Determination Hardness and Conductivity of Water

The method includes analytical methods for total hardness and chloride content for soft water and de-ionized water analysis. Analytical tests in this method can be substituted by commercially available water test kits for the respective tests, selected for the appropriate specification levels and sensitivity. An analytical test method for free chlorine is D0005559

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Example water test kits - Hardness Merck 114652, Chloride - Merck 111106, Free Chlorine - Merck 114670

Section A - Total hardness of water

section B - Determination of chloride in water

Section C – Determination of Free Chlorine

Section D - Conductivity of water

Total Hardness of Soft water

Total hardness - The hardness of water is caused by the presence of alkaline earth, primarily calcium and magnesium. Recently the statutory unit has been introduced for water hardness - "the sum of the alkaline earth" - is expressed in mol/m3. The following conversion table provides a summary of the international units used to assess water hardness.

ANALYTICAL METHOD OF TOTAL HARDNESS

Principle

EDTA forms a chelated soluble complex when added to a solution of certain metal cations. If a small amount of a dye such as Eriochrome Black T is added to an aqueous solution containing calcium and magnesium ions at a pH of about 10, the solution will become wine red. If EDTA is then added as a titrant, the calcium and magnesium will be complexed. After sufficient EDTA has been added to the complex all magnesium and calcium, the solution will turn from wine red to blue. This is the endpoint of the titration.

Reagents for total hardness

EDTA, sol 0.01 M - Weigh 3.7225 g of Disodium EDTA dihydrate in 1000 ml of distilled water.

BUFFER SOLUTION pH 10 - Dissolve 54 g of Ammonium Chloride in 350 ml of concentrated Ammonium Hydroxide. Dilute to 1000 ml with distilled water.

INDICATOR - Mix together 0.2 g of Eriochrome Black T and 200 g of NaCl to prepare a dry powder mixture.

Total hardness test procedure 

Read all safety data sheets and take appropriate precautions as necessary. The aliquot of a sample taken for the titration should require less than 15 ml of EDTA titrant. The duration of titration should not exceed 5 minutes measured from the time of the buffer addition. Depending on the water hardness, dilute a known volume of the sample (20-30 ml) to about 50 ml with distilled water in a 150 ml beaker. Add 1-2 ml of buffer solution and an appropriate amount of powder indicator. Titrate with EDTA, under stirring, until the last reddish tinge disappears from the solution.. The color of the solution at the endpoint is blue under normal conditions.

Expression of results/Calculation of hardness

Hardness as mg/l CaCO3 =V1 x A x 1000/ V2

Where

V1 = ml titration for sample

A = mg CaCO3 equivalent to 1.00 ml EDTA titrant (1ml = 1mg CaCO3)

V2 = ml of the sample

Determination hardness of water and Conductivity of water

Determination of Chloride content in water.

Principle

Read all safety data sheets and take appropriate precautions as necessary.

This method determines the amount of chloride by direct titration with silver nitrate and uses the secondary precipitate of AgCrO4 as the indicator. It requires a neutral or nearly neutral solution.

Reagents

Calcium Carbonate (100%)

Dilute Acetic Acidc Acid to 100 ml with distilled water

Potassium Chromate (5% aqueous) -- 5 grams potassium chromate to 100 ml with distilled water 0.02M Silver Nitrate solution

The procedure of chlorine content

Transfer 100 ml of the sample to a conical flask.

Measure the pH of the sample using a suitably calibrated pH meter. Record the value.

If the pH lies outside the range of 6.5 to 9.0, adjust with calcium carbonate or dilute acetic acid, measuring the amounts added.

Add 1 ml of 5% w/v potassium chromate solution and titrate with 0.02M silver nitrate solution to a reddish-brown color.

Carry out a blank determination using 100 ml of distilled water, adding the same quantities of acetic acid or calcium carbonate used in the sample.

Expression of results/ Calculation of Chlorine

Chloride, ppm as NaCl = (T - B) x 11.69

Where:

T = Volume of 0.02M silver nitrate required by the sample

B = Volume of 0.02M silver nitrate required by the blank

Free Chlorine test in water

Use an appropriate water test kit. Alternatively use analytical test method for free chlorine is D0005559

What is conductivity of the water?

Principle

When all the ions have been removed water has an extremely low conductance and ions such as Carbonate ions, chloride ions, calcium, and sodium ions increase the conductance depending on their concentration. Measuring the conductance gives a good indication of the ion concentration as the higher the reading the more impure is the water. Resistivity is the .reciprocal of conductivity.

Conductivity - micro siemens Resistivity - mega ohms Approx. total dissolved solids -

Conductivity testing Procedure of water

Use a conductivity meter with an appropriate range and sensitivity checking the calibration of the instrument first. Ensure that the cell is thoroughly washed with boiled and cooled de-ionized water before testing each sample.

Expression of results

Express results as micro siemens (conductivity) or mega Ohms (resistivity) as appropriate.

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