Abstract

Studies of inorganic carbon cycle in natural waters provide important information on the biological productivity and buffer capacity. Determination of total inorganic carbon, alkalinity and dissolved carbon dioxide gives an indication of the balance between photosynthesis and respiration by biota, both within the water column and sediments, and carbon dioxide transfers from the water column to the atmosphere. There are few methods to measure and distinguish the different forms of inorganic carbon, but all require a measure or an indirect quantification of total inorganic carbon. A direct measurement of TIC in water is made possible by the introduction of electrolytic generated hydroxide eluent in ion chromatography which allows to detect a chromatographic peak for carbonate. The advantage of this method is that all the inorganic forms of carbon are converted in carbonate at eluent pH and can be detected as a single peak by conductivity detection. Repeatability of carbonate peak was evaluated at different levels from 0.02 to 6 mequiv. l⁻¹ both in high purity water and in real samples and ranged from 1 to 9%. The calibration curve was not linear and has to be fitted by a quadratic curve. Limit of detection was estimated to be 0.02 mequiv. l⁻¹. Accuracy has been estimated by comparing ion chromatography method with total inorganic carbon calculated from alkalinity and pH. The correlation between the two methods was good (R² = 0.978, n = 141). The IC method has been applied to different typologies of surface waters (alpine and subalpine lakes and rivers) characterised by different chemical characteristics (alkalinity from 0.05 to 2 mequiv. l⁻¹ and pH from 6.7 to 8.5) and low total organic carbon concentrations. This analytical method allowed to describe the distribution of TIC along the water column of two Italian deep lakes.

Keywords: Ion chromatography; Total inorganic carbon; Freshwaters

Article Outline

1. Introduction

2. Experimental

2.1. Materials and chemicals

2.2. Ion chromatographic determination

2.3. Total alkalinity determination

2.3.1. Calculation of total inorganic carbon

2.4. Total carbon determination by high temperature catalytic oxidation

3. Results and discussion

3.1. Calibration curve

3.2. Repeatability and detection limit

3.3. Comparison between calculated TIC and IC measured TIC

3.4. Comparison with TIC measured by a TOC analyser

3.5. An example of application of IC method in lake limnology

3.6. Preliminary tests with a different stationary phase: Ion Pac AS19

4. Conclusions

References