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) |

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

For the probe geometry R = ÏL/A where Ï = is the specific resistivity of the water.

R = 1/G where G = electrical conductance

Ï = 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,Â 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) is approximately = 0.5*EC(microSiemens/cm

Test

- 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 ( if yor instuctor requests it).