand was observed. No trend of error with calcium concentration was detected in this series. Three titrations were run in which less than 2.2 mg. of calcium were present (initial concentration less than 0.001.11). The amounts of calcium present were 2.110, 1.877, and 1.831 mg. The amounts found, using the titrations at higher calcium concentrations as standardizations, were 2.071, 1.818, and 1.640 mg., respectively. I t appears that low results are obtained if the concentration of calcium is less than millimolar.
70-
EXPERIMENTAL
The apparatus was relatively simple. A large H-cell with a volume of approximately 100 ml. in one arm was used. This arm was connected by a frit)teddisk backed by agar gel to a saturated calomel electrode in the other arm. The circuit consisted of the saturated calomel electrode, the sample solution, and a dropping mercury electrode. The potential was applied and the current was measured with a Sargent manual polarograph Model 111. A 5-ml. semimicroburet was used to add the reagent. Yitrogen was bubbled through the solution for at least 2 minutes after each increment of reagent. Analytical grade sodium dihydrogen T'ersenate, manufactured by the Hach Chemical Co., was used. The zincate indicator solution was prepared by dissolving 0.01 mole of zinc oxide in a minimum of hydrochloric acid, diluting to approximately 1 liter, and adding 1 mole of potassium hydroxide and 0.5 mole of potassium chloride. The potassium chloride was added as additional supporting electrolyte, but does not appear to be necessary. RESULTS
.Itypical titration curve of 5.5 mg. of calcium in 50 ml. of solution containing 10 nil. of zincate indicator solution with 0.0531 Eodiuni dihydrogen 17ersenate is shoxvn in Figure 1. Because of the nature of the titration curve (constant current before the end point), it proved more satisfactory to interrupt the flow of nitrogen for each mensurement. rather than to use the previously described cell ( 7 ) designed for continuous passage of nitrogen, which limits the precision of current measurements. Preliminary experiment,s were run to establish the optimum hydroxyl ion concentration. If the hydroxyl ion concentration is too Ion., a precipit'ation of zinc hydroxide n-ill occur. Even in the abscnce of precipitate, the hydroxyl ion concentration must be high enough to prevent a premature end point due to reaction between zincate and I-ersenate prior to the calcium end point (Equation 1). If the concentration of hydroxyl ion is too high, the reaction between zincate and Versenate beyond the end point (Equation 2 ) will not be complete. Khen the concentration of hydroxyl ion was 100 to 140 times that of the zincate, good results were obtained. In nearly all tit'rations, a slight decrease of diffusion current near the end point was observed. Best results were obtained if current readings before 70y0and be)-ond 110% were estrapolsted to the end point. The concentration of zincate was chosen to yield a sufficientljlarge diffusion current for convenient measurement and yet to be sufficiently sensitive as an indicator. A concentration of 0.00231 zincate in the sample solution prior to titration proved convenient and was used throughout the present work. -4 series of titrat'ions was carried out using known amounts of calcium (3.166 t'o 11.12 mg.) in 50 ml. of solution, adding 10 ml.
50-
30-
IOI
,
I
IO
,
1
20
1
,
I
4c
30
K L OF VERSENATE
F i g u r e 1. Typical T i t r a t i o n Curve
Table I.
T i t r a t i o n of Calcium in Presence of l l a g n e s i u m
IJO-ml. sample, 10
1111. of zincate, titrated with 0.05.1f Verrenate) Del.., Ca Found, n-eight of 3Ig, Ca Present. 3Ig. Der. P.P.T. 11%. Mg.
0,493 1.478
4 693
5,273
.j.2.54
1.478
4.693 6.531 5,Zi.i .5 . 3 3 1 3.275 3.701
4 72.5 .i, ,548
1.478
1.9il 1.971 2 464
2,927
4.883
5,212 j.567 .i 307 . 3 io9
-0,008
-0,021 +0.032 -0,008 -0.053 +0.016 +0.032 +0.008
-1.7 -4.0 8
-1;.
-1.4 -10.0 f1.9 +G. I -2.2
The effect of magnesium is shon-n in Table I. The cnlculated amounts of calcium found are based upon standardization by the same method in the absence of magnesium. The standmi deviiition for these results is 5.6 parts per thousand, which is not significantly different from the results in the absence of magnesium. There is no noticeable systematic error as judged by the algebraic sum of the deviations. I n the presence of magnesium, a precipitate was observed, but it had no noticeable effect on t.he c:tlcium end point. The diffusion current of zincate \vas decreased appreciably in the presence of magnesium. For example, when the concentration of magnesium was 0.002431, the diffusion current due to the 0.002.11 zincate was 7.2 pa. as compared n-ith 11.5 pa. in the absence of magnesium. This decrease in diffusion current could be due to a coprecipitntion of zinc on the magnesium precipitate 01' to the formation of a magnesium zincate precipitate. COMPARISON WITH OTHER METHODS
Diehl, Goetz, and Hach ( 4 ) determined calcium in the presence of magnesium by an indirect volumetric procedure using
ANALYTICAL CHEMISTRY
558 Versenate as the titrating reagent. I n waters containing up to 1400 p.p.m. of calcium carbonate, the authors claimed an accuracy within 2 p.p.m. Betz and No11 (3), in a direct titration with Versenate, obtained an accuracy of 2y0 in the concentration range of 100 to 1200 p.p.m. Hahn ( 5 ) determined calcium by an alkalimetric titration with Versenate. The titration error for 100 ml. of water containing 50 to 400 mg. of calcium carbonate per liter was less than 5 mg. of calcium carbonate per liter when indicators were used and less than 1 mg. of calcium carbonate per liter in potentiometric titrations. Barreto ( 8 ) suggested a colorimetric method for calcium. Tyner (1.4) made a systematic study of this method and concluded thst there was a systematic error of +5% (relative) as compared with the stsndard Association of Official Agricultural Chemists volumetric procedure (1). Le Peintre (9),by rigorous control of pH, found no systematic error. West, Folse, and lllontgomery (15) determined calcium by flame spectrophotometry. For a concentration of 90 p,p,m. a standard deviation of 4.06 p.p.m. was obtained. At 50 p.p.ni. the standard deviation was 2.82 p.p.m. and a t 10 p.p.m. it was 3.28 p.p.m. Saifer and Clark ( I f ) describe a turbidimetric method for ralcium. I n the concentration range of 0.004 to 0.28 mg. of calcium per 10 ml., they obtained an average error of 2 ~ 4 % . The method described in this paper seems to be more accurate than the optical methods, but not so sensitive. Its accuracy compares favorably with that of most of the volumetric methods. PROCEDURE
Use 45 to 50 ml. of the sample solution containing not less than 2.2 mg. of calcium. Add to the sample solution 10 ml. of 0.01[11
zincate solution in 1 to 1.4M potassium hydroxide and 0.5X potassium chloride. If the sample was put in solution by adding excess acid, adjust the p H of the solution to nearly neutral before adding the zinc solution; or increase the hydroxyl ion concentration in the indicator solution in order to be sure the solution is sufficiently alkaline before the titration is begun. Remove oxygen from the solution by bubbling it with oxygen-free nitrogen. The solution is then ready to be titrated. The potential applied is - 1.700 volts va. the saturated calomel electrode. Take at least four readings both before and after the end point. Bubble the solution with nitrogen for at least 2 minutes after each addition from the buret. LITERATUKE CITED
(1) Assoc.
Offic.Agr. Chemists, “Officialand Tentative Methods of
Analysis,” 5th ed., p. 127, 1940. SOC. b r a d agron. (Aio de Janeiro), 8 , 351
(2) Barreto, A., Bol.
(1945). (3) Beta, J. D., and Noll, C. d.,J . Am. Water Works Assoc., 42, 49 (1950). (4) Diehl, H., Goeta, C. A., and Hach, C. C., Ibid., 42, 40 (1950). (5) Hahn. F. L., Anal. Chim. Acta. 4, 583 (1950). (6) Kolthoff, I. IM., and Lingane, J. J., “Polarography,” 2nd ed., p. 504, New York, Interscience Publishers, 1952. (7) Laitinen, H. A., and Burdett, L. W., ANAL. CHEM.,22, 833 (1950). (8) Latimer, W. Id.,“Oxidation Potentials,” 2nd ed., p. 170, New York, Prentice-Hall, 1952. (9) Le Peintre, M., Compt. rend., 231, 968 (1950). (IO) Ringbom, il., and Wilkman, B., Acta Chem. Scand., 3, 22 (1949). (11) Saifer, A,, and Clark, F. D., IXD.EKG.CHEM., ANAL.ED., 17, 757 (1945). (12) Schwarrenbaoh, G., and Ackermann, H., Helv. Chim. Acta, 30, 1798 (1947). (13) Schwaraenbach, G., and Freitag, Eisi, Ibid., 34, 1503 (1951). (14) Tyner, E. H., ANAL. CHEM., 20, 76 (1948). (15) West, P. W., Folse, P., and Montgomery, D., Ibid., 22, 667 (1950). RECEIVED for review June 29, 1953. rlccepted October 23, 1953
Determination of Fluoride Ion Using a Monohydroxy Azo Dye-Thorium lake JACK L. LAMBERT Kansas State College, Manhattan, Kan.
most successful colorimetric methods (1, 3) for the deterT mination of fluoride ion in small concentrations have been those using chelation compounds (lakes) of zirconium with 1,2HE
dihydroxyanthraquinone dyes. Fluoride ions, which form very stable complex ions with zirconium, react to disp1ac.e the dye molecules. Quantitative determinations are made by measuring the ratio of the displaced free dye to the color of the unrlianged lake. Other colorimetiic methods involve the bleaching of colored ferric complexes or pertitnnates by their reaction with fluoride ion to form stable, colorless complex ions (3). The work described here was undertaken to develop a colorimetric method in which the intensity of co101 developed in the treated sample would be dirertly proportional to the concentration of fluoride ion. -4n insoluble thorium-Amaranth lake supported on filter paper was found to exchange dye molecules rapidly for fluoride ions, producing solutions of Amaranth Tvhich closely approximated Beer’s law. I t is a justified assumption that a chelation compound in which ring closure with the zirconium atom was effected through two adjacent hydroxyl groups in the alizarin-type dyes would require either a stepwise replacement by two fluoride ions or (unlikely) a three-body collision of two fluoride ions and a zirconium-dye molecule. Either should result in a relatively slow reaction, which is the case in methods based on this type of reaction. The azo dyes studied in this work ran effect a ring closure with tho-
rium or zirconium through only one hydroxyl group and a coordination bond with a nitrogen atom of the azo linkage. The metalnitrogen (dye) bond should be much the weaker of the two bonds, and the breaking of the metal-oxygen (dye) bond should set free the dye molecule. Evidence supporting this idea is, first, the lack of known stable complex ions involving thorium-nitrogen or zirconium-nitrogen coordination bonds and, second, the very rapid reaction observed between fluoride ion and the thorium and zirconium lakes of the monohydroxy azo dye used in this work, x-hich indicates a bimolecular reaction. REAGERTS AND EQUIPMENT USED
Thorium nitrate, tetrahydrate, reagent grade, 1% solution. Amaranth, certified food color grade, 0 2% solution. Standard fluoride solution, 100 p.p.m., 0.221 gram of C.P. grade sodium fluoride per liter of solution. Filter paper, Whatman No. 42, 5.5-cm. diameter circles. Evaporating dishes, porcelain, Coors NO. 2 (90-mm. diameter). Interval timer. Spectrophotometer, Beckman Model DU, IO-mm. Corex cells.
Of the dyes a t hand, only Amaranth, Ponceau SX, and Acid Alizarin Red B contained just one hydroxyl group not adjacent to an oxygen-containing chelating group, and formed insoluble lakes with thorium and zirconium. The lakes of the other two dyes are much less satisfactory than the thorium and zirconium lakes of Amaranth, and the tinctorial powers of the free dyes are
