NTA as a Masking Reagent

Nitrilotriacetic acid (NTA) is essentially one half of an EDTA molecule and forms somewhat weaker complexes than EDTA. Calculations of aM. the ratio of free metal ion to all forms of the metal in solution, showed that manganese(II), magnesium(Il), and the alkaline earths are not complexed significantly by NTA below a pH of about 

Iron(II) is not complexed at pH 2.0 or below, although some oxidation to iron 

The ion-chromatographic determination of manganese(II) with NTA masking of other metal ions looked particularly promising. Addition of a constant excess of NTA to the sample and varying the pH showed an essentially constant peak height for man-ganese(ll) between pH 2.0 and 3.0 [14]. For subsequent experiments, excess NTA was added to the sample and the pH was adjusted to 3.0. 

Both peak heights and retention times are reproducible with excess NTA in the sample the calibration curves are also linear. For example, a calibration curve for manganese(II), with a constant excess of aluminum(lll) (5.0 mM) masked by 10 mM NTA was linear over a range of 0.020 to 0.50 mM manganese with a correlation coefficient of 0.999. 

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