An analytical approach for alkalinity measurement with a small volume of
natural water by chemical equilibrium formula and geochemical modeling
Abstract
Spectrophotometry using a small sample volume has been developed to
measure the alkalinity (or bicarbonate). However, the experimental and
calculation processes are complicated, and the atmospheric
CO2 has to consider in preparing standard solutions.
This study aims to quantify the dissolution of the atmospheric
CO2 when using spectrophotometry. Also, the effect on
the standard solution in titrating that uses to make calibration curve
are calculated to understand the effects of the different
CO2 concentrations in a laboratory. The difference
between the bicarbonate concentrations and the calculated ones based on
the updated chemical equilibrium formula was from 0.038 to
5.4×10-6 mg/L. The maximum difference was found at pH
5.0 in the 10 mg/L HCO3- standard
solutions. The bicarbonate concentration without the atmospheric
CO2 reaction (C1) and with the atmospheric
CO2 reaction (C2) was calculated by the PHREEQC. The
difference between C1 and C2 ranged from 0.01 to 0.02 mg/L, but the
calculated bicarbonate concentrations between the HCl titration and the
PHREEQC output were certainly different, which ranged from 3.1 to 11.5
mg/L at the pH 4.3 endpoint. In contrast, at the pH 4.8 endpoint, the
difference was significantly decreased from 0.8 to 1.3 mg/L. The effect
of the increasing atmospheric CO2 by human breathing in
a laboratory is only 0.05 mg/L in the standard solutions when titrating.
From the results of this study, the experimental and calculation
processes to correct the bicarbonate concentration by the effect of the
atmospheric CO2 in a laboratory may be omitted if
natural waters are targeted.