Question

Describe the principles of measuring F_, Cl_, NO_3 , and SO2_ 4 in water by anionic IC with Na2CO3__NaHCO3 as the eluting solvents

Answer 1

Ans

Ion chromatography (IC) is a process that separates and detected the ions and polar molecules based on their affinity to the ion exchanger. The advantages of these methods are low cost of analyses, relatively simple and cheap apparatus, and hence a possibility of use in most laboratories and another advantages of ion chromatography includes the short time needed for analyses, possibility of analysis of small volume samples, high sensitivity and selectivity, and most importantly – a possibility of simultaneous separation and determination of a few ions, or ions of the same element at different degrees of oxidation, which provides more comprehensive information for the sample studied.

In Anion IC, the resin is coated with cations will bind to negatively charged molecules, displacing the counter-ion.

Anion exchange chromatography retains anions using positively charged functional group:

R-A⁺X^- + M⁺ Y^- = R-A⁺Y^- + M⁺ + X^-

Separation and determination of F^-, Cl^-, NO^-3 , and SO₂^_4 ions in water by anion IC is carried out in anion-exchange columns filled with a suitable exchanger and using a proper eluent (e.g. water solution of Na₂CO₃ /NaHCO₃) and most often conductometric or UV detection. Nitrogen ion determination by ion chromatography is accompanied by determination of other anions present in the sample, such as: fluorides, chlorides, phosphates, bromides, and sulphates. The main problems are related to the proper separation of NO2 - from Cl- ions. Irrespective of the column used, the retention times of the ions (related to their structure, ionic radius, selectivity against the exchanger) are close and in environmental samples with chloride ions concentrations usually a few times higher than those of nitrate ions, the peak assigned to NO₂^- can be masked by a large peak assigned to Clions. The problems related to the separation of the pairs of Cl- /NO₂^- ions can be solved by optimizing the conditions of analysis, i.e. changing the composition of the eluent, type or pH of eluent, the intensity of its flow, type of column and detector.

The use of a classical conductometric detector and water solution of Na₂CO₃ /NaHCO₃ as eluent, the maximum ratio of the concentrations of the ions Cl^- /NO₃^- ensuring good performance is 200:1. With water solution of NaCl as eluent and UV detector, this ratio increases to 200,000:1, and in the system with NaCl as eluent and an amperometric detector it increases up to 1,000,000:1.