as the second wave of the nickel-DCH complex. EFFECT OF PH. The analytical method for D C H does not require a close control of p H a t p H approximately 12. The maximum pH reading that could be obtained with as much as 4 to 5 grams of H M D in 25 ml. of aqueous solution is about 12.2. I n a pH range extending from 11.7 to 12.2 constant diffusion current and halfwave potentials were obtained. Below p H 11.7, the diffusion current increases rapidly with changes in pH. ilt pH 9, the amount of diffusion current obtained for a given amount of D C H is approximately three times the current obtained a t p H 11.7. Below pH 9 hydrated nickel ions are present and produce interfering polarograms. Figure 1 shows polarograms for the nickel-DCH complex a t pH 12.1, 11, and 10. The shift in half-wave potentials to more positive values with decreasing pH is also shown in Figure 1. There is an obvious loss in the sensitivity of the method described above on carrying out analyses a t high p H values, but this is necessary to eliminate interferences from other impurities present in H M D which are discussed below. IXTERFERENCES. The only known substance a t present which offers interference to the method is aminom e t hy1 c y c 1o p e n t y l a m in e (Ah1C) , Nickel ions react with this impurity in
HMD, t o form a complex that reduces a t a slightly more positive voltage than the nickel-DCH complex, and could possibly lead to erroneously high D C H contents. The diffusion current produced by the nickel-AMC complex is also very sensitive to p H and a t values near 12, fortunately, the interference from AMC is eliminated, because the diffusion current is suppressed to negligible values. The AMC concentration would have to be several times the DCH concentrations to offer any serious interference to D C H analysis in the pH range of 11.7 to 12.2. The nickel-ARIC complex reduction always produced a polarogram with a maximum, but it could be eliminated by an 0.0015~0 concentration of an alkylaryl polyether alcohol. Gelatin would not suppress the maximum. The nickel complex technique also could be used as a method for determining A?tlC in HMD, but it would be most difficult to analyze for AMC in the presence of any DCH. I n the course of the analysis of refined H M D a t various pH values, an unknown nickel-amine complex was detected a t a pH 10 and lower. This reduction wave occurs a t a more positive voltage than the nickel-AMC wave, and it could interfere with an attempted analysis of AMC, but the wave is sufficiently removed from the nickelD C H wave not to offer any interfer-
ence. The unknown nickel-amine complex was found to be identical with the nickel-HMD complex which could be eliminated a t high p H values. Ammonium ions as high as 0.1M in the supporting electrolyte were found not to interfere in the analysis for DCH a t p H values above 11.7. Figure 2 shows typical polarograms obtained for nickel complexes of DCH, AMC, and HMD a t p H 10. Precision. The standard deviation of the method is *5y0 of the amount of DCH present in the concentration range of 0.3 to 1.0 mg. per 25 ml. of supporting electrolyte solution. Below 0.3 mg. per 25 ml. of electrolyte solution the standard deviation is approximately *lo% of the amount present. ACKNOWLEDGMENT
The author expresses his appreciation to E. D . Smith, who supplied some of the standard samples for this study, and to R. C. McNutt, Kho performed many of the analyses. LITERATURE CITED
(1) Horton, A. D., Thomason, P. F., Kelley, M. T., ANAL. CHEM.27, 269
(1955).
RECEIVED for review October 20, 1958. Accepted January 30, 1959. Contributiori No. 55, Research Center, The Chemstrarid Corp.
Polarographic Determination of Nitrilotriacetic Acid in (Ethylenedinitrilo) tetraacetic Acid ROGER L. DANIEL and R. BRUCE LeBLANC Texas Division, The Dow Chemical
Co., Freeport,
b The stabilities of the cadmium(ll) chelates of nitrilotriacetic and (ethylenedinitri1o)tetraacetic acids are so different that separate polarographic waves are produced on solutions of these two chelates. The wave of the cadmium nitrilotriacetic acid chelate precedes that of the cadmium (ethylenedinitri1o)tetraacetic acid chelate. The wave height for the former chelate is directly proportional to the concentration and is used as a quantitative measure of the nitrilotriacetic acid content of samples of (ethylenedinitril0)tetraacetic acid and its salts. Nitrilotriacetic acid, 0.03%, can still b e detected in (ethylenedinitri1o)tetraacetic acid samples.
S
methods exist for the determination of nitrilotriacetic acid
EVERAL
Tex.
(ETA) in (ethylenedinitri1o)tetraacetic acid (EDTA). Xone are satisfactory for low concentrations. The difference in the titers of the titration of an EDTA sample to a Tiron and a potentiometric end point gives a maximum figure for the nitrilotriacetic acid content (6). This method can detect no less than 1% nitrilotriacetic acid in 99% EDTA. With infrared analysis one can detect a minimum of 1% nitrilotriacetic acid in 99% EDTA (9). This analysis is performed on the dry acid only. A sample in any other form mould first have to be converted to the dry acid before this technique could be applied. Spectrophotometric methods on the metal ion chelates of these compounds It was found that were investigated. the bismuth(II1) ,copper(I1) ,and nickel(11) chelates do not have sufficient dif-
ference in wave lengths between the absorption bands of the nitrilotriacetic acid and the EDTA chelates for a satisfactory analysis ( 5 ) . It was estimated that about 10% nitrilotriacetic acid is the lower limit of detection in EDT-4 by this technique. Davies and Furness (3) reported on a polarographic method of analyzing the copper(I1) chelates of nitrilotriacetic acid and EDTA a t p H 8.2 The difference in the half-wave potentials of the nitrilotriacetic acid and the EDTA chelates was not sufficient to allow for analysis of mixtures of the tlvo. Daniel and LeBlanc ( 2 ) found that in the pH range of 7 to 10 there is sufficient separation of the half-wave potentials of the copper chelates to allow as little as 1% nitrilotriacetic acid to be detected in EDTA. Table I shows that, of the metals VOL. 31, NO. 7, JULY 1959
1221
Table 1. Instability Constants of Chelates of Nitrilotriacetic Acid and EDTA
(7,8) PK
Metal Ion lIri(11)
Fe(I1) Co(I1)
WIIi
C[;(II) Zn(I1) Cd(1I)
Pb(I1)
La(II1)
NTA
chelate
EDTA chelate
7.4 8 8 10.6 11.2 12.6 10.4 9.5 11.8 10 3
13 4 14 2 16 1 18.4 18.3 16 1 16.4 18 2 15 4
ApK 6 0 5 4 5 5 7.2 5.7 5.7 6.9 6.4 5.1
1' C. in a supporting electrolyte of EDTA were standardized by potentio0.1M potassium nitrate. KO maximum metric titration with standard ferric suppressor was necessary. Eastmsn nitrate (6). white label nitrilotriacetic acid, D ~ F analytical reagent disodium Versenate EXPERIMENTAL purified by the method of Blaedel ( I ) , A study mas made of the half-wave analytical reagent cadmium nitrate, and potentials of the chelates of cadmium a t analytical reagent potassium nitrate various pH's (Figure 1). The half-wave were used throughout the work. potential of the cadmium-nitrilotriacetic Solutions of nitriloacetic acid and
to
9
listed, cadmium(I1) and nickel(I1) have the largest ApKk between their nitrilotriacetic acid and EDTA chelates. Therefore, they should have the largest difference between the half-wave potentials of their nitrilotriacetic acid and EDTA complexes. The polarographic reduction of cadmium(I1) is usually reversible and the reduction of nickel(I1) is not (4). For these reasons the polarography of the cadmium(I1) chelates of nitrilotriacetic acid and E D T A was investigated.
7
5
4
3
I I
I -09
-0 8
APPARATUS A N D REAGENTS
I
I I I 0 -I I -I 2 -I 3 E d . IS S C E , Y O L T S -I
I / -1
4
0
6 10 16 WAVE H E I G H T
Figure 1. pH vs. Ed.#. for CdNTA- Figure 2. Wavz and CdEDTA-2 chelates height vs. pH for CdNTA- chelate
The polarograph used was a Leeds & Northrup Type E Electrochemograph. All polarograms were run at 25' f
Cd EDTA' I
I
I
I
I €de
Figure
I
I
I
I
I
VS S C E ,VOLTS
3. Polarogram of solution 1 0 3 4 in CdNTA- and 1 0-3M in CdEDTA-2 with a little excess Cd" ion Supporting electrolyte, 0.1 M KNOs.
1222
I
ANALYTICAL CHEMISTRY
N o maximum suppressor
acid chelate is iiidependent of pH above pH 5.5 and is equal to -0.97 volt us. the saturated calomel electrode (S.C.E.). Below p H 5.5 the half-wave potential decreases markedly to less negative values. The wave for the cadmium-EDTA chelate is poorly formed. Its diffusion coefficient is only about one t m t h that of the cadmium-nitrilotriacetic acid chelate. Only an estimate can be made of its half-wave potential. Above p H 5.5 the half-wave potential of this chelate is constant and about - 1.4 volts us. S.C.E. I3clow a p H of 5.5 the half-wave potcwtial drops to less negative values more sharply than does the half-wave potential of the cadmium-nitrilotriacetic acid chelate. Figure 2 shows the results of a study of the diffusion current of the cadmiumnitrilotriacetic acid chelate at various pH’s. Above pH 5.5 the diffusion current is independent of pH. Below this pII the diffusion current decreases, probably because of instability or a change in the form of the chelate in the low p H range. On the basis of the above information it was decided that a p H of about 10 would be satisfactory for a polarographic analysis of the cadmium-nitrilotriacetic acid cheIate in the presence of the cadmium-EDTA chelate. Calculations with the K,, of cadmium hydroxide showed that about 1O-jiM cadmium ion can be present in a solution at p H 10. Putting this figure into the equation for the instability constant of the cadmiumnitrilotriacetic acid chelate (Table I) shomd that this is in excess of the amount of cadmium necessary to complex all the nitrilotriacetic acid present in the concentration range a t which the samples were to be analyzed. The presence of a slight excess of cadmium ions in the solution can be detected by a small wl’ave at -0.60 volt vs. 8.C.E. due to the aquated eadmium ion. Figure 3 is R polarogram of a solution of cadmium-EDTA chelate and cadmium-nitrilotriacetic acid chelate with a small excess of cadmium ions a t a p H of IO. The small wave a t -0.60 volt is due to cadmium ions. The wave a t -0.97 volt is due to the cadmium-nitrilotriacetic acid chelate, while the poorly formed wave a t - 1.4 volts is due to the cadmium-EDTA chelate. i A plot of log id-i vs. Ed.s.for the cad-
P
I
t2
j t o e
4 - 0 4
4 - 0 8
4.1
Figure
6
I
4. Plot of log id
appear, indicating that excess C ~ ~ I I ~ I I I I I I nitrate was added. If no precipitate appears, more cadmium nitrate must bc added. The volume of the solution is adjusted to 100 ml. in a volumetric flask. This solution is uscd as a stock solution to make the dilutions for thc polarographic analysis. Ten milliliters of the stock solution and 10 ml. of 1M potassium nitrate are diluted to 100 ml. and the polarogram is obtained in the usual way from -0.4 to -1.3 volts. No maximum suppressor is necessary. The p H of this final solution is not critical. Because of dilution and carbon dioxide absorption it may drop a little. It can drop as low as 7 without undesirable effects.
-i
VS.
Ed.,. for CdNTA- chelate at pH 10
mium-nitrilotriacetic acid chelate at p H 10 is shown in Figure 4. Unlike the aquated cadmium ion, the cadmiumnitrilotriacetic acid- ion does not give a straight line; therefore, the reduction is not reversible. In spite of this, the diffusion current of the cadmium-nitrilotriacetic acid chelate is proportional to the concentration and small amounts of nitrilotriacetic acid can be analyzed polarographically in the presence of EDTA. A procedure for preparing samples was developed that can be varied as the sample varies. A sample weight is so chosen that a readable wave for the cadmium-nitrilotriacetic acid complex is obtained t o 10-6M concentration). For samples in the range 0.1 to 3% sodium nitrilotriacetate, a 5gram sample is adequate. The sample is diluted in two stages. First, it is put into 75 ml. of water, and the p H of the solution is adjusted to 10 with 1M nitric acid or 1M potassium hydroxide. A IM cadmium nitrate solution is added a drop at a time. The p H mill drop as the cadmium nitrate is added. When one drop of cadmium nitrate solution does not change the p H more than 0.1 unit, a sufficient amount has been added to complex all the nitrilotriacetic acid and EDTA. One or two drops in excess are then added. The p H is adjusted again to 10 with potassium hydroxide. At this point a slight precipitate of cadmium hydroxide should
The height of the cadmium-nitrilotriacetic acid wave (half-wave potential -0.97 volt) is measured and the concentration of the nitrilotriacetic acid obtained from a calibration curve of diffusion current us. concentration. The calibration curve for cadmium-nitrilotriacetic acid in the presence of cadmiuniEDTA is linear over the concentration range of 10-5 to 10-3M. Polarographic analysis of cadmiumnitrilotriacetic acid chelate allows for the determination of as little as 0.05 mole % nitrilotriacetic acid in EDTA (0.03 n-eight %). Accuracy is to 1575 of the amount prescnt or k0.03 actual %, Yhichever is the greater. LITERATURE CITED
W.J., Knight, H. T., SAL. CHEY.26,741 (1954). (2) Daniel, R. L., LeBlanc, R. B., Doa Chemical Co., Freeport, Tex., unpublished work. (3) Davies, W.C., Furness, W., Sbornik Mezin8rod. Polaroqraf. Sjezdu, Praze, 1st Congr., Pt. I, 28-50 (1951). (4) Kolthoff, I. hl., Lingane, J. J., “Polarography,” Vol. 11, pp. 486, 509, 2nd ed., Interscience, New York, 1952. (5) LeBlanc, R. B., Dow Chemical Co., Freeport, Tex., unpublished work. (6) MacNeil, R. L., Tynam, J. F., Dow Chemical Co., Frainingham, Mass., “Analysis of Diethylenetriamine Pentaacetic,, Acid-Sitrilotriacetic Acid Mixtures, Analytical Procedure, 1956. (7) Schwarzenbach, G., Freitag, Elsi, Helu. Chim.Acta 34, 1492-1502 (1951). (8) I b i d , pp 1503-8. (9) Spell, H. L., DOW Chemical Co., Freeport, Tex., private communication. (1) Blaedel,
RECEIVEDfor review August 22, 1958. Accepted November 5, 1958. Southncst Regional Meeting, ,ICs, San Antonio, Tex., December 1958.
VOL. 31, NO. 7, JULY 1959
1223
