Polarographic Determination of N,N‘-Ethylenediglycine and Nitrilotriacetic Acid in (Ethylenedinitril0)tetraacetic Acid R. BRUCE LeBLANC Texas Division, The Dow Chemical Co., Freeport, Tex.
b The polarographic half-wave potentials of the cadmium(l1) chelates of two common impurities in industrial (ethylenedinitri1o)tetraacetic acid, N,N’ethylenediglycine and nitrilotriacetic acid, occur at potentials too close to give resolution of the two waves. The diffusion current of this double wave can be used as a measure of the sum of the two components. The N,N‘ethylenediglycine i s converted in hot acid solution to a cyclic imide which does not form a stable chelate with cadmium(l1) under the conditions of the analysis. This affords a method of determining the nitrilotriacetic acid directly and the N,N’-ethylenediglycine by difference.
T
cadmium(I1) chelates of nitrilotriacetic acid (NTA) and (ethylrnedinitrilo) tetraacetic acid (EDTA) in aqueous solution a t a pH of 10 have sufficiently different half-wave potentials to allow for the analysis of as little as 0.03% nitrilotriacetic acid in (ethylenedinitrilo) t.etraacetic acid (2). When the N,N’-ethylenediglycine (EDDA) chelate of cadmium(I1) is present, its wave is not resolved from that of the cadmium(I1)-NTA chelate. Only the sum of the two wave heights can be measured. Figure 1 is a polarogram of a solution of the cadmium(I1) chelates of these three components. The wave a t -0.6 volt is due to the aquated cadmium ion; the one at -0.85 volt, to CdEDDA; the one a t -0.95 volt, to the CdNTA- ion; and the wave going off scale a t - 1.3 volts, to the CdEDTA-2 ion. This polarogram shows that accurate measurement of the wave heights of the individual species CdEDDA and CdNTA- would be difficult. N,N’-Ethylenediglycine in hot acid solution will be transformed to a cyclic imide according to the equation (3): HE
Cd f f
I
1
I
Ede VS
Figure 1.
I
I
I
I
S C E , VOLTS
Polarogram of solution of cadmium chelates
CdEDTA-2, 5mM; CdNTA-, 0.5mMI CdEDDA, 0.087mM1 KNO,, 0.1 M
The reaction is reversed in hot ,basic solution. The cyclic imide(1) does not form a stable chelate with cadmium(I1) a t a pH of 10, thus &ording a means of analyzing mixtures of NTA and EDDA by the following technique: The sum of the wave heights of thc cadmium(I1) chelates of the two com-
pounds is measured (Figure 1). A sample of the mixture of NTA and EDDA is treated with hot acid to convert the EDDA to (I). The wave height of the cadmium(I1) chelate of the NTA is obtained with this resultant solution, and the wave height of the cadmium(I1) chelate of EDDA is obtained by difference. APPARATUS AND REAGENTS
A Leeds & Northrup Type E Eleetrochemograph with dropping mercury electrode and H-cell was used for all the work. The nitrilotriacetic acid was Eastrnan 1840
ANALYTICAL CHEMISTRY
White Label material which assayed 1 0 0 ~ oby caustic titration. The N,N’-ethylenediglycine was prepared from a modification of Bersworth’s method (1). Formaldehyde, sodium cyanide, and ethylenediamine were reacted according to his procedure. The pH of the product solution was adjusted to 4.5 with hydrochloric acid instead of 6.5 with sulfuric acid, and the EDDA was precipitated from an acetone-water mixture. Recrystallization several times from acetone-water mixture gave a product which assayed 86.0% by caustic titration. There was 14.0% sodium chloride as the impurity. The cadmium nitrate, nitric acid, potassium nitrate, sodium hydroxide, and disodium (ethylenedinitri1o)tetraacetate were all of analytical reagent grade.
A second EDTA sample of approsiniately the same size is accurately weighed and dissolved in about 20 ml. of water. The pH is adjusted to 1.5 to 2.0 with 1M hydrochloric acid. This solution is carefully boiled on a hot plate for 20 minutes, cooled, and diluted to 50 ml. The pH is adjusted to 10 with 0.1M potassium hydroxide. This solution is then treated as described for the first sample. The polarogram obtained on this solution gives a wave for the cadmium-ETAonly. The wave height is used to calculate the NTA content of the EDTA. The wave height of the cadmiumNTA- is subtracted from the combined wave height of the cadmium-EDDA and the cadmium-NTA- to obtain the wave height of the cadmium-EDDA. From this difference the EDDA content of the EDTA can be calculated.
EXPERIMENTAL
A calibration curve of wave height us. concentration was made for the cadmium-EDDA chelate over thr cont o 10-3M. centration range of 5 X The polarograms were obtained on solutions which were 0.005M in cadmium-EDTA-?, 0.1M in potassium nitrate, O.Olyo in gelatin, and of various cadmium-EDDA concentrations. The pH of all solutions was 10. A calibration curve for the cadmiumNTA chelate was made under the ssme conditions. Concentration is proportional to wave height for both chelates. The diffusion coefficient of the cadmium-EDDA chelate is 90% of that of the cadmium-NTA chelate. A procedure for analyzing EDTA samples for impurities is as follows:
A sample containing about 0.5 mmole of EDTA is accurately weighed and dissolved in about 60 ml. of water. The pH is adjusted to 10 with 0.1M nitric acid or O.1M potassium hydroxide. A 0.1M solution of cadmium nitrate is added slowly with the pH maintained a t 10 until a slight turbidity due to cadmium hydroxide develops. This indicates that sufficient cadmium nitrate has been added. Ten milliliters of 1M potassium nitrate and 10 ml. of 0.1% gelatin are added and diluted to 100 ml. in a volumetric flask. The polarogram is obtained on this solution between -0.5 and -1.3 volts us. S.C.E. The wave height of the double wave of cadmium-EDDA and cadmium-NTA- is measured in the usual way.
DISCUSSION
The analytical method was checked by analyzing some standard blends of EDDA, NTA, and EDTA. Table I shows the results of three of these blends. In general, the accuracy of the analysis of NTA and EDDA is better than +loyo of the amount present. The limit of detection of both EDDA and KTA is in the range of 0.01 mmole per liter. Accuracy of the analysis for NTA is good in all ratios of NTA to EDDA. Accuracy is good for EDDA nt low ratios of NTA to EDDA but poor a t high ratios. In the first blend shown in Table I the ratio is about 10 to 1 and the error in the EDDA analysis is about %yo. If the ratio is much larger, EDDA could not be detected with any certainty.
termincd in EDTA by a slight modification of the above procedure. When boiled a t a pH of 12, it will be tranaformed to EDDA. The increase in the height of the double wave of EDDA and KTA would be D measure of the cyclic imide. N,N-Etheylenediglycine also forms a cyclic imide when boiled in acid (3): H+ HZN--C~H,-N(CH~COOH)~+ Hz Hz
c-c
/
HN
\
S-CH&OOH
+ HzO
(2)
C ‘J
//
0
Hz
The cadmium chelate of A’,.Vethylenediglycine has a polarographic half-wave potential of -0.90 volt L I S . S.C.E. It would interfere with the analysis of N,N’-ethylenediglycinc, but it has not been found in industrial EDTA products. If present, its i n t w ference could be circumvented becausci its cyclic imide will not re-form K,Kethylenediglycine when boiled in a solution a t a pH of 12. Depending on the method of prrparation, EDTA can have a variety of impurities. Some products contain (ethylenedinitri1o)triacetic acid. This compound can be detected in EDTA by this polarographic procedure. Thc cadmium(I1) chelate of this compound has a polarographic half-wave potential of -1.2 volts us. S.C.E. It does not interfere with the analysis for nor is its analysis interfered with by any of the impurities encountered in this work. Calibration was not made for this compound.
Table I.
Analysis of Blends of EDTA, EDDA, and NTA (Concentration of EDTA was 5mM) EDDA, mM NTA, mM
Added
Found
Added
Found
0.043 0.22 0.43
0.03 0.20 0.43
0.50
0.51 0.24 0.04
LITERATURE CITED
(1) Bersworth, F. C., U. S. Patent 2,558,923 (July 3,1951). (21 Daniel. R. L.. LeBlanc. R. B.. ANAL. CHEM. 3 i , 1221 (1959). ’ (3) LeBlanc, R. B., unpublished work. .
0.25 0.05
The cyclic imide(1) could be de-
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RECEIVEDfor review April 27, 1959. Accepted July 23, 1959. Division of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959
VOL 31, NO. 11, NOVEMBER 1959
1841
