Specific Conductivity Measurement (Using Electrical Conductivty to Calculate Total Dissolve Solids)

In this exercise you will be recording measurements of electrical conductivity of threee water samples, converting them to an estimate of Total Dissolve Solids (TDS) and then assuming the TDS is a measurement of salt (NaCl) content, you will estimate the sodium and chloride concentratios and determine if drinking water standards are exceeded.

The TDS is a measure ot the total amount of cations and anions in water in mg/l. The chart below gives you an idea of the range of TDS for different waters.

 

Water  TDS ( mg/l)
Fresh <1,000
Brackish 1,000- 15,000
Sea Water 30,000 – 40,000
Brine >40,000

If an electric current is induced in water, the ions carry the current (unlike a wire where eletrons flow).  The higher the TDS, the higher the current. An electrical conductivity meter measures the TDS indirectly by inducing a current between two voltage contacts separated by a constant length (L)  and having a constant crossectional area for current flow (A).

Recall Ohms Law  V =IR ( V = voltage, I = current and R = resistance)  can be written R = V/I

For the probe geometry R = ρL/A   where ρ = is the specific resistivity of the water.

R = 1/G   where G = electrical conductance

ρ = 1/EC  where EC = electrical conductivity

Making the Substitutions we get  G = EC(A/d)  or EC = GL/A = L/RA = LI/VA

So by measuiring the current (I)  for a fixed V, A and L,  EC can be found.   For a fixed system geometry, the EC of the water (independent) of the probe geometry is found by calibrating the instrument reading to a standard solution of know conductivity before making measurments.

For water where the major ions are sodium and chloride, the TDS (mg/l) ≈ 0.5*EC (μSiemens/cm = μS/cm

Test

  1. Open the EC Measurement Spreadsheet
  2. Watch the video and record the measurements
  3. Convert the EC measurments to TDS
  4. Convert TDS to Na and Cl Concentration
  5. Determine if the water standards wound be exceeded ( 28 mg/l for Na, and 250 mg/l for CL)
  6. Convert the Spreadsheet to a PDF and submit it using the form below.

EC Measurment Spreadsheet

 

EC Data

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