I am working on carbonate impurities method per USP for sodium bicarbonate. The assay is using carbonate apparatus (resemble to chittick apparatus) where the absorbtion of CO2 correlates with the carbonate amount in the sodium bicarbonate. I have problem to establish baseline of not more than 0.2 mL volume increase. We got continuous CO2 absorbance, so were were unable to measure sample. When the temperature of the saturated carbonate solution is raised to above 28C, I was able to stop the volume increase to no more than 0.2 mL after 5 attempts (5 cycles). However, after sample is applied, similar problem occured, after more than 5 attempts, we still could not get the volume increase of not more than 0.2 mL. Any suggestion and possible explanation. Also, we sent the same samples to different laboratories, the results were varied 10-40x between labs.