A method for measuring free CO sub(2) in upland streamwater using headspace analysis

• Published in: Journal of hydrology (Amsterdam) Vol. 166; no. 1-2; pp. 1 - 14
• Main Authors: Hope, D, Dawson, JJC, Cresser, MS, Billett, M F
• Format: Journal Article
• Language: English
• Published: 01.01.1995
• Subjects: Freshwater; Q1; Water analysis
• ISSN: 0022-1694

Existing titration-based methods for the measurement of dissolved free CO sub(2) are indirect and require the measurement of a number of other determinands (e.g. pH); they may under-estimate free CO sub(2) concentrations, because analysis is carried out frequently in an open vessel from which some free CO sub(2) may be lost prior to measurement. Here, a method of head-space analysis is described; this minimises CO sub(2) loss and provides a more direct technique for determining free CO sub(2) in low ionic strength, organic-rich upland streamwaters. A sample of streamwater is collected in a sealed flask and a headspace is created by pumping out a known volume of sample, replacing it with CO sub(2)-scrubbed air. After equilibration of CO sub(2) between the remaining water and the headspace, the concentration of CO sub(2) in the headspace is measured using an Infra Red Gas Analyser. The concentration of free CO sub(2) in the original sample is then calculated using Henry's law. This method measured free CO sub(2) in standard solutions containing 1-10mg/l free CO sub(2) to within 0.1 mg/l. The method was used to measure free CO sub(2) in streamwater from 19 sites on the River Dee in north-east Scotland and the results were compared with those reported for streams elsewhere. Free CO sub(2) concentrations measured by headspace analysis were significantly higher than those found using acidimetric titration.