After converting the nitrate into a purple dye, and measuring the absorbance of the purple dye on a spectrophotometer, a standard curve is used to convert the absorbance into concentration.

to make a standard curve, samples with known concentrations of no3- are run on the spectrophotemeter. the samples with known concentrations are called standards. a linear regression is then performed to relate the concentration of no3- to measured absorbance values.

here is a spreadsheet containing a simulated data set. there are standards and their related absorbance values, and there are samples from two sites that were diluted, prior to processing and measuring their absorbances. the groundwater originates from the upslope site, and the hope is that the microbes in the soil are removing the no3- from the groundwater before it reaches the downslope site.