This method is used for the determination of calcium carbonate as a measure of alkalinity in water and wastewater. The EPA range of this method is 10 to 200 mg/L, however this method is also applicable to other ranges.


Samples to be determined for "alkalinity" are mixed with a methyl orange indicator solution that is weakly buffered at a pH of 3.1. Sample alkalinity causes the color of the indicator solution to change from red to yellow. Photometric measurements are performed at 540 nm. The decrease in absorbance at the wavelength is proportional to the sample alkalinity.


Samples with pH values less than 3.1 cannot be determined by this method. Residual chlorine bleaches the indicator solution and therefore interferes. This interference can be removed by treating the samples with sodium thiosulfate. Samples with background absorbance at the analytical wavelength may also interfere. Turbid samples must be filtered prior to determination. This method is not approved by the National Pollution Discharge Elimination System (NPDES) for colored or turbid samples.


System Method # Range(s)
Flow System A011  2 - 500 mg/L
Astoria 2 A011  2 - 500 mg/L
rAPID-T D011  5 - 200 mg/l