Determination and differentiation of nitrilotriacetic acid and ethylenediaminetetra-acetic acid

‘r&ma.

1966. vol.

13. pp. 103 to 106.

Pergamon Press Ltd.

Printed in Northern Ireland

DETERMINATION AND DIFFERENTIATION OF NITRILOTRIACETIC ACID AND ETHYLENEDIAMINETETRA-ACETIC ACID D. L. FUHRMAN@,G. W. LATIMER, JR.? and JACQUELINE BISHOP Pittsburgh Plate Glass Company, Chemical Division, Corpus Cbristi, Texas, U.S.A. (Received 2 June 1965. Accepted 3 September 1965)

Summary-Ethylenediaminetetra-acetic acid (EDTA) and nitrilotriacetic acid (NTA) can be differentiated and determined by titration with metal ions to visual metallocbromic dye end-points. EDTA can be determined without interference from NTA, either by titrating with copper(I1) at pH 5 using PAN indicator, or by titrating with iron(II1) at pH 6 and 70” using Tiron indicator. The total chelating power (EDTA + NTA) can be determined either by titrating with lead(H) at pH 4.4 using dithizone indicator, or by titrating with iron(II1) at pH 3.5 using Tiron indicator; NTA is determinedby difference. The lowest concentration at which NTA can be determined in EDTA by titration to the iron(III)-Tiron end-point is about 1 wt. %. The apparent stability constants of the iron(III)-Tiron complexes under the conditions of the titration at pH 3.5 and pH 6 have been determined using the method of continuous variations. INTRODUCTION ALTHOUGH a number

of methods exist for the determination of nitrilotriacetic acid (NTA) in the presence of ethylenediaminetetra-acetic acid (EDTA),le2e3 onlj two procedures have been described for their differentiation and determination.* In these two procedures, the solid, apparently anhydrous, acids are dissolved in 1: 1 water: pyridine. In one instance the solution is titrated potentiometrically with zinc using a mercury-on-platinum VS. calomel electrode pair. In the other, EDTA is titrated with mercury(I1) and the total chelon content titrated with copper( both end-points being detected with a mercury-calomel electrode pair. In the zinc titration, two in the mercury-copper titration, NTA is determined potential breaks are observed; by difference. As a result of work done on systems containing water, NTA and EDTA (the NTA content ranging from 1 to 20 ‘A of the EDTA), we have found it possible to differentiate and distinguish the two chelons using metallochromic indicators and entirely aqueous systems. EXPERIMENTAL Reagents Metal solutions. Copper and lead solutions were prepared by dissolving the metals in nitric acid,

evaporating off the nitrogen oxides and diluting to volume. Two iron solutions were prepared. For the titrimetric work, Fe(NO,),*9H,O was dissolved in water containing 10 ml of cont. nitric acid and the resultant solution diluted to volume. For the stability constant studies, iron wire was dissolved in hydrochloric acid and the solution made to volume. (No positive steps were taken to oxidise the iron, but excellent agreement between absorbance values at pH 3.5 was obtained when another iron solution to which bromine water had been added to ensure complete oxidation was used.) Appropriate aliquots of iron solution were taken such that when diluted to 1 litre the final iron concentration was t Present address: 4.081 South 1500 East, Salt Lake City, Utah, U.S.A. 103

D. L. F~JHIM#AN, G. W. LATIMER,JR. and JACQUELINE BISHOP

104

either 5 x lO+kf or 25 x IO-*.&f. The metal solutions were standardised against EDTA by wellknown procedures, i.e., copper with PAN fl~Z-p~dyl~o)-Z-nap~thol] indicator’ at a pH of 5, lead at pH 6 (hexamethylenetetramine buffer) using Xylenol Orange,* iron at pH 3.5 and 70” using Tiron* (disodium 1,2-dihydroxybenzene sulphonate). Cheioa soiufions. Both NTA and EDTA solutions were prepared by suspending the reagent in water and adding sodium hydroxide until the compound dissolved. The EDTA was standardised by titration with zinc at pH 10 using Eriochrome Black T ~di~tor.s The NTA was standardised both alkalimetricaIly6 and potentiometrically against iron $ the respective purities were 1004 and 98.0%. The latter value has been used to judge the succeeding work. ZmZicators. A number of indicators and metal titrants were examined for use in these titrations. Although published data’0 were used as a guide, the variation in stability constants according to the experimental conditions made it necessar y to screen a large number of systems. Those surviving the initial qualitative tests and used in subsequent work were PAN, 0.1% w/v in ethanol; dithizone, 0.25 % w/v in ethanol; and ‘I’iron, 2 % w/v in water. The Tiron solution used in the stability studies was 5 X IO-%f.

The buffer consisted of 163 g of anhydrous sodium acetatellitre and 225 ml of glacial acetic acidf Ike. All reagents were of highest purity. Distilled or demineralised water was used throughout. AppW&US pfZ meter, Beckman Zeromatic ~~ctro~~oto~ter. Beckman Model B or Beckman DK-2 Preliminary screening studies

Investigations were conducted using different cations to titrate either NTA or EDTA directly to various indicator end-points. The results of those studies are presented in Table I. TAB= I.-Dmacr

Chelon

Titrant

Indicator

NTA

Zn(II)

Eriochrome

TM’RATION OF NTA ANDEDTA TO VISUALEND-POINTS

Xylenol Grange Cu(II)

PAN

Pb(II)

Dithizone

Fe@)

Tiion

EDTA Cu@I,l

PAN

Fe(IIQ

Tiron

Experimental conditions NH. buffer, pH 10, no end-point He~~ylenete~~ine bulTer, pH 6, no endpoint HOAc, pH 5, PAN at endpoint NaOAc-HOAc buffer, pH 44,l: 1 EtOH: H,O, fair end-point Hexamethylenetetramine buffer, pH 6, Tiron at end-point NHINO., pH 35, 70*, excellent end-point HOAc, pH 5, good endpoint H~methylenetetr~ne buffer, pH 6, 70”, excellent end-point NH,NOa, pH 3.5, 70”, excellent end-point

Literature reported stability constants Chelon-titrant Indicator-t&rant lo*5

7.1 6.2

12.7

16

11.5

-

15.9, 8.4

15.2, 9.1 10

18-S

16

25.1

15*2,9-l 10

From Table I, there appear four possible ways of anal sing mixtures of EDTA and NTA: 1. Determine the total chelating power of one aliquot or the sampleeither by titrating with lead(H) at pH 4-4 in 1: 1 EtOH:H,O using dithizone as indicator, or by titrating with iron at pH 3-5 and 70” using Tlron as indicator. 2. Determine the EDTA separately on another aliquot either by titrating with copper(H) at pH 5 using PAN as indicator, or by titrating the iron(III) at H 6 using Tiron as indicator. In either instance NTA would be determined by dirperence.

Determination

and differentiation

of NTA and EDTA

105

Quantitative determination of EDTA and NTA Known mixtures were prepared of solutions of EDTA and WA; aliquots of these synthetic samples were taken for the determination of EDTA and NTA. Procedure 1. A portion of the EDTA-NTA mixture was placed in a beaker, 1 ml of glacial acetic acid and three drops of PAN added and the EDTA titrated with standard copper@). Another portion was placed in another beaker, sufficient ethanol added to give a 1: 1 v/v mixture of EtOH : H,O, one drop of thymol blue added and the solution neutral&d with 1M sodium hydroxide. Five ml of buffer and 2 ml of dithizone were then added and the mixture titrated with standard lead to obtain the total chelating power, Le., EDTA + NTA. In addition, weighed amounts of NTA were titrated directly with lead in 1: 1 EtOH: HzO. The results are shown in Table II. Procedure 2. A quantity of the NTA-EDTA mixture was placed in a beaker, 2 ml of cont. nitric acid added and the pH adjusted to 6 with solid hexamethylenetetramine. Three drops of Tiron were added, the solution heated to 70” and the EDTA titrated with standard iron(III). A second quantity was taken, 2 ml of cont. nitric acid added and the pH adjusted to 35 with cont. aqueous ammonia, the solution heated to 70” and total chelating power determined with iron( In addition, weighed amounts of NTA were titrated directly with iron(III) under the same conditions. The results are shown in Table III. TABLE

II.-THE

Taken, mg 131

-

DETERMINATION OF NTA

EDTA by Cu Found, Recovery, % W 134 133 133 -

TABLEIII.-THE

Taken, mg 182

-

102 101 101

Taken, W 0.92

50l

Recovery, %

182 182 182 182 182 -

100 100 100 100 100

AND

Taken, W 36.6 18.3 9.3 4.6 1.2 25.0 20.0

NTA by Pb Found, Recovery, % V 0.88 0.94 0.90 2.39 2.43 4.79 4.88

2.50

DETERMINATION OF NTA

EDTA Found, “zg

EDTA BY TITRATIONTO Cu-PAN END-POINTS

AND

96 102 98 96 97 96 98

AND

Pb-DITHIZONE

NTA in EDTA, wt. % Calculated Found 6.3 -

6.4 -

EDTA BY TITRATIONTO F~(III)-TI~oN END-POINTS NTA Found, mg 37.0 19.0 9.7 4.8 1.1 24.6 24.6 19.5

Recovery, % 101 104 104 104 92 98 98 98

NTA in EDTA, wt. % Calculated Found 16.8 9.1 4.9 2.5 0.7 -

16.9 9.5 5.1 2.6 0.6 -

Apparent stability constants The method of continuous variations11 was used to estimate the stability constants of the iron(m)Tiron complexes at both pH 3.5 and pH 6. Into each of a series of ten lOO-ml beakers was placed, in order, 0, 5, 10, 15, 20, 25, 30, 35, 40 and 45 ml of 5 x lo-‘M Tiron solution and 50,45,40,35,30,25,20,25,10 and 5 ml of 5 X 10-&M iron solution. Two ml of cont. nitric acid were added to each beaker and the pH adjusted to 3.5 with cont. aqueous ammonia. The solutions were transferred to lOO-ml volumetric flasks and diluted to volume using a solution containing 20 ml of cont. nitric acid (adjusted to pH 3.5 with aqueous ammonia) per litre. In the same manner, the stability constant at pH 6 was estimated using the 5 x lO_‘M Tiron solution and a 2.5 X lo-‘M iron solution. The weaker iron solution and different aliquot sixes were used because the colour at pH 6 is more intense than the colour at pH 3.5. Into each of a series of ten lOO-ml beakers, each of which contained 50 ml of water and 2 ml of cont. nitric acid was placed, in 9

106

D. L. F UHRMAN, G. W. LATIMER, JR. and JACQUELINEBJSHOP

order, 0, 1,2, 3,4, 5, 6, 7, 8 and 9 ml of iron solution and 10, 9, 8, 7, 5,4, 3, 2 and 1 ml of Tiron. The pH was adjusted to 6 with solid hexamethylenetetramine, the solutions transferred to 100-ml volumetric Basks and diluted to volume. The absorption spectra were obtained on both sets of solutions and continuous variations plots prepared; these plots are shown in Figs. 1 and 2. The data presented in Figs. 1 and 2 confirm previous workI* that a 1: 1 Tiron:iron complex is formed at pH 3.5 (blue colour) and a 2: 1 Tiron: iron complex is formed at pH 6 (violet colour). The measured values obtained from Figs. 1 and 2, used to calculate apparent stability constants (Log Kh) according to the directions of Diehl and Lindstrom,rr are shown in Table IV.

-

600 My ,.A-., C’ ‘\

k-----\

-\ i k ‘\@

s50

‘\ ‘\

MlJ

A--Y

0

‘\

500 MP _-

*

a-__

,

.

.t\ ‘\ \\ ‘\ \ ‘!

\ ? ‘\ P

‘b\’‘\

\;.

MOLE

FIG. l.--Continuous

%

\

\\

FE

variations plot of iron-Tiron

at pH 35.

DISCUSSION

The reason for screening a number of indicator-metal systems is apparent on examination of Table I. Xylenol Orange and Eriochrome Black T, which were expected to be satisfactory as indicators for the titration of NTA with zinc, were not. The different literature1°s12*13Kh values reported for the iron-Tiron complex are probably caused by variations in experimental conditions. Because there was an indicator end-point, we conclude that under the conditions of our experiments at pH 6,

107

Determination and differentiation of NTA and EDTA 0.30

550 /-’ r

0.20

M/J a.

\

I r

.

\ z

i”‘

600

M/l

L

\

0.1 0

4\ \\ \\

.

0 4c 1 MOLE FIG.

2.-Continuous

TABLE IV.-APPARENT

Pf-f 3.5 6

Average fraction dissociated (d) 0.354 0.137

f

)

o/o FE

variations plot of iron-Tiron at pH 6.

STABILITYCONSTANTS* OF Fe(III)-TmoN

Cl 1.25 x 1O-4 1.25 x 10-b

(1 - d)/daC, 4.17 x 10’ 3.73 x 106

COMPLEX

Log Kh

Std. dev.

4.6 6.3

0.15 (n = 3) 0.53 (n = 2)

* Calculated from the relationship K h = (1 - d)/d2C, as presented by Diehl and Lindstrom.” the iron-Tiron complex is weaker than the iron-EDTA complex, but stronger than the iron-NTA complex. However, at pH 3.5, the iron-Tiron complex is the weakest of the three. Because NTA, apparently, was partly titrated with lead in 1: 1 v/v mixtures of EtOH : H,O using dithizone, the stability constant of the lead-dithizone complex is evidently less than that of lead-NTA; these data have not previously been published. Although EDTA was titrated with copper to a PAN end-point, the amount which

could be titrated was limited to the equivalent of 20 mg of copper. 20 mg of copper are used, PAN gives false end points.’

When more than

108

D. L. FUHRMAN, G. W. LA~IMER,JR. and JACQUELINE BISHOP

The reason for the low NTA results obtained with lead (Table II) is unknown. During the titration, however, a precipitate formed, sometimes before and sometimes after the end-point. If precipitation occurred before the end-point, the indicator change was obscured. To minimise this problem, very small samples were taken. The iron-Tiron system (Table III) provided much sharper end-points. The stability constants reported here for the iron-Tiron system have not taken into account the effect that hexamethylenetetramine buffer may have on the system. The lower limit (at 92 % NTA recovery) of the iron-Tiron differential titration using visual indicators is effectively about 1 wt. % NTA in EDTA (not 1 wt. % NTA in the system). Photometric end-point detection might reduce the value below lx, but lower NTA contents could probably be determined more satisfactorily by polarography. Zusammenfassung-Athylendiamintetraessigs~ure (EDTA) und Nitrilotriessigslure (NTA) konnen durch Titration mit Metallionen bis zu visuell erkennbaren metallochromen Farbumschlagen unterschieden und bestimmt werden. EDTA kann ohne Storung durch NTA entweder durch Titration mit Kupfer(I1) bei pH 5 mit PAN als Indikator oder mit Eiscn(II1) bei pH 6 turd 70” mit Tiron als Indicator bestimmt werden. Die Gesamtmenge an Chelatbildner (EDTA + NTA) kann durch Titration mit Blei(I1) bei pH 4,4 mit Dithizon oder mit Eisen(II1) bei pH 3,5 mit Tiron bestimmt werden; NTA wird aus der Differenz bestimmt. Die niedrigste Konzentration, bei der NTA in EDTA durch Titration bis zu den Eisen(III)-Tiron-Endpunkten bestimmt werden kann, ist etwa 1 Gewichts- %. Die scheinbaren Stabilitatskonstanten der Eisen(III)-Tiron-Komplexe wurden mit der Methode der kontinuierlichen Verlnderungen bestimmt. R&arm&-L’acide Bthylenediamine t&ac&ique (EDTA) et l’acide nitrilotriacetique (NTA) peuvent etre differencies et doses visuellement a l’aide d’ions metalliques, en presence de colorants sensibles. EDTA peut dtre dose sans que NTA apporte une gene soit par le cuivre(I1) a pH 5 en utilisant l’indicateur PAN, soit par le fer(II1) a pH 6 et a 70” en presence de Tiron. Le pouvoir complexant total (EDTA + NTA) peut &tre determine soit par dosage a l’aide du plomb(I1) a pH 4,4 en presence de dithizone, soit par titrage avec le fer(II1) a pH 3,5 avec le Tiron comme indicateur; NTA est obtenu par difference. La plus faible concentration a laquelle NT’A peut dtre determine dans EDTA par dosage a l’aide de Fe(III)-Tiron est voisine de 1% (p/p). Les constantes de stabilite apparente des complexes Fer(III)-Tiion ont et6 d&ermin&ss par la methode des variations continues. REFERENCES 1 R. B. LeBlanc, Analyt. Chem., 1959,31, 1840. a R. L. Daniel and R. B. LeBlanc, ibid., 1959, 31, 1221. * R. N. P. Farrow and A. G. Hill, Analyst, 1965, 90,241. 4 S. Siggia, D. W. Eichlin and R. C. Reinhart, Analyt. Chem., 1955, 27, 1745. L F. J. Welcher, The Analytical Uses of Ethylenediaminetetra-acetic Acid. Van Nostrand Company, Princeton, U.S.A., 1958, p. 149. e G. Schwarzenbach, Complexometric Titrations. Interscience, New York, 1957, p. 56. ’ F. J. Welcher, The Analytical Uses of Ethylenediaminetetra-acetic Acid. Van Nostrand Company, New Jersey, U.S.A. 1958, p. 272. BIdem. ibid.. D. 50. @Idem; ibid.; b. 225. lo L. Meites, Handbook of Analytical Chemistry. McGraw-Hill, New York, 1963, pp. 145, 3-101 and 3-109. I1 H. Diehl and F. Lindstrom, Analyt. Chem., 1959,31,414. i* A. E. Harvey, Jr. and D. L. Manning, J. Amer. Chem. Sot., 1950,72,4488. la The EDTA Titration, Nature and Methods of End-Point Detection. J. T. Baker Chemical Company monograph, 1957, p. 9.