Determination of Conductivity of Water

DETERMINATION OF CONDUCTIVITY OF WATER METHOD

Electrolytic conductivity is the capacity of ions in a solution to carry electrical current and is the reciprocal of the solution resistivity. Current is carried by inorganic dissolved solids, such as Chloride, Nitrate, Sulphate and Phosphate anions and cations such as Sodium, Calcium, Magnesium, Iron and Aluminum. The determination of conductivity is actually performed by measuring the resistance occurring in an area of the test solution defined by the probe design. A voltage is applied between the two electrodes immersed in the test solution, and the voltage drop caused by the resistance of the solution is used to calculate its conductivity per centimeter. The basic unit of conductivity is the siemens (or mho), the reciprocal of the ohm in the resistance measurement. Normally used sub-units are millisiemens/cm and microsiemens/cm.

APPARATUS AND REAGENTS:

Conductivity-Meter

Conductivity Solution: 84 µS/cm

Weigh 0.0400 g of reagent grade NaCl dried at 110°C, transfer into a 1000-mL volumetric flask, dissolve in de-ionized water, dilute to one kg and mix thoroughly.

Conductivity Solution: 1413 µS/cm

Weigh 0.7000 g of reagent grade NaCl, transfer into a 1000-mL volumetric flask, dissolve in de-ionized water, dilute to one kg and mix thoroughly.

Conductivity Solution: 12,880 µS/cm at 25°C

Weigh 7.2300 g of reagent grade NaCl, transfer into a 1000-mL volumetric flask, dissolve in de-ionized water, dilute to one kg and mix thoroughly.

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PROCEDURE:

1. Turn the Conductivity-Meter ON; wait for five minutes for the warm-up period.

2. If calibration of instrument is required, calibrate the instrument according to manufacturer’s “Instruction Manual”.

3. Cool the sample to room temperature (in case of alkaline sample, add 2–3 drops of Phenolphthalein indicator, swirl to mix and add Acetic acid drop-wise with constant stirring until the sample turns to its original colour).

4. Rinse the Conductivity-probe with de-ionized water and immerse in the water sample without touching the sides of the beaker.

5. Move the probe up and down and tap it on the beaker to free off any air bubbles from the electrode.

6. Select the appropriate range, beginning with the highest range and working down. Read the conductivity of water sample.

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NOTE:

If the conductivity of the sample exceeds the calibration range of the instrument, dilute the sample and measure the conductivity of diluted sample. Calculate the conductivity of original sample by multiplying the measured conductivity with dilution factor.

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CALCULATION:

Conductivity of Sample = Displayed by the instrument.

In case of Sample dilution:

Conductivity of original Sample = Conductivity of diluted sample × {1 + (volume of distilled water added / volume of original sample)}

T.D.S of water sample (ppm) = Conductivity (µS/cm) × 0.666