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Alkalinity measurements have used spectrophotometric methods to measure concentrations from small samples, especially for HCO 3 − concentrations in terrestrial water. Nevertheless, the methods still pose challenges such as experimental and calculation complexity, along with the necessity of considering atmospheric CO 2 when preparing standard solutions. This study improved theoretical aspects of the spectrophotometric method based on chemical equilibrium equations combined with PHREEQC, yielding a new approach to alkalinity measurement. The differences between the experimentally obtained HCO 3 − concentration and the calculated value based on the updated chemical equilibrium equation were 0.038–5.4 × 10−6 mg/L. PHREEQC quantified the leaching of atmospheric CO 2 into the samples during experimentation. The effect was negligible at 0.01–0.02 mg/L HCO 3 −. The effect of increased atmospheric CO 2 because of human respiration in the laboratory was estimated as 0.05 mg/L HCO 3 −. These results suggest that the laboratory's experimental and computational steps for atmospheric CO 2 correction can be omitted when natural terrestrial water is targeted. We applied our new approach to various terrestrially derived natural waters (limestone cave drip water, hot spring water, fresh submarine groundwater discharge, groundwater, and river water) for simulations at different titration endpoints (pH 4.8 and 4.3). Differences between the values obtained by hydrochloric acid titration of HCO 3 − concentrations and those calculated using PHREEQC were smaller: less than 1%–7% different concentration for most samples. This study established a method for analyzing small samples of various natural terrestrial waters. • A method for alkalinity measurements (as bicarbonate) was tested. • The method uses spectrophotometric analysis of water samples with small volume. • Correcting bicarbonate concentrations for atmospheric CO 2 in labs is unnecessary. • Necessary sample volumes were 0.1 mL or 0.3 mL, which are useful for many fields. [ABSTRACT FROM AUTHOR]