I need to calculate total dissolved solids (TDS) of various groundwater samples from chemical analyzes. I have concentrations of ions/molecules in mg/L. I’ve found various formulas to calculate TDS but the ions/molecules I have never match. I don’t have a lot knowledge in chemistry.

Here are the three equations I have found. (As i said, I found many different equations and I don't know which one I should use):

Eq. 1 : TDS=Na(+) +K(+) +Ca(2+) +Mg(2+) + Fe^(2+) +HCO3(-) +Cl(-) +SO4(2-) +NO3(-)

Eq.2 : TDS=0.6*Alkalinity (as CaCO3) +Na(+) +K(+) +Ca(2+) +Mg(2+) +Fe(2+) +Cl(-) +SO4(2-) +NO3(-) [1]

Eq.3 : TDS=cations +anions +SiO2

Problems/questions I have:

•I don’t have the concentration of HCO3(-).

•I have the concentration of CaCO3. In natural waters, alkalinity (mg/L CaCO3) is HCO3(-)+CO3(2-) +OH(-) +H(+). At pH under 8.3, could I simply convert the concentration of CaCO3 in HCO3(-)? If yes and if the pH is over 8.3, is there a way to calculate the concentration of both HCO3- and CO3(2-)?

•I don’t have the concentration for NO3-. I only have the concentration for NO3-+NO2. Is there a way to isolate the concentration of NO3(-)?

•I don’t have the concentration for SiO2. I only have the concentration for Si. Could I simply use the concentration of SiO2?

•The pH of my water samples varies from 6 to 9.

•Also, I don't have the samples so I cannot titrate the water or measure electrical conductivity.

Thank you!


[1] Federation, W. E., & American Public Health Association. (2005). Standard methods for the examination of water and wastewater. American Public Health Association (APHA): Washington, DC, USA.

  • $\begingroup$ I am sorry this is not the way it works. Do you remember the mathematical requirement that if we have 9 unknowns we need 9 independent linear equations. Nobody solves such equations for TDS. Here is the step-by-step process wikihow.com/Calculate-Total-Dissolved-Solids $\endgroup$ – M. Farooq Apr 15 at 1:54
  • $\begingroup$ Sorry, english is not my first language. I meant that I don't know which equation is better. Also, I don't have the water samples, so I cannot use electrical conductivity or titrate the water samples. $\endgroup$ – Rachel Apr 15 at 2:04
  • 2
    $\begingroup$ No problem with English and no need to say sorry for that. Without conductivity you $cannot$ measure TDS easily. None of the equations are useful or officially accepted ones. Could you clarify what is the context of your problem? Also clarify what information do you have. $\endgroup$ – M. Farooq Apr 15 at 2:08
  • $\begingroup$ Can you get hold of this book "Environmental Laboratory Exercises for Instrumental Analysis Frank M. Dunnivant". It will explain everything to you. $\endgroup$ – M. Farooq Apr 15 at 2:18
  • 1
    $\begingroup$ I would like to calculate TDS of groundwater samples to compare it to values of TDS measured in wetlands. Then, I would like to establish if there is a connection between groundwater and wetlands. I foud the second equation in the Standard methods for the examination of water and wastewater so I think it is possible to calculate TDS from cations and anions. And thanks, I'll try to get the book! $\endgroup$ – Rachel Apr 15 at 2:23