Simultaneous determination of nitrilotriacetic acid, ethylenediaminetetra-acetic acid and diethylenetriaminepenta-acetic acid as their methyl ester derivatives by GLC

Water Research Pergamon Press 1972. Vol. 6, pp. 871-876. Printed in Great Britain

SIMULTANEOUS ACETIC ACID ACID

ACID,

AND

DETERMINATION

OF NITRILOTRI-

ETHYLENEDIAMINETETRA-ACETIC

DIETHYLENETRIAMINEPENTA-ACETIC

AS THEIR

METHYL

ESTER

DERIVATIVES

BY GLC

LARS RUDLING Swedish Water and Air Pollution Research Laboratory, S-114 28 Stockholm, Sweden (Received 7 Februat3' 1972) Abstract--A specific method for the determination of nitrilotriacetic acid (NTA), ethylenediaminetetra-acetic acid (EDTA) and diethylenetriaminepenta-acetic acid (DTPA) is reported. The aqueous sample, after evaporation to dryness, is treated with a methanol/ borontrifluoride mixture. The resulting methyl ester derivatives are analysed by GLC. Concentrations down to 0-01 mg 1-1 for EDTA and DTPA, and 0-2 mg 1-1 for NI'A, can be determined.

NITRILOTRIACETICacid (NTA), ethylenediaminetetra-acetic acid (EDTA) and diethylenetriaminepenta-acetic acid (DTPA) are used as complexing agents for various applications, e.g. in detergents (NTA and EDTA) and in some pulp bleach processes (DTPA). When these compounds are discharged to community sewage works or to other types of receiving waters, it will be of interest to study their behaviour, especially since EDTA and DTPA are supposed to be non-biodegradable. Several methods have been published for the analysis of NTA, EDTA and DTPA. The specificity is low, however, since these methods are based on spectrophotometry or polarography without preceding separation. Two gas chromatographic methods for the determination of NTA have recently been published. The methods are based on the conversion of NTA to the propyl ester (CHAU and Fox, 1971) and 2-chloroethyl ester derivatives (RUDLING, 1971). However, the EDTA and DTPA derivatives are not sufficiently volatile for simultaneous determination. This communication presents a method in which NTA, EDTA and DTPA simultaneously are analysed by GLC with FI-detector as their methyl ester derivatives. EXPERIMENTAL

GLC was carried out by using a Varian 1400 Gas Chromatograph with a flame ionization detector. Analyses were made at a gas flow rate of 30 ml helium min- 1 on a glass column (100 × 0.2 cm i.d.) containing 5 percent (w/w) of OV-17 on Aeropak (100-120 mesh). The injector and detector temperatures were maintained at 285°C. The column temperature was programmed from 150 to 285°C with 10°C min -1. The column was regularly conditioned by injections of 10-20 tzl of Silyl-8-column conditioner. All reagents should be of highest purity. Esterification reagent: Boron trifluoride in methanol 10 per cent (w/v). Buffer solution:

Add 10 M NaOH to 1 M KH2PO4 until pH 7.0 is reached. 871

872

LARS RUDLING

Internal standard:

CDTA/cyclohexandiamine-(1.2)-tetraacetic acid/0.500 g 1-1 The acid is dissolved by adding an equal amount of NaOH.

PROCEDURE

Preparation of the sample The sample has to be analysed immediately or otherwise stored in a frozen state. The sample (ca. 10 ml) is filtered through filter paper (Munktell OOH or equivalent) and, if necessary, adjusted to pH 7. The filtrate (ca. 7 ml) is extracted with an equal volume of chloroform for 1 min in order to remove compounds which could interfere in the analysis. Part of the aqueous layer (5.0 ml) is transferred to a 10-ml ampoule, the internal standard solution (25 ~1) is added and the ampoule is immersed in an oil bath at 100°C. Nitrogen gas is flushed, by means of a needle, through the ampoule at a rate of about 100 ml rain- 1. After evaporation to dryness, the esterification reagent (1.0 ml) is added and the ampoule is sealed and placed in an ultrasonic bath at 100°C for 40 min. After cooling, chloroform (1.0 ml) is added and the mixture is transferred to a test tube containing the buffer solution (3.0 ml). After shaking for 1 rain and centrifugation, part of the chloroform layer (0.6--0.8 ml) is transferred to a glass vial and the chloroform is evaporated in a desiccator at 20°C under reduced pressure (100 mm Hg). The residue is dissolved in chloroform (50 ~1) and part of this quantity (5 ~1) is injected into the gas chromatograph. The concentration is calculated by using the standard curve.

Preparation of the standard curve Known concentrations of NTA, EDTA and DTPA in tap water were analysed as above and the ratio of the individual peak area and the peak area of CDTA, used as an internal standard, was plotted against the original concentration in the sample (FIG. 1).

DTPA

NTA

EDTA

E'

f

0"1

0-2

0"3

0.4

0'5

0'6

0"7

FIG. 1. Standard curve. The original concentrations (mg 1- t) of NTA, E D T A and D T P A in the sample is plotted against the ratio of peak area of NTA, E D T A and D T P A resp./peak area of internal standard (CDTA).

Simultaneous Determination of NTA, EDTA and DTPA

873

RESULTS AND DISCUSSION The method has been tested on tap water and sewage water after spiking with known amounts of NTA, E D T A and DTPA. In TABLE 1 and FIG. 2 the results of the analysis of raw sewage are summarized. Owing to the background concentrations of N T A and E D T A in the raw sewage it was not possible to spike with concentrations lower than 0.1 mg/1. However, the data demonstrate that the recovery for E D T A and D T P A is quite consistent in the range of 0.1-1.6 mg 1-1. With no background concentration, as for tap water, it has proved to be possible to estimate a concentration of 0.01 mg 1-1 for E D T A and DTPA. TABLE

1.

RAW

SEWAGE

SPIKED

WITH

Added concentration mg 1-~ NTA

EDTA

NTA,

EDTA

and

DTPA

Recovery, corrected for initial concentration (%) DTPA

NTA

EDTA

DTPA

94 89 92

92 92 98 95 93

92 88 109 80

1 "6

1.6

1-6

0'8 0"4 0"2

0"8 0.4 0"2 0"1

0.8 0-4 0"2 0.1

97

Concentration before addition of NTA 1-1 mg 1-1, EDTA 0.12mg 1-a, DTPA 0.02 mg 1-1.

Concentrations lower than 0.2 mg 1-1 of N T A are difficult to analyse owing to the poor chromatographic properties of the trimethyl ester of NTA. If low concentrations have to be determined, it is more advantageous to use the tripropyl ester derivative or the tri-2-chloroethyl ester derivative.

CDTA

DTA

I

I /

NTA DTPA

i

t

L

i 20~ . . . . 25.~

tG

i ! ' : I i i----Temp, proarommed

Temp ,

h

I

L

lO°Imin

i

I

-

'

~

I

Time,

rain

15C

°C

FIG. 2. Raw sewage spiked with NTA, EDTA and DTPA. Added concentration 1"6 mg 1-1 of each (see TABLE1).

874

LARS RUDLING

In order to test the interference from different metal ions, a tap water solution containing 0.2 mg I-* of each of NTA, E D T A and D T P A was analysed with different metal ions added. In each test the investigated metal ion was added to a concentration of 2 mg 1-~. N o significant disturbance from the investigated metal ions (Cu 2+ , Cd z +, Zn 2+, Ni 2+, Fe 3+) was observed. TABLE 2. EDTA mg l-1 Sewage works

Raw sewage

Outgoing sewage

0.14 0.05 0.17

0.13 0.06 0.18

Henriksdal Loudden .~keshov

CDTA NTA

x16 ZO r 2185 T

i

i

I L~

x64 [

I

I

1

I

IO I

Time, I

I

t

I

I

Temp. programmed I 0 * / rain Temp.,

rain

I

~J

150

°C

FIG. 3. Raw sewage .~,keshov. Calculated concentration of NTA 1"1 mg 1-1, E D T A 0.17 mg 1- t.

CDTA

EDTA x 16

. '

I

'

t

20 ' ' 285"

'

x64 1~..~1

/

I0 ' ' ' ' Z i Temp. ~ogrommed Temp.,

' ' i i.~..~ I0" / rain

~0. I

time,

rain.

I

°C

FIG. 4. Outgoing sewage /~keshov. Calculated concentration of NTA 0"20 mg l-1, EDTA 0"18 mg 1-1.

Simultaneous Determination of NTA, EDTA and DTPA

875

Since E D T A is s u p p o s e d to be n o n - b i o d e g r a d a b l e , different d a y - n i g h t s a m p l e s f r o m s o m e sewage w o r k s in S t o c k h o l m were a n a l y s e d . F r o m the results in TABLE 2 a n d FIGS. 3 a n d 4 it c a n be seen t h a t there is no significant d e g r a d a t i o n o f E D T A in a c o m m u n i t y sewage w o r k . TABLE 3. PRECISIONTEST. REPLICATEANALYSISOF TAP WATERSPIKEDWITH NTA, EDTA and DTPA. CONCENTRATION1"0 mg 1- t oF EACH NTA Percental standard deviation n = 10 Percental deviation at 95 % confidence, n = 10

4-7 5=3-3

EDTA

DTPA

2"8 5=2'0

11"6 ___8"3

T h e p r e c i s i o n o f t h e m e t h o d was tested b y a n a l y s i n g 10 replicates o f a t a p w a t e r s o l u t i o n c o n t a i n i n g N T A , E D T A a n d D T P A with a c o n c e n t r a t i o n o f 1.0 m g 1-1 o f e a c h (TABLE 3). T h e i d e n t i t y o f t h e N T A a n d E D T A p e a k in t h e gas c h r o m a t o g r a m s o b t a i n e d f r o m a s a m p l e t a k e n at t h e A k e s h o v sewage w o r k s was a l s o c o n f i r m e d b y c o m b i n e d gas c h r o m a t o g r a p h y - m a s s s p e c t r o m e t r y analysis.

REFERENCES CHAU Y. K. and Fox M. E. (1971) J. Chrom. Sci. 9, 271-275. RUDLING L. (1971) Water Research. 5, 831-838.