current research
Determination of Trace Quantities of Nitrilotriacetic Acid by Differential Cathode-Ray Polarography Badar K . Afghan and Peter D. Goulden Department of Energy, Mines and Resources, Inland Waters Branch, Water Quality Division, Ottawa 3, Ont., Canada
rn Two methods for the determination of nitrilotriacetic acid
in natural water and detergents are proposed, depending upon the nature of the sample. Determination of “free” NTA in natural water is based on the formation of a lead-NTA complex in alkaline medium at pH 8.0 using differential cathoderay polarography. Determination of “total” NTA is based on releasing NTA from metal complexes under acid conditions. The subsequent addition of EDTA and return of the sample to pH 8, preferentially bind the metals with EDTA. This frees NTA from metal complexes and the released NTA is then determined by lead-NTA method. The procedure for the determination of NTA content in detergents utilizes bismuth-NTA reduction wave at p H 2.0. The method is capable of detecting as little NTA as 10 pgiliter without any preconcentration of the sample.
N
itrilotriacetic acid (NTA)is beginning to replace phosphates in detergents-already some detergents marketed in the U.S. contain NTA. It is anticipated that the volume of NTA discharged into water will eventually equal that of phosphates. Because NTA is biodegradable (Swisher et al., 1967; Pfiel and Lee, 1968), the final concentration of samples taken for analysis is likely to be low; therefore, a method for determining very low concentrations is needed. There are several methods for the determination of NTA described in the literature but none is satisfactory for concentrations at pg/liter levels. Clinckermaille (1968) determined NTA in detergents by titration with cupric chloride using chrom azurol S as indicator. Phosphates interfere in this method and are removed by precipitation with tin(I1). Horacek and Pribil(l969) reported a consecutive potentiometric determination of NTA and other complexans by titration with iron(II1) chloride at pH 4 to 5 in the presence of ferroin and 1,lO-phenanthroline. West and coworkers (Hoyle and West, 1959; Hoyle et al., 1961) investigated polarographic resolution and semiquantitative estimation of different complexans, including NTA. Polarographic determination of NTA in EDTA has been studied by Daniel and LeBlanc (1959) and LeBlanc (1959), who deter-
mined NTA down to 0.03% in EDTA using the cadmium-NrA reduction wave in alkaline medium. The above method has been further improved by Farrow and Hill (1965), who used cathode-ray polarography (single cell) and increased sample weight to determine NTA content down to 20 mg/liter. Although sensitivities greater than lOWM can often be obtained by the use of cathode-ray polarography, the major limitations in achieving this are impurities in reagents, contamination during chemical procedure prior to polarography, charging current drawn by Helmhotz double layer at the cathode surface, etc. However, these difficulties can be easily eliminated by use of differential cathode-ray polarography. This technique utilizes a two-cell operation and eliminates all cell currents not relevant to the analysis. The twin-cell circuitory enables a resultant signal from the reference cell, containing all the reagents, to be balanced with the signal obtained from the cell containing the sample solution. Therefore, full use of current sensitivity of the instrument can be made to measure very small quantities of NTA. NTA is a powerful chelating agent which ordinarily will complex the so-called heavy metals in natural waters, with possible effect on the toxicity (Pickering and Henderson, 1966; Sprague, 1968) of these elements to aquatic life and on their transport in the water. The analytical method developed will distinguish between NTA complexed by these heavy metalssuch as copper, iron, lead, zinc, etc-and the alkali salts of NTA together with NTA complexed by the alkaline earth elements. In this communication, the NTA complexed by the alkaline earth elements is referred to as “free NTA.” This amount plus the NTA complexed by heavy metals is referred to as the “total NTA.” Methods are described for their determination using differential cathode-ray polarography. The method is capable of detecting concentration of 10 pg/liter as [N(CH2COOH)3] without any preconcentration. The procedure for the determination of NTA content in detergents utilizes bismuthNTA reduction wave at pH 2. This procedure is free of interferences and does not require separation of phosphates prior to the determination of NTA. Experimental Apparatus. Davis differential cathode-ray polarograph type A-1660, manufactured by Southern Analytical Ltd., Surrey, England, was used. The capillaries used in each of the two cells are matched so that when the mercury drop rates Volume 5, Number 7, July 1971 601
are adjusted (to 11 sec + 0.2) the signals from the two cells with the same solutions in them are effectively identical. A mercury pool reference electrode is employed during the analysis. Reagents. Bismuth stock solutions (lO-3M and 10-4M in 1M hydrochloric acid). NTA-Na2 stock solution (100 pgiliter as N(CH2COOH)8: Prepare 2.5 pg/liter standard NTA solution every day. Glycine/sodium hydroxide buffer pH 8.0 (pH 1 = 0.05M). Detergent solution: dissolve 0.25 g of detergent in deionized water and dilute to l liter. Preservation and Pretreatment of Water Samples. Acidify the samples to pH 1.0 with hydrochloric acid. Digest the sample at 90°C for 2 hr and neutralize to pH 7 before determining NTA. Acidification of samples prevents the further biodegradation of NTA in the samples and digestion hydrolyzes any tripolyphosphate present which also reacts with lead to give a reduction wave very close to lead-NTA wave. Procedure. (A) DETERVINATIOS OF FREENTA USING LEADAS REAGENT. Transfer 0 to 2.5-ml aliquots of standard NTA (2.5 mgjliter) solution to 25-ml volumetric flasks. Add 1 ml of 50 ppm lead solution to all the solutions except blank, 2.5 ml of glycine buffer pH 8.0, 2.5 ml of 1Msodium chloride, and dilute to 25 ml. Transfer 5 ml of blank containing no NTA to cell 2 and an equivalent amount of the lowest standard to cell 1. Bubble nitrogen through solution at equal rate for 10 min. Lower the capillaries, ensuring the skirt forms a water seal and degas for further 5 min. Polarograph the solutions between -0.5 to - 1.0 V vs. mercury pool reference electrode. The applied potential affects the surface tension of the mercury and, hence, alters the area of electrode. For quantitative work, it is essential that the start potential be kept the same for samples and their corresponding standard solutions. Measure the peak height of lead-NTA at peak potential (-0.9 V f 0.025) using suitable shunt scale and amplification scale setting. Convert observed peak height to the peak height at maximum sensitivity. Prepare a calibration curve by plotting peak height at maximum sensitivity vs. concentration of NTA. Prepare a sample solution containing suitable amount of sample as described above and corresponding blank without any lead. In some instances, where a sample contains a large excess of organic materials such as humic acid, higher concen-
b 09
o
trations of free lead in the solution are necessary for quantitative recovery of NTA. Polarograph the solution and read concentration of NTA from the calibration curve. (B) DETERMINATION OF TOTAL NTA. Add suitable aliquots of sample to a 50-ml beaker. Add 5 ml of 1M hydrochloric acid, 1 ml of 1 0 - 4 ~EDTA-Na2, 5 ml of 0.05M glycine, and dilute to 40 ml with distilled deionized water. Adjust the pH of the solution to 8.0 with sodium hydroxide and add 1 ml of 10F3Mlead solutjon. Transfer the contents of the beaker to 50-ml volumetric flask and dilute to the mark with deionized water. Prepare a blank in the same way, except lead, and polarograph the solution as recommended in the previous procedure. (C) DETERMINATION OF NTA CONTENT IN DETERGENTS USINGBISMUTH-NTACOMPLEX. Add 0- to 2.5-ml aliquots of standard NTA (1 X 10-4M)solution to 50-ml beakers. Add 1 ml of 10-3M bismuth stock solution to all beakers except the blank, 4 ml of 1M sodium chloride, and dilute to approximately 40 ml. Adjust p H of the solutions to 2.0 + 0.05 using dilute potassium hydroxide or hydrochloric acid. Transfer the contents of each beaker to 50-ml volumetric flask and dilute to the mark with deionized water. Polarograph the solution as recommended in procedure A and obtain polarogram between 0.0 to -0.5 V vs. mercury pool reference electrode. Peak potential of bismuth-NTA complex is -0.3 V f 0.02. Prepare a calibration curve by plotting peak height at maximum sensitivity vs. concentration of NTA. Transfer suitable aliquots of the detergent solution and proceed as above to obtain the NTA content. Results and Discussion
Effect of pH. A study was made of the nature of metal nitrilotriacetates of lead and bismuth at various pH’s (Figure 1). The results obtained showed that peak potentials of leadNTA moved to increasingly negative potentials with increases in pH. This suggests a more stable chelate formation at higher pH. The peak potential of lead-NTA complex is independent of pH between 6 to 9 and is equal to -0.9 V vs. mercury pool references electrode. However, above pH 9 the peak potential moves slightly to more positive values with the decrease
b
04-
-> J
Q w z
LO.8
0 3 .
CL
0.7
I
I
0
2
0.2
I 4
I 6
I 8
I IO
PH
Figure 1. Effect of pH on peak potential of (a) bismuth-NTA complex at pH 2 and (b) lead-NT.4 complex at pH 8 602 Environmental Science & Technology
I
I
2
4
I 6
I 8
I IO
I 12
PH
Figure 2. Effect of pH on peak height of A, bismuth-iWT.4 complex at pH 2; and .,lead-NTA complex at pH 8
0.9
0.8 h
4
v 2
-
4 c
tc
'
0.7
0.6 C O N C E N T R A T I O N O F CYANIDE IN MOLES PER M O L E OF L E A D - N T A C O M P L E X
Figure 3. Effect of cyanide ion concentration on 1 X lO-4M leadNTA complex at pH 8 using procedure A
0.5
0.6
from the heavy metal complexes, and the addition of cyanide to nitrilotriacetate complexes of cobalt, copper, iron, nickel, and zinc liberated NTA,which was then available for determination by the lead-NTA method. However, the reduction potential of lead-NTA wave, using a mercury pool as a reference electrode, shifted to a more positive value. This shift was proportional to free cyanide ion concentration in solution. Figure 3 shows the effect of increasing cyanide ion concentration on lead-NTA wave using mercury pool as an anode. This drift is characteristic of a mercury pool electrode, since similar solutions examined by ordinary dc polarography using external silver-silver chloride reference electrode did not alter the reduction characteristics of lead-NTA wave. This indicated that the shift in peak potential of lead-NTA wave, in presence of cyanide, might be due to a shift in potential of the mercury pool reference electrode. The use of silversilver chloride electrode was found to be possible in the system used and would enable cyanide to be used as the masking agent. However, the problems involved in the manipulation of such a system do not seem to justify the use of cyanide, particularly, as described below, it is found that EDTA acted as a satisfactory masking agent to release NTA from heavy-metal complexes and could be used in conjunction with a mercury pool electrode. EDTA forms stronger complexes with heavy metals than does NTA, and its use as a masking agent was investigated. A solution containing metal-NTA complexes at pH 8 was treated with an excess of EDTA in order to release NTA. Release of NTA was measured by lead-NTA method. The amount of lead added was sufficient to complex the released NTA and any excess EDTA not complexed with the heavy metals originally present in the sample. In alkaline medium, EDTA did not displace complexed NTA (Figure 4a). The procedure used was to acidify the sample to pH 1, add EDTA,and then bring pH back to 8. Under these
0.8 POTENTIAL (-V)
Table I. Recovery of NTA from Spiked Samples in Presence of Other Heavy Metals Foreign ions present, Pro- NTA found, 10-fold excess cedure mg/litera Recov,
Figure 4. Typical polarotraces showing the effect of EDTA in releasing NTA from a mixture of metallic nitrilotriacetates of cobalt, copper, iron, nickel, and zinc, each 2 X 10-6M (a) Released NTA measured when EDTA was added at pH 8. (b) Released NTA measured according to procedure B
in peak height which might indicate either the instability of the kad-NTA complex, or the formation of different species at high pH range. In case of bismuth-NTA complex there was a precipitate above pH 4, indicating the hydrolysis of the complex at higher pH values. Figure 2 shows the variation of peak height with lead and bismuth nitrilotriacetate complexes with PH. Effect of Masking Agents. In our preliminary investigations, NTA formed strong complexes with heavy metals-e&, cobalt, copper, iron, manganese, nickel, and zinc. The addition of lead at pH 8 does not displace NTA from these complexes. Therefore, to determine total NTA with use of the lead-NrA complex, a method had to be devised to release any complexed NTA from heavy metals, since these metals are normally present in natural waters. The use of masking agents which would form stronger complexes with heavy metals than with NTA was investigated. The masking agents investigated were cyanide, EDTA, and other aminopolycarboxylicacid. A solution containing different metallic nitrilotriacetates and cyanide at pH 10 was heated to 80°C to form cyano complexes and release coordinated NTA. Cyanide did displace NTA
A B
0.00 0.51
0.00 102
A B
0.00 0.48
0.00 96
Cadmium
A B
0.00 0.50
0.00 100
Iron 111
A B
0.15 0.52
30 104 64 108
Copper Cobalt
A
0.32
B
0.54
Nickel
A B
0.00 0.48
0.00 96
Zinc
A B
0.00 0.50
0.00 100
Copper, cobalt, cadmium, iron 111, manganese, nickel, and zinc
A B
0.00 0.48
0.00 96
Manganese
Q
O S mg/liter
O f NTA
was added t o each sample listed above.
Volume 5, Number 7, July 1971 603
Table 11. Reduction Characteristics of Other Complexans Structurally Related to NT.4 Using Lead and Bismuth Methods Peak potential vs. Metal ion, Complexail,a mercury pool rcf. 4 x Io-j!M lO-5M Procedure electrode Lead Lead Lead Lead Lead Lead
NTA
Bismuth Bismuth Bismuth Bismuth Bismuth Bismuth
NTA
DCyTA D TPA EGTA HEDTA
EDTA
DCyTA DTPA EGTA HEDTA EDTA
A A A A A A C C C C C C
-0.90 V NWb
NW
-0.86 -1.15 -1.73 -0.31 -0,57 -0.48 -0.41 -0.44 -0.50
NTA,nitrilotriacetic acid; DCYTA, 2-diaminocyclohexaiie-N,N,N’,N’tetraacetic acid; DTPA, diethylenetriaminepenta acetic acid; EGTA, ethylene glycol-bis(p-aminoethyl ether)N,N-tetraacetic acid; HEDTA, N-hydroxyethyl ethylene diamine triacetic acid; EDTA, ethylene diaminetetra acetic acid. No wave obtained. a
*
conditions there was complete exchange of NTA from heavy metal complexes with EDTA (Figure 4b). None of the heavy metal-EDTA complexes produce a reduction wave in the region of the kad-NTA wave, and hence the use of EDTA did not result in any interference with the determination of NTA using leadNTA complex. It was also shown that other aniinopolycarboxylic acids such as DCYTA and DPTA gave similar results to EDTA in displacement of NTA but offered no benefit over the use of EDTA. Table I shows the quantitative recovery of NTA in presence of other heavy metals with the above procedure, each result is the average of five determinations. Nature of the Complex. Continuous variations and mole ratio plots obtained from lead-NTA and bismuth-NTA com-
plexes suggest that the metal-to-NTA ratio is 1 to 1. Figures 5 and 6 show continuous variation and mole ratio plots for IO-jM bismuth and lead complexes at the optimum conditions described above. The reversibility of lead and bismuth nitrilotriacetate comagainst E for plexes was studied by plotting Iogl~[(i)/(i~-i)] bismuth and kad-NTA complexes at pH 2 and 8, respectively. The results indicated reversible behavior of these complexes with n = 2 and 3 for lead and bismuth, respectively. Reversibility of these complexes was further verified by examining the above solutions, containing bismuth and lead complexes, with increasing temperature. Peak potentials of the waves did not change with temperature. Cathode-ray polarography with reverse potential sweep, proposed by Davis and Shalgosky (1960) was also used to confirm the reversible behavior of these complexes. Interferences. The proposed methods, using lead and bismuth, have been tested in the presence of major ions present in natural waters, metal ions which form complexes with NTA, closely related aminopolycarboxylic acids, amino acids, pyro-, hexameta-, tripolyphosphate. Using lead-NTA method, ethylene-glycol-his(p-aminoethyl ether)-N,N-tetraacetic acid (EGTA)and tripolyphosphate interfered by giving a reduction waves with peak potentials very close to that of lead-NTA wave. However, the peak height of lead-EGTA complex is only 20 of the lead-NTA wave at equimolar concentrations using the recommended procedure. Interference of tripolyphosphate is eliminated by digesting the sample at p H 1 for 2 hr. In the presence of sulfate, lead is precipitated, which can be prevented by the addition of citrate, oxalate, or tartarate. Therefore, those samples which precipitate on the addition of lead should be treated with one of the above ions. In some natural waters there are high levels of organic materials which appear to make the lead unavailable for complexation with NTA. Under these conditions (as described in the procedure), it is necessary to add large amounts of lead for the quantitative recovery of NTA. In the case of bismuth-NTA method, tripolyphosphates d o not produce any prewave close to that of the bismuth-NTA reduction wave and aminopolycarboxylic acids give reduction
L
M O L E RATIO O F NTA TO METAL
Figure 5. Continuous variation plot of 0 , bismuth-NTA complex at pH 2; and A, lead-NTA complex at pH 8
0 0
04
0 8 I 2 16 M O L E S O F M E T A L P E Q M O L E OF N T A
2 0
Figure 6. Mole ratio plot of 0 , bismuth-NTA complex at pl-I 2; and A,lead-NTA complex at pH 8 604 Environmental Science & Technology
I
I
0 25 mg/LITER
NTA
Figure 7. Calibration curves for NTA: 0 , using lead-NTA complex according to procedures A and B; A, using bismuth-NTA complex according to procedure C
waves at more negative potentials. However, the peak potentials are not as well separated as in the case of lead chelates of these chelating agents. Table I1 shows the peak potentials obtained for bismuth and lead chelates with various aminopolycarboxylic acid. Iron I11 also forms NTA complex at pH 2, resulting in a prewave at -0.23 V vs. mercury pool. This wave is close to that of bismuth-mA. However, addition of ascorbic acid hydrox-
0 50 POTENTIAL (-V)
I .o
0 75
Figure 8. Typical polarotracesof 10-eM NTA using(a) bismuth-NTA complex according to the procedure C and (b) lead-NTA complex according to procedures A and B
ylamine hydrochloride to reduce iron I11 to iron I1 eliminates this prewave. Therefore, if the presence of iron I11 is suspected in samples, it should be reduced by ascorbic acid or hydroxylamine hydrochloride before the determination of NTA by bismuth method. Calibration Curves and Precision. Figure 7 shows the calibration curves obtained for NTA from 0.05 to 0.25 mg/liter using lead and bismuth methods. Typical polarotraces for
Table 111. Analysis of Lake Water Samples from Winnipeg Area NTA
Lake water no. Procedure L-227
L-227
L-241' (IM)
L-241' (IM)
303 (IM)
A
B
A
B
A
NTA
added found mg/liter
NTA
Recov,
Lake water no. Procedure
0.000 0.050 0.100 0.200
0.000 0.054 0.102 0.200
108 102 100
0.000 0.050 0.100 0.200
0.045 0.048 0.980 0.204
96 98 102
0.000 0.050 0.100 0.200
0.015 0.050 0.960 0.202
100 96 101
305
0.000 0.050 0.100 0.200
0.021 0.048 0.106 0.198
96 106 99
305
0.000 0.050 0.100 0.200
0.000 0.048 0.100 0.196
303 (IM)
304 (IM)
96 100
98
304 (IM)
B
A
B
A
B
NTA
added found mg/liter
Kecov Z
0.000 0.050 0.100 0.200
0.000 0.050 0.095 0.204
100 95 102
0.000 0.050 0.100
0.000 0.049 0.104
98 104
0.000 0.050 0.100
0.000 0.049 0.104
98 104
0.000 0.050 0.100 0.200
0.000 0.046 0.100 0.204
92 100 102
0.000 0.050 0.100 0.200
0.015 0.050 0.980 0.200
100 98 100
a 2.5 ml of 100 ppm lead was added instead of 1 ml of 50 ppm lead as recommended.
Volume 5, Number 7, July 1971 605
Table IV. Analysis of Detergent Formulations Using Bismuth Method NTA
Detergent no. 1,
found,
z
I . .
Amount added Amount found mg 1 .o 1 .o 2.0 2.0 5.0 5.1
2
15.2'
1.0 2.0 5.0
1. o 2.1 5.1
3
...
1.o 2.0 5.0
1. o 2.1
1. o 2.0 5.0
1.o 2.1 4.9
1.o 2.0 5.0
1. o 2.1 5.1
4
...
5
(L
10-6M NTA using lead and bismuth are shown in Figure 8. The identical curves were obtained on synthetic lake water. A series of experiments with various concentrations of NTA showed that peak heights were reproducible to within a graticule division irrespective of the number of scans on the solutions. The precision of the methods was calculated by determining the percentage standard deviation from the multiple analyses of a series of solutions, each containing 50 pg/ liter of NTA. The coefficient of variation was 5.4 and 3.8 for the lead and bismuth methods. Analysis of Lake Water Samples and Detergents. Tables 111 and IV show the results obtained for lake water samples from the Winnipeg area and NTA content of different detergent formulations.
5.1
6
12.4a
1.0 2.0 5.0
1 .o 2.0 5.1
7
5.6'
1 .o 2.0 5.0
1. o 2.1 5.0
Nitrilotriacetic acid is expressed as HsX. In detergent formulation
NassTA, H20 is added; therefore, above values should be multiplied
Literature Cited
Clinckemaille, G. G., Anal. Chin2 Acta 43,520 (1968). Daniel, R. L., LeBlanc, R. B., Anal Chirn. Acta 31,1221 (1959). Davis, H. M., Shalgosky, H. I . , J . Polarogr. SOC.6,12 (1960). Farrow, R. N. P., Hill, A. G . ,Analyst 90,241 (1965). Horacek, J., Pribii, R., Talunfa16,1495 (1969). Hoyle, W., Sanderson, I. P., West, T. S., J . Electronal. Chetn. 2, 166 (1961). Hoyle, W., West, T. S., Talantn 2,158 (1959). LeBlanc, R. B., Anal. Clrern. 31,1840 (1959). SCI.TECHNOL. 2, 543 (1968). Pfiel, B. H., Lee, G. F., ENVIRON. Pickering, Q. H., Henderson, C., Air W a f . Pollut. Znt. J . 10,453 (1966). Sprague, J. B., Nuture220,1345 (1968). Swisher. R. D.. Crutchfield. M. M.. Caldwell. D. W.. ENVIRON. SCI.TECHSOL. 1, 820 (1967).
by 1.44 to get correct values.
Receiced for reciew July 16, 1970. Accepted Dec. 24, 1970.
Classification of Organics in Secondary Effluents M e n a h e m Rebhun
and J o s e p h a M a n k a
Technion-Israel Institute of Technology, Sanitary Engineering Laboratories, Haifa, Israel
w The composition of soluble organics in secondary effluents was investigated. A fractionation procedure was applied enabling recovery and quantitative determination of humic substances as well as other chemical groupings present in secondary effluents. This procedure made possible the classification of the total organic content of secondary effluents. Forty to 5 0 z of the organics was classified as humic substances (humic, fulvic, and hymathomelanic acids), the fulvic acid being the major fraction of this class. The remainder of the organic matter consisted of (in percent): ether extractables, -8.3; anionic detergents, -13.9; carbohydrates, -11.5; proteins, -22.4; and tannins, -1.7.
T
he permanent increase of pollution in existing freshwater sources and the need to reuse waste water in many parts of the world are focusing attention on more efficient methods of contaminant removal. Effluents from the common biological treatment processes contain considerable
606 Environmental Science & Technology
amounts of organics. These organics are responsible for taste, odor, and color in contaminated water supplies; some of them may have toxic effects on the biota of freshwater bodies. In the case of waste water reuse for human consumption, attention has been given to the aesthetic as well as physiological effects of the organic residuals (McCallum, 1959; Ottoboni and Greenberg, 1970). Improvement of effluent (waste water) quality is planned mainly by application of physicochemical treatment processes. Since existing treatment plants apply biological methods, it is mostly their effluent that will be subjected to chemical and physical treatment, in this case tertiary treatment. Residual organics affect and are affected by many of the tertiary physicochemical processes (Dean, 1969). Better understanding of the composition and characteristics of the residual organics in secondary effluents, as well as in effluents after various stages of tertiary chemical treatment, would aid better design and operation of physicochemical treatment processes and the assessment of the significance of the organics in water quality. Little has been reported on the composition of secondary effluents, the most comprehensive reports available are those
