| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 101 === This study investigates effects of temperature, iodide concentration, oxidants and suspended solid on the colorimetric determination of oxygen in natural waters. Since the measurement involves the mixture of two iodine species (i.e. I2 and I3-), the equilibrium between them becomes a crucial factor to the precision of the method. A high concentration of iodide in the pickling reagent was found essential to ensure a stable [I_3^- ]/[I_2 ] ratio. Besides, it was also found that elevated temperature can cause a lower signal of molecular iodine, but increase the absorbance of triiodide ion. An over-all increment of 0.4% per 1 ℃ has been observed for a freshwater sample having oxygen concentration of ca. 250 μM. Therefore, temperature deviation can not only change the equilibrium constant but also alter the spectra of both iodine species. To solve this problem, the spectrophotometer has been modified using a thermostat unit which ensures the temperature difference between samples be kept within 0.5 ℃. Iodate, hydrogen peroxide, nitrite which are known to exist in natural environment all interfere the oxygen measurement. A quick reverse pickling procedure can be used to identify the iodate interference. Addition of sodium azide is always suggested to remove overestimate from nitrite. Since the sampling for oxygen usually excludes filtration therefore suspended solid in a sample can be another source of error. Re-measurement of the absorbance of the same sample after adding sodium thiosulfate reagent to remove the iodine color is considered the most effective way to correct for the interference from turbid water.
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