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||Â TDS ( mg/l)|
|Sea Water||30,000 – 40,000|
If an electric current is induced in water, the ions carry the current (unlike a wire where eletrons flow).Â 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)Â and having a constant crossectional area for current flow (A).
Recall Ohms LawÂ V =IR ( V = voltage, I = current and R = resistance)Â can be written R = V/I
For the probe geometry R = ÏL/AÂ Â where Ï = is the specific resistivity of the water.
R = 1/GÂ Â where G = electrical conductance
Ï = 1/ECÂ where EC = electrical conductivity
Making the Substitutions we getÂ G = EC(A/d)Â or EC = GL/A = L/RA = LI/VA
So by measuiring the current (I)Â for a fixed V, A and L,Â EC can be found.Â Â 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) â 0.5*EC (Î¼Siemens/cm = Î¼S/cm
- Open the EC Measurement Spreadsheet
- Watch the video and record the measurements
- Convert the EC measurments to TDS
- Convert TDS to Na and Cl Concentration
- Determine if the water standards wound be exceeded ( 28 mg/l for Na, and 250 mg/l for CL)
- Convert the Spreadsheet to a PDF and submit it using the form below.