A METHOD FOR MEASURING FREE CO2 IN UPLAND STREAMWATER USING HEADSPACEANALYSIS

Existing titration-based methods for the measurement of dissolved free CO2 are indirect and require the measurement of a number of other det erminands (e.g. pH); they may underestimate free CO2 concentrations, b ecause analysis is carried out frequently in an open vessel from which some free CO2 may be lost prior to measurement. Here, a method of hea d-space analysis is described; this minimises CO2 loss and provides a more direct technique for determining free CO2 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 v olume of sample, replacing it with CO2-scrubbed air. After equilibrati on of CO2 between the remaining water and the headspace, the concentra tion of in the headspace is measured using an Infra Red Gas Analyser. The concentration of free CO2 in the original sample is then calculate d using Henry's law. This method measured free CO2 in standard solutio ns containing 1-10 mg 1-1 free CO2 to within 0.1 mg l-1. The method wa s used to measure free CO2 in streamwater from 19 sites on the River D ee in north-east Scotland and the results were compared with those rep orted for streams elsewhere. Free CO2 concentrations measured by heads pace analysis were significantly higher than those found using acidime tric titration.