0.27, respectively. The influence of some products of degradation of NTA and some chemical species expected to be present in natural and industrial waters was also investigated. Iminodiacetic acid, glycolic acid, glycine, dodecylbenzene sodium sulfonate, sodium tripolyphosphate, and calcium ion did not interfere at least in molar concentrations 100 times that of NTA. Ortho- and m-cresol, and iron(II1) ion did not interfere if present at the same molar level of NTA. Aluminum ion and sodium lauryl sulfate can be tolerated in molar concentrations 10 times that of NTA. Dissolved chlorine proved to be the only serious interference of all species considered. Figure 3 presents a working curve for NTA with a known constant amount of EDTA. This figure implies that by add-
ing the adequate amount of Mn(I1) and selecting appropriate reference potentials, the procedure can be used for the determination of NTA in samples of aminopolycirboxylic acids (or other complexing agents) inhibiting the catalytic effect of manganese (i.e,, CDTA, DTPA, EDTA, and HEDTA).
RECEIVED for review April 24, 1972. Accepted July 19, 1972. Paper presented at the 1972 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1972. Work supported by the National Science Foundation (Grant GP-28207) and the College of Arts and Sciences, Oklahoma State University.
Analysis of Mixtures of Aminopolycarboxylic Acids by Chemical Kinetics. Parts per Billion of Nitrilotriacetic Acid in Water Lee C. Coombs, John Vasiliades, and Dale W. Margerum' Department of Chemistry, Purdue Unigersify,Lufuyette, Ind. 47907
Aminopolycarboxylic acids2 are detected and determined, individually or in mixtures, by the reaction of cyanide ion with their nickel(l1) complexes in basic solution. The procedure is based on the large differences in the rate of formation of tetracyanonickelate ion. Simultaneous kinetic determinations of twocomponent (NTA and EDDA) and three-component (NTA, EDDA, and EGTA) mixtures are accomplished by on-line regression analysis of stopped-flow spectrophotometric data. As little as 10 ppb of NTA in natural water samples can be detected. Trace amounts of NTA in EDTA and a four-component (EGTA, HPDTA, HEEDTA, and EDTA) mixture also are determined offline.
grade, can contain NTA (from less than 0.01 to as high as 3 7 9 (3, 4). LeBlanc ( 5 ) was able to analyze EDDA-NTA mixtures polarographically by converting EDDA to its cyclic amide form at low pH. However, a method of analysis which is generally applicable to mixtures of aminopolycarboxylic acids and which is sensitive to trace levels has not been available heretofore. In the present work, we report a kinetic method which is capable of distinguishing a variety of aminopolycarboxylate ligands from one another and of detecting the ligands down to about 10-8Mconcentrationlevels. Previous work in this laboratory has demonstrated the applicability of simultaneous kinetic analysis to the determination of mixtures of metal ions (6-8). Similar data handling The ANALYSIS OF MIXTURES of strong complexing multidentate techniques are used for the analysis of aminopolycarboxylates. ligands, such as NTA, EDTA, and their derivatives, has been The reaction rates can be adjusted over a wide range so that a difficult problem, particularly at low concentrations. A either conventional or stopped-flow mixing can be used. For potentiometric method using Zn(I1) in 1 :1 pyridine-water convenience we have used on-line computer processing of determined 20-80x NTA in NTA-EDTA mixtures in 0.1M stopped-flow data (9) and a simplified linear least squares concentration ( I ) . A polarographic technique using Cd(I1) method developed for a small computer (8). Data acquisition complexes was reported by Daniel and LeBlanc (2) and modiis rapid and accurate. An immediate evaluation of the data fied by Farrow and Hill (3, 4). This procedure was used to is possible giving analytical concentrations within a few determine to 10-3M NTA in mixtures with EDTA. minutes. Some systems were run at slower rates and Commercial samples of EDTA, depending on the source and monitored with a strip chart recorder. The chemical system developed for the simultaneous kinetic Correspondenceto be addressed to this author. analysis is based on the conversion of the aminopolycarboxyThe aminopolycarboxylic acids and their abbreviations are as ~ O ~ ~ O WNTA, S N(CHXOOH),; EDDA, HOzCCHzNHCHKHr lates, L"-, to their nickel complexes and the observation of NHCHKOzH; EDTA, (HOKCH~)?NCH~CH~N(CH~CO~H)U; the rate of reaction of the nickel complexes with cyanide ion EGTA, ( H O ~ C C H ~ ) ~ N C H , C H ~ ~ C H ~ C H ~ O CtoH give ~ O aC common H ~ C ~ product, ~ H ; tetracyanonickelate. The stoiIDA, NH(CHrCO?H),; MIDA, CHaN(CH2C02H)z; HPDTA, chiometry is given in Equation 1. (HOrCCHr)2NCHzCH(OH)CH~N(CH-?COzH)r; CyDTA, (HOy CCHI)~N(CF,H~~)N(CH?CO~H)~; HEEDTA, (HO?CCH,)(HOCH,( 5 ) R. B. LeBlanc, ibid., 31, 1840 (1959). (6) J. B. Pausch and D. W. Margerum, ANAL.CHEM.,41, 226
(1) S . Siggia, D. W. Eichlin. and R. C. Reinhart, ANAL.CHEM., 27, 1745 (1955). (2) R. L.Daniel and R. B. LeBlanc, ibid., 31, 1221 (1959). (3) R. N. P. Farrow and A. G. Hill, A m / y s t (Loudon), 90, 210 (1965). (4) Ibid., p 241.
(1969). (7) D.W. Margerum, J. B. Pausch, G. A. Nyssen, and G. F. Smith, ibid., p 233. (8) B. G. Willis, W. H. Woodruff, J. M. Frysinger, D. W. Margerum, and H. L. Pardue, ibid., 42, 1350 (1970). (9) B. G. Willis, J. A. Bittikofer, H. L. Pardue, and D. W. Mar-
gerum, ibid., p 1340.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
2325
nickel-aminopolycarboxyla te-cyanide complex (Equation 6), 2-
l-
which in general, forms rapidly and has a low absorbance compared to the final reaction product. The value of the first-order rate constant, k o h s d , depends on the cyanide ion concentration as expressed in Equation 7 and shown in Figure 1.
0-
-I
-
B 2-2-
-0
-3 -
-4
-
-5
-
-6
The slope in Figure 1 can vary from unity at high cyanide ion concentrations (where k o h s d = k;[CN-I) to three at low cyanide ion concentrations (where k o b s d = K1K2k3[CN-]9). A wide range of reaction times and variation of relative rates are possible by adjusting the cyanide ion concentration, The data in Figure 1 indicate that all the ligands except IDA and MIDA could be determined kinetically in the presence of each other. In the present work, we have attempted to show the applicability of the reaction system to a limited number of determinations.
I
-6
-5
-4
-2
-3
log
-I
0
I I
CCN-I,
Figure 1. Cyanide ion dependence of the observed first-order rate constant (sec-') for the formation of Ni(CN)42- from nickel aminopolycarboxylate complexes at 25 "C, pH 10.8 + 0 . 2 , ~= O.lM(KaCI0,) NiLen
+ 4CN-
-f
Ni(CN),2- f Lff-
EXPERIMENTAL
(1)
The high stability of Ni(CN)42- (log a4 = 30.5) and its characteristic three absorption bands with large molar absorp= 7.50 X tivities (€287 = 1.16 X lo4, ~ 2 = 8 ~4.63 X lo3, 1O2M-I cm-l) make it an excellent product for the detection of traces ofNiL2-n. The kinetics of formation of Ni(CN)42- have been studied in detail (10-12), and have been shown to follow a similar path for all the aminopolycarboxylates. The reaction of NiCyDTA2- with cyanide has recently been studied in our laboratory and follows the same mechanism as the other aminopolycarboxylates. In each case, the transition state in the reaction mechanism has three cyanide ions, but the observed reaction rate varies from first order to third order in cyanide ion. The order dependence in cyanide ion depends upon the extent of formation of mixed cyanide complexes prior to the rate-determining step (rds). The mechanism is given in Equations 2-5 where NiL(CN)'-n and NiL(CN)2n-
+ CN-
NiL2--n
K1
NiL(CN)'-n (rapid)
(2)
K2
+ CN- Jr NiL(CN)*%-(rapid) NiL(CN)2n- + CN- -%Ni(CN),L('+')- (rds) Ni(CN),L(ff+l)-+ CNNi(CN)42- + L"- (rapid) NiL(CN)l-ff
(3)
(4) (5)
may be present in appreciable concentrations, depending upon the cyanide ion concentration. Cyanide ion is present in excess so that the observed reaction rate is first order in the (10) D. W. Margerum, T. J. Bydalek, and J. J. Bishop, J . Amer. Cl7em. Soc., 83, 1791 (1961). (11) L. C. Coombs and D. W. Margerum, Znorg. Chern., 9, 1711 (1970). (12) L. C . Coombs, D. W. Margerum, and P. C. Nigam, ibid., p 2081.
2326
Apparatus. A Durrum-Gibson stopped-flow spectrophotometer with a 2-cm observation cell was used for the rapid reactions (8). For slower reactions, a Cary Model 16 spectrophotometer with a Varian 6-2000 strip chart recorder, using a Cary Model 1626 recorder interface to make it compatible with the spectrophotometer was used to record the data. This system can be read to 0.0003 absorbance unit. For on-line data acquisition, the output of the stopped-flow and the Cary 16 were interfaced to a Hewlett-Packard 2115A computer as described previously (9). Another stopped-flow system capable of mixing thermostated reactants within a fraction of a second in a 5-cm observation cell was constructed for use with the Cary 16. This system was useful for reactions where long term stability is needed for slower reacting components. The mixing and observation system was constructed from Pyrex (Corning Glass Works), Teflon (Du Pont), and Kel-F (3M Co.) and assembled using epoxy cement. Hamilton Teflon needles (3 mm i.d.) were used for all connecting tubing. The mixing chamber (Kel-F) was a four-jet tangential type. Inlet and outlet ports were 3-mm i.d., jets were 2-mm i.d., and the flow channel was 5.5-mm i.d. The optical path length of the cell was 50 mm and the total volume of the flow channel plus mixing chamber was 1.5 ml. Flow velocities were about 0.4 meter per second with a flow-through time of about 200 msec. Mixing was 99% for solutions of the same density and poorer for solutions with large differences in density. The cell windows were optical quartz (1 mrn thick). The fastest data acquisition rate used for this mixing system was 500 msec per datum point. Reagents. Ethylene glycol bis(aminoethy1)tetraacetic acid (EGTA, 98 minimum) and N-hydroxyethylethylenediaminetriacetic acid (HEEDTA, 98 minimum) were obtained from G. Frederick Smith Chemical Co. Nitrilotriacetic acid (NTA) and 1,3-diamino-2-hydroxypropanetetraacetic acid (HPDTA) were obtained from Aldrich Chemical Co, Ethylenediamine-N,N'-diacetic acid (EDDA) was obtained from K & K Laboratories, and ethylenediaminetetraacetic acid (EDTA) was obtained from J. T. Baker Chemical
z
z
co. EGTA was recrystallized twice by dissolving the acid in hot water with the aid of NaOH and precipitated by adjusting the solution to pH 2 with dilute HC104. NTA and HPDTA were recrystallized once by the above procedure and twice
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
from water. HEEDTA and EDDA were purified by procedures described previously (13). The EDTA was used without prior purification. A 0.1M borate buffer was prepared from reagent grade boric acid and sodium borate. Nickel perchlorate was prepared from reagent grade nickel carbonate and perchloric acid and recrystallized twice from water. Sodium cyanide stock solutions (0.1M) were prepared from reagent grade sodium cyanide and standardized by an argentimetric method prior to use. Sodium perchlorate was twice recrystallized and used for ionic strength control. Stock solutions of the nickel complexes (0.01M) were prepared by adding a slight excess of Ni(C104)*to a weighed amount of ligand dissolved in water and removing the excess as Ni(OH)2 at pH 11 by Millipore filtration. The solutions were adjusted to pH 9 with HCIOl for storage. All nickel aminopolycarboxylate solutions were standardized by adding a tenfold excess of NaCN solution and measuring the absorbance of Ni(CN)42at 267 nm and 285 nm after appropriate dilutions. Ligand solutions also were standardized with a copper(I1) nitrate solution using the mole ratio method. Water samples for the NTA determinations were obtained and treated by J. P. Haberman, The Procter and Gamble Co. They were taken from the Ohio River and the City of Cincinnati water system. The treated samples were run through an ion-exchange column to remove all metals present. The column was made of 20 ml of Bio-Rad Chelex 100 Na+ form, 50-100 mesh and 40 ml of Bio-Rad AG 50W-X8 H+ form, 50-100 mesh. Procedures of this type have been used for concentrating metal ions from environmental samples. NTA recovery from such a procedure has been shown to be greater than 90% (14). Procedure. Trace amounts of NiNTA- in NiEDTA2were determined by using the rapid mixing device incorporated in a Cary 16 spectrophotometer. The NiEDTA2solutions (pH 11) containing various amounts of NiNTAwere introduced into one syringe and 0.02M NaCN at pH 11 was placed in the other syringe. The solutions were mixed by a manual push using 3-5 ml from each syringe. A microswitch at the end of the push triggered the Cary 16 recorder. The absorbance-time plots were recorded and the concentration of NiNTA- was determined from the absorbance at specified times as outlined in the results. A four-component mixture of NiEGTA2-, NiHPDTA2-, NiHEEDTA-, and NiEDTA2- was prepared from stock solutions of the complexes and run on the Cary 16 at 10-2M cyanide concentration and pH 11. Data were taken using the H-P computer but the calculations were not done on-line. Two-component NiNTA- and NiEDDA mixtures, and three-component NiNTA-, NiEDDA, NiEGTA2- mixtures, were prepared from stock solutions of the complexes and run on the Durrum stopped-flow using the H-P computer and the REDKAN program (B), at cyanide concentrations of 5 X and 1 X 10-2M, respectively, and pH 11. Each run used about 0.3 ml of the sample mixture. The fastest data rate used was 5 msec per point. REDKAN is a computer program written in BASIC which provides on-line data acquisition and performs a regressive differential kinetic analysis for two components. The program begins by clearing the data buffers and matrix elements into which the derived parameters are summed. The data acquisition routine sums each datum into the buffer location making ensemble averaging of multiple runs possible. Values for the rate constants and t b (molar absorptivity x cell path), all of which have been calibrated for the day’s runs, are then input. The defining parameters for the operation of the data acquisition routines are input as initial rate, final data rate, switch point at which data rates are (13) N. E. Jackobs and D. W. Margerum, Iuorg. Chem., 6, 2038 ( 1967). (14) J. P. Haberman, ANAL.CHEM., 43, 63 (1971).
-
015-
$EDTA wlth I% NTA
E
\Forrnolian
of Ni(CN$- from NI(EDTA)(CN)~~
Pure EDTA
or0
10
20
x)
40
50
I
€0
Time ( s e d
Figure 2. A recorder trace obtained for the determination of NTA in EDTA The sample containing 6.1 X lO-’M NiNTA- reacted to form Ni(CN)42-within the response time of the system (
