Detection and Variable Time Kinetic Determination of Micro and Submicrogram Amounts of Nitrilotriacetic Acid Horacio A . Mottola and Garrg L . Heath’ Department of Chemistry, Oklahoma State Unicersity, Stillwater, Okla. 74074
The analytical use of the modifying effect of nitrilotriacetic acid (NTA) on the oxidation of Malachite Green cation by periodate ion, catalyzed by low concentrations of manganese(l1) ions has been extended to the detection and determination of NTA. A variable time kinetic method of analysis has been developed which allows detection and determination of ppm and fractions of ppm of NTA. This method uses photometric monitoring and digital electronic collection of the time elapsed as the system evolves between two preestablished chemical compositions. The effect of some metal ions, other aminopolycarboxylic acids, some products of degradation of NTA, and some other species expected to be present in detergent formulations, natural water, and industrial waters was also evaluated. The procedure is easily amenable to monitoring and repetitive analysis.
RECENTINTEREST
I N NITRILOTRIACETIC ACID (NTA) has mainly resulted from the fact that its trisodium salt has been proposed and used as a partial substitute for polyphosphate in heavy duty detergent formulations. In December 1970, the sale of detergents containing NTA was halted in the United States because of potential toxicity of NTA. The reports o n NTA toxicity are, however, confusing and point to the need for further research in this area. Biochemical and biological studies are surely being planned and pursued to evaluate the effects of NTA on the bio-environment. These considerations point to the relevance of the analytical chemistry of NTA and the potential usefulness of sensitive tests for this aminopolycarboxylic acid. Actually, NTA has received little attention as an analytical reagent beyond its occasional use as masking agent or in a few seldom used complexometric titrations ( I ) . Interest on the determination of NTA arose around 1965 as a result of the observation that small amounts of NTA have a n adverse effect on the quality of color change in some complexometric titrations (2, 3). Infrared analysis (4), polarography (4, 5 ) , and complexometric and redox titrations with potentiometric end-point indication (6-8) have also been applied for the determination of NTA in ethylenediamine-N,N,N’,N’tetraacetic acid (EDTA). More recently the determination of NTA has been focused o n samples of granular detergents, natural waters, and sewage. Complexometric titrations (including potentiometric end-point detection, use of metal ion-selective electrodes, and photometric end-point indicacation), indirect colorimetry, polarography, thermometric 1 Present address, Department of Chemistry, Southwest Missouri State College, Springfield, Mo. 65802.
(1) G. Schwarzenbach and H. Flashka, “Complexometric Titrations,’’ 2nd ed., Methuen & Co. Ltd., London, 1969, pp 9 and 10. (2) R. N. Farrow and H. G. Hill, Analyst (London),90, 210 (1965). (3) R. G. Monk, ibid., 91, 597 (1966). (4) R. L. Danield and R. B. LeBlanc, ANAL.CHEM., 31,1221 (1959). ( 5 ) R. B. LeBlanc, ibid., p, 1840. (6) D. L. Furman, G. W. Latimer, Jr., and J. Bishop, Talunta, 13, 103 (1966). (7) H. Holzapfel and K. Dittrich, ibid., p 136. (8) J. HoraEek and R . Piibil, ihid., 16,1495 (1969). 2322
titrations, gas and ion-exchange liquid chromatography, have been used as analytical approaches for the determination of NTA in these samples. A stopped-flow kinetic determination has also been reported (9). Most of these procedures are cited in recent contributions to the literature ( I 0-1 4 ) . The present investigation was prompted by the observation that NTA modifies the rate of catalyzed oxidation [Mn(II) as catalyst] of Malachite Green cation by periodate ion (15, 16). This effect has been used to noticeably increase the sensitivity of manganese(I1) determination in solution (17, 18). This paper reports the use of this modifying effect for the detection (at the submicromolar level) and determination of NTA (micro and submicrogram amounts). The kinetic procedure herein described provides a selective chemical probe for NTA. Part of this selectivity arises from the relatively low p H a t which the NTA effect can be detected. This reduces the possibility of interferences from other metal ions and complexing agent species. Ethylene-glycol bis(2-amino-ethyl ether)tetraacetic acid (EGTA) and NTA both increase considerably the catalytic effect of Mn(I1). The effect of NTA, however, is much more pronounced than that exhibited by EGTA. Determination was accomplished by means of a variabletime kinetic procedure (19) using photometric monitoring and digital electronic collection of the time elapsed as the system evolves between two preestablished chemical compositions. The procedure was tested in the presence of some products of degradation of NTA and other species expected to be present in detergent formulations, natural waters, and industrial waste waters. The proposed method compares well in detectability, sensitivity, and selectivity with those previously published and is easily amenable to monitoring and repetitive analysis. EXPERIMENTAL Apparatus. A flow-photometric system similar to one described previously (16) was used for monitoring absorbance changes, The output from the Beckman DB spectrophotometer was sent to a double switch network (Figure 1) assembled (9) D. W. Margerum and L. C. Coombs, Abstracts, 160 National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1970, paper No, 15, Division of Analytical Chemistry. (10) L. Rudling, Water Res., 5,831 (1971). (11) G. F. Longman, M. J. Stiff, and D. K. Gardiner, ibid., p 1171. (12) B. K. Afghan, P. D. Goulden, and J. F. Ryan, ANAL.CHEM., 44,354 (1972). (13) J. E. Longbottom, ibid., p 418. (14) N. Vanwelssenaers and G. G. Clinckemaille, A d Chin?. Arfa, 58,243 (1972). (15) H. A. Mottola and H. Freiser, ANAL.CHEM.,39, 1294 (1967). (16) H. A. Mottola, ibid., 42, 630 (1970). (17) H. A. Mottola and C. R. Harrison, Tulunfu, 18, 683 (1971). (18) H. A. Mottola, MPApplications Notes, 6, 17 (1971). (19) W. J. Blaedel and G. P. Hicks, in “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, C. N. Reilley, Ed., Interscience, New York, N.Y., 1964, p 126.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
from MP-modular units (McKee-Pedersen Instruments, Danville, Calif.). The chopper-stabilized operational amplifier was used to make the source impedance negligible, improve stability, and multiply the signal by a factor of ten. The operating characteristics of the double switch network have been described previously (18). Daily, before measurements were taken, the potentials corresponding t o zero absorbance reading (sample and reference cell with purified water), EL,and Err were adjusted t o read pre-established constant values in a Heath Universal Digital Instrument (Model EU-805) operating in the DVM mode. As a n alternative to the double switch circuit, the corresponding absorbance 6s. time curve may be obtained from a strip chart recorder and the elapsed time computed manually. As expected, however, this approach results in poorer accurac.y and reproducibility. Reagents and Solutions. The water used through this work was purified by ion exchange and double distillation. The possible metal content of samples of aminopolycarboxylic acids was reduced as described earlier (16). Malachite Green oxalate (Eastman White label) was used without further purification. The phosphate-acetate buffer used was also described elsewhere (16). The dodecylbenzene sodium sulfonate and sodium lauryl sulfate (K & K Laboratories) of practical grade were purified by evaporating the oily layer separated by adding isopropyl alcohol to a saturated aqueous solution. All other chemicals were of reagent grade. Procedure. Samples containing at least 1 nanomole of NTA and ranging between fractions of 1 ml t o 5 ml are added t o the reaction vessel. One milliliter of Malachite Green solution (15 mg/100 ml) and the Mn(I1) solution are then added and the volume is adjusted to 10 ml with concentrated acetate-phosphate buffer (16) to give a final p H of approximately 3.5. One milliliter of a 10% sodium periodate solution is instantaneously injected with the help of a hypodermic syringe to initiate the reaction. Even though the instantaneous injection of periodate provides a very simple and convenient means of initiating the reaction, it was observed that the addition of the periodate solution (concentrated solution t o assure pseudo-zero order dependence o n this reagent) at a constant rate (0.0006 m1;’sec was found satisfactory) resulted in a n improvement of precision. Addition of the periodate at a constant rate can be performed by a buret such as that described in reference (16). A more accurate and reproducible buret has been described recently (20). All results reported here were obtained a t constant room temperature with a variation of 1--3 “C. The short time required for determination makes close temperature control unnecessary.
*
RESULTS I ~ N D DISCUSSION Variable Time Procedure and NTA Effect. Most kinetic catalytic methods are oriented toward the determination of low concentrations of metal ion species in solution by either the so-called “constant time” or “variable time” (fixed concentration) procedures (19, 21, 22). Both are essentially integral methods in which a finite change in reactants and products is measured. If the finite change is very small and close to the initial time of reaction, the approaches qualify as differential (initial-rate) methods (22). Theoretical and experimental factors influencing the accuracy of these two procedures have been examined recently (23) indicating that
(20) H. Hall, B. E. Simpson, and H. A. Mottola. Anal. Biochern.. 45,453 (1 972). (21) K . B. Yatsimirskii, “Kinetic Methods of Analysis,” Pergamon Press, Oxford, 1966. (22) H. B. Mark, Jr., and G. A. Reclinitz, “Kinetics in Analytical Chemistry.” Interscience, New York, N.Y., 1968. (23) J. D. Ingle, Jr., and S. R. Crouch, ANAL. CHEM.,43, 697 (1971),
-
*
1N5221 1N5243
10 M
(-1 OAl
II
I
R1
OA2
t o input timer
R,:
E”+-+++-i
\
*
1N5223 1N5221
I
1 OA3
Figure 1. Circuit of double switch network and impedance matching amplifier OAl: MP-1031 0.42 and OA3: MP-1006A ,Eu and ET.taken from a MP-1008 millivolt source R,, RP,RS, R4, and R;: 10K
the variable time procedure is superior in the case of a nonlinear response of the reaction monitored signal. Consider the generalized case: C
R
+ X -+
Products
(1)
in which R is the monitored species, C is a catalytic species, and X is present in a large excess to assure pseudo-zero order dependence o n it. Addition of a complexing agent t o the system characterized by Reaction 1 may result in inhibition, true metal-complex catalysis, or ligand promotion (24). Analytical applications of inhibition have resulted in the development of some analytical methods centered around the concept of catalytic end-point indication (16). O n the other hand, true-metal catalysis and/or ligand promotion have been rather scarcely used for analytical purposes (25, 26). Assuming first-order dependence with respect to R and initial catalyst concentration, accounting for the fact that the uncatalyzed reaction proceeds simultaneously with the catalyzed one, and if one considers the case in which the ligand is the limiting species for complextion, the following generalized expression may be written :
-_ d[R1 dt _= kdR1
+ k,([Cl,
- [C*Ll)[RI
+ k,’[C*LI[Rl
(2)
where k,, k,, and k,‘ are constants for the system and experimental conditions considered and for the uncatalyzed and catalyzed paths, respectively. If true metal-complex catalysis is assumed and C*L is virtually constant in a given run, with C* representing a higher oxidation state for the catalytic species, rearranging and integrating between tl and ty gives: [C*L]
=
K At(k,’ - k,)
-
(ku
+ kc[Clo)
k,’
- k,
(3)
in which: K = ln([R11/[R12)= constant, if [Rll and [Rlz are kept constant from run to run; At = time needed by the system to change from the chemical composition characterized by [R]I t o that characterized by [RIy,respectively. Since the metal ion species is in excess, the equilibrium concentration of C*L would be proportional or for all practical purposes (24) A. E. Martell, Pitre Appl. Clwm., 17, 129 (1968). (25) P. R. Bontchev and B. Evtiinowa, Mikrocliim. Acta, 1968, 492. (26) P. R. Bontchev, A. Alexiev, and I. Dimitrova, ibid., 1970, 1104.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2323
concentration is negligible:
’’.o
9 1 70
d[R1 - k,[R] t k,’[R][C*L] dt
1
/*’
and [C*L] ‘v [C]
a
cNTA (CURVE A xio‘,
(4)
CURVE B ~ 1 0 ’ ~ )
Figure 2. Working curves for determination of NTA according to procedure, except as noted: A . Analytical concentration of Mn(I1): 5.00 X M. Reference potentials corresponding to 0.800 and 0.430 absorbanceunit B. Analytical concentration of Mn(I1): 1.00 X 10-sM. Reference potentials corresponding to 0.560 and 0.350 absorbanceunit
7.0
K ku -7 -- At k ,
k,’
Equations 3 and 5 show that the analytical application of the variable time procedure is the same for both reaction mechanisms except in the case of promotion with C, >> CL. Even in this case judicious selection of the values of [Rl1 and [R]z may lead to conditions of analytical significance. Considering that Mn(1I) is in at least tenfold excess with respect to NTA in the particular case of the results reported for NTA at the 10-7M level, the NTA effect appears to be true metal-chelate catalysis with metal complexation providing the rate limiting species. Calculations based on equilibrium considerations (27) show that the ratio of complexed (by NTA) to uncomplexed Mn(I1) at a pH -3.5 varies roughly from 5.5 x 10-j to 5.5 x 10-7 for the experimental conditions utilized to construct the working curves of Figure 2. The same ratio for Mn(III), however, varies from 7 X lo7to 7 X l o 5for the same conditions. The formation constant for the Mn(II1)-NTA complex used in these calculations was equal to 3.2 X 10’9 and estimated from the values for Mn(I1)-NTA and extrapolation of results reported for the Mn(II1) complexes of EDTA, rruns-1,2-diaminocqclohexanetetraacetic acid (CDTA), and hydroxyethylethylene diaminetriacetic acid (HEDTA) (28). This value compares well with a recently reported one of 1.8 x l o z o taking into account oxidation-reduction reactions of Mn(I1) with the chelating agent (29). Since the Mn(II1)NTA complex is relatively more stable in solution than that of EDTA or DTPA (29), it seems reasonable to postulate its participation in the catalytic cycle. Strong complexation of the lower oxidation state of manganese, Mn(II), as exemplified by the effect of EDTA (15), leads to inhibition by the ligand. Determination of NTA. Working curves (l!At us. NTA concentration) obtained in the range of 0.2 to 2 pg and 2 to 20 pg of NTA gave straight lines in agreement with Equation 3. The sensitivity for these determinations can be expressed as the slopes of these lines and is approximately equal to 0.01 sec-l pg-l for the determination a t the lO+M level and 0.004 sec-I pg-l at the lO-+M NTA. These working curves were obtained according to the described procedure with the only difference in a Mn(1I) concentration of 1 x 10-jM in detecting the lO-’M NTA level and one of 5 x 10-6M when determining the 10-6M level. This change was introduced to permit resolution of the times recorded at the highest concentrations of NTA determined and allow an extension of the method to this higher concentration. These changes in Mn(I1) concentration and adequate selection of reference potentials (absorbances) allow the determination of NTA within certain concentration levels without sample dilution. Seven (out of seven) individual determinations obtained with different sets of solutions and on different days gave average values of 2.99 and 7.05 for 3.00 and 7.00 micromolar solutions with standard deviations of 0.1 3 and
k
3.0
100
3.00
5.00
7.00
9.00
cNTA x107w Figure 3. Working curve for determination of NTA in presence of a given amount of EDTA according to procedure, except as noted : Mn(I1) concentration: 1.5 X 10-5M. Reference potentials corresponding to 0.620 and 0.250 absorbance unit. EDTA concentration: 1.0 X lO+M
equal to the analytical concentration of the ligand, CL. Since the second term in the right hand side of Equation 3 is constant, CL a ( ] / A t ) , resulting in the desired relationship between At and the total concentration of the sought-for species (complexing agent) to apply the variable time procedure for ligand determination. If ligand promotion is assumed, the metal complex would provide a path requiring a lower activation energy than the uncatalyzed and even metal-catalyzed reaction, but in doing so the ligand is destroyed. Considering again C, >> CL, as soon as the ligand is totally destroyed, the over-all rate would tend toward the rate of the metal-catalyzed reaction. In the other extreme situation, namely C,
