5.3. Chelating agents ADA and NTA
There exist several direct and indirect analytical methods for DTPA and EDTA, including chromatography, voltammetry, potentiometry, spectrophotometry and AAS [100]. As an exact, practical and versatile technique, chromatography is probably the most widespread. Gas chromatographic (GC) methods are far more sensitive than liquid chromatographic (LC) methods [100]. A main benefit of GC is the fact that the same method can be used during the whole operation cycle of the chelating agents, from their addition into the process to the final environmental fate. As drawbacks of the GC methods, a required derivatization step is time consuming and, in contrast with LC [101, 102] and capillary electrophoresis CE [103, 104], only total amounts can be determined.
Since GC analytics of DTPA and EDTA have been well established earlier [100, 105, 106], this study focused on the development of analytical methods for ADA [X] and NTA [XI]. Use of NTA has attracted criticism because it is considered to be poorly biodegradable [24], and it has been classified as possibly a carcinogenic substance [107]. On the other hand, it has been observed to degrade well in normal waste water treatment [35, 108] and, in a long-term study, it has been concluded that NTA is an ideal substitute for EDTA from the environmental point of view [109]. In any case, it has been used for a long time in the EU area (mainly in detergents), which increases the need for a sensitive analytical method.
In the GC-procedures developed in this dissertation [X, XI], the water sample is first evaporated to dryness, followed by the addition of the esterification reagent consisting of alcohol and sulphuric acid. Further, a sample is extracted to toluene and neutralized with KHCO3. The organic phase is dried with Na2SO4 before the GC-NPD analysis. The optimum sample pretreating conditions for both analyses are presented in Table 7. For ADA, the analysis of lake sediment samples, after extraction with NaH2PO4, was also studied.
Table 7. Optimized sample pretreating conditions and analytical parameters [X, XI].
| ADA | NTA | |
|---|---|---|
| Optimized sample pretreating conditions | ||
| Sample amount | 20 ml | 10 ml |
| Derivatization - conditions and added reagent | 86 °C, 2 h | 78 °C, 6 h |
| 1 ml | 3 ml | |
| 5 vol% H2SO40.5 mg/l HDAN | 5 vol% H2SO45 mg/l HDAN | |
| Ethanol | 0.05 vol% acetic acid catalyst | |
| Ethanol or propanol | ||
| Extraction | 1.5 ml toluene | 0.75 ml toluene |
| Neutralization | 10 ml 1 mol/l KHCO3 | 20 ml 1 mol/l KHCO3 |
| Statistical data | ||
| Reproducibility | Lake water 4.1-14.2 % | Distilled water under 2.1 % |
| Sea water 5.7 –21.7 % | ||
| Waste water 4.4 –12.3% | ||
| Sediments 9-30% | ||
| Repeatability | Distilled water under 7% | Distilled water under 2.1 % |
| Detection limit | Distilled water 2.0 µ/l | Distilled water 6 µg/l |
| Waste water 2.7 µg/l | ||
| Lake water 2.5 µg/l | ||
| Sea water 2.9 µg/l | ||
| sediment 21 µg/g | ||
| Linear range | 0-10 mg/l | 0.006-50 mg/l |
| Correlation coefficient 0.998-0.999 | Correlation coefficient 0.9995-1 | |
| Observed interferences of metals | Cd(II), Ni(II), Pb(II), Cu(II), Hg(II), Mn(II), Cr(III) : no interference at 1mmol/l | Cu(II) and Mn(II): no interference under 100 mg/l |
| Fe(III): interference at concentrations over 10 mg/l | Fe(II):slight interference at concentrations over 10 mg/l | |
| Recovery | Natural waters 49-111% | Lake water 105-123 % |
| Sediments: 19-21% | Waste water 113-124 % | |
The procedure for ADA is simple and suitable for the analysis of different water samples. Preparation of propyl ester instead of ethyl ester may be useful when the resolution needs to be improved. The influence of metals is low; iron disturbs only in concentrations of heavily contaminated waste waters matrices. Table 7 reveals that the recovery of the extracted sediment samples is low, only 19-21 %. Hence the extraction step should be improved.
The good reproducibility and recovery values of the determination of NTA suggest that the procedure is applicable for routine analysis of waste waters and for a natural aquatic environment. Importantly, the low detection limit at the sample amount of 10 ml shows the suitability of the method for trace analysis and the limit of determination can be further lowered by using higher sample volumes. This procedure may also allow simultaneous determination of DTPA, EDTA, ADA and NTA, as the high resolution in Figure 10 reveals.
