Here is an example of how to calculate concentrations for a measurement procedure that uses a digestion followed by a dilution and a colorimetric test. This example uses the Total Kjeldahl Nitrogen (TKN) test. A similar procedure would be used for Total Phosphorus (TP), except that TP does not include a distillation step.
I ran an ammonia standard curve and received the following absorbance values:
I digested 2.5 mL of untreated sewage and 100 mL of treated effluent (ie, Vorig was 2.5 mL and 100 mL). After digesting and distilling (but before running the ammonia test), I diluted each to 50 mL (ie, Vtest was 50 mL for both).
Then I ran the test on the samples and got the following values:
The "Calc Conc" column gives the ammonia concentration of the diluted sample.
To get the original concentrations of the treated and untreated samples, use our old friend CV=CV.
Corig*Vorig = Ctest*Vtest
Where:
Corig = The concentration of the original sample (this is the value we are trying to get)
Vorig = The volume used for digestion
Ctest = The concentration from the test (calculated based on the standards)
Corig*Vorig = Ctest*Vtest
Vtest = The volume to which the samples were diluted before the final test
In this case:
Corig = Ctest*Vtest/Vorig
For the Untreated sample:
Corig = 1.22 mgN/L*50mL/2.5mL = 31mgN/L
For the Treated sample:
Corig = 0.613 mgN/L*50mL/100mL = 0.31mgN/L