How does SpecE8 calculate TDS?

[Herdis Schopka]

I have a really basic question: How does SpecE8 calculate TDS? The value for TDS that I get from SpecE8 are consistently higher than the ones I get summing up the concentrations of input species (Na+, K+, Mg++, Ca++, Cl-, SO4--, HCO3-, SiO2(aq)), such that TDS_sum = 0.73 * TDS_spece8 + 63, R^2 = 0.94. Does anyone know what's going on?

 

Thanks,

Herdis

[Tom Meuzelaar]

I have a really basic question: How does SpecE8 calculate TDS? The value for TDS that I get from SpecE8 are consistently higher than the ones I get summing up the concentrations of input species (Na+, K+, Mg++, Ca++, Cl-, SO4--, HCO3-, SiO2(aq)), such that TDS_sum = 0.73 * TDS_spece8 + 63, R^2 = 0.94. Does anyone know what's going on?

 

Thanks,

Herdis

 

Hi Herdis:

 

GWB calculates TDS according to the following formula:

 

TDS = (Mass_solution - Mass_solvent) / Mass_solution

 

This formula gives a correct conversion from mass fraction to molal units, and vice versa.

 

As an aside- TDS is not a precise measurement of a well-defined physical quantity. Instead, it is a standardized test, the results of which vary depending on the extent to which volatiles like H2O and CO2 are lost from or retained in the residuum during desiccation. There will always be discrepancies between lab tests and theoretical calculations, and between calculations that account for or neglect the distribution of H2O.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.