How to determine chloride ion concentration in water
Chloride Mohr Method (0-200 ppm)
The chlorides of calcium, magnesium, sodium iron and other cations normally found in water are extremely soluble. Since no precipitation occurs, the chlorides present in boiler and cooling waters are usually proportional to the cycles of concentration.
In industrial water conditioning, the principal application of the chloride test is in the control of blow-down in boiler and cooling systems. In addition, the chloride test is also suitable to accurately calculate the rate of boiler blowdown provided that the chloride concentration of the feed water is of sufficient magnitude for acurate determination. The chloride test is unsuitable for blow-down calculation only in those cases where the feedwater chloride is quite low, such as 0.5ppm or 1.0 ppm. In this range, a slight analytical error in determining feedwater chloride will cause an appreciable error in the calculated rate of blow-down. Another use for the chloride determination is to estimate the per cent make up present in boiler feedwater. In open recirculating cooling water systems, the chloride test is used in determining cycles. The chloride test can be of advantage in the control of ion exchange softeners: to determine if the regeneration salt of a sodium unit has been washed out completely. Finally, the chloride test is
useful in determining condenser leakage, especially aboard ships or other places where seawater is used for condensing purposes. If leakage is occurring, the condensate will show increased chloride.
Theory of Test
The titrimetric method for chloride uses a standard solution of silver nitrate as the titrant and chromate ion as the indicator. The silver ion in the titrant reacts with the chloride in the sample, forming insoluble silver chloride. At the endpoint, excess silver ion combines with chromate and produces insoluble brick red silver chromate. Since silver chloride is more insoluble than silver chromate, the silver chromate does not form until all the chloride has combined. In the sulfite modification of this test, hydrogen peroxide is used to oxidize the sulfite to sulfate which, unlike sulfite, does not interfere with the chloride test, EDTA also interferes with this test by complexing the silver titrant, this interference also may be removed by oxidation with hydrogen peroxide.
Apparatus Required
Buret, automatic, 25 ml - 1
Casserole, Porcelain, 210 ml. - 1
Cylinder, Graduated, 50 ml - 1
Flask, Erlenmeyer, 250 ml - 1
Funnel, three inch diameter, Glass, Fluted. - 1
Stirring Rod, Glass - 1
Chemicals Required
Hydrogen Peroxide,
3% Phenolphthalein Indicator
Potassium Chromate Indicator
Silver Nitrate, 1ml = 1 mg Cl
Sulfuric Acid, 0.02N
Procedure for Test
If the phenolphthalein or methyl orange alkalinity has been determined, the same sample may be used directly for the chloride test without further neutralization. If the phenolphthalein or methyl orange alkalinity has not been determined the sample must be properly neutralized with sulfuric acid.
For the neutralization step, take a 50 ml sample of the water to be tested, add four to five drops of phenolphthalein indicator, and then add sufficient 0.02N sulfuric acid to neutralize to the colorless side of phenolphthalein as described under alkalinity. In this case, it is not necessary to record accurately the amount of acid required for the neutralization.
After neutralization, add five drops of potassium chromate indicator to the neutralized sample, which will then turn into a bright yellow color. Slowly add silver nitrate from the buret to the sample, stirring constantly, until one drop produces a permanent reddish color. This initial reddish color is to be taken as the endpoint (not the brick red color that will develop if additional silver nitrate is added).
A blank of 0.2 ml for this strength silver nitrate solution should be subtracted, as this is the quantity of silver nitrate required to produce the endpoint with distilled water.
Modification When Sulfite is Present
When the sulfite content of the water tested exceeds 10 ppm the sulfite should be oxidized to sulfate by hydrogen peroxide to avoid interference with the chloride test. After neutralization to the colorless side of phenolphthalein, add 2.0 ml of hydrogen peroxide and stir well. Add potassium chromate indicator and titrate as outlined above.
This procedure should not be used on a sample already neutralized to methyl orange. If methyl orange alkalinity has been determined, discard the sample and neutralize a fresh sample to the phenolphthalein endpoint only, before adding the hydrogen peroxide.
Modification When Edta is Present
When the EDTA content of the water tested exceeds 50 ppm, the EDTA should be oxidized by hydrogen peroxide to avoid interference with the chloride test.
Place a 50-ml sample of the water to be tested into the 250 ml Erlenmeyer flask. After neutralizing the water to the colorless side of phenolphthalein, add 25 ml of hydrogen peroxide and stir well.
Insert the stem of the fluted funnel into the flask; place the flask on a hot plate and boil the solution until 1 to 2 ml remain. Add 50 ml of distilled water. Readjust the pH to the colorless side of phenolphthalein. Add potassium chromate indicator and titrate as outlined above.
Calculation of Results
FORMULA:
ppm chloride as Cl = (ml silver nitrate - ml blank) X strength AgNO3 mg/ml x 1000 ml sample
Using a 50 ml sample and silver nitrate of the strength 1 ml = 1 mg Cl. Chloride in parts per million as Cl is equal to the ml of silver nitrate required minus a blank of 0.2 ml, multiplied by 20. When using a 50 ml sample and silver nitrate of the strength 1 ml = 5 mg Cl, chloride in parts per million as Cl is equal to the ml of silver nitrate required multiplied by 100.
Limitations Of Test
This method of chloride determination is unaffected by sulfate, total alkalinity, phosphate, silica, iron and color in the concentrations normally encountered in industrial waters. Bromide, iodide and cyanide register as equivalent chloride concentrations. EDTA sulfide, thiosulfate and sulfite ions interfere, but can be removed by treatment with hydrogen peroxide.
Orthophosphate in excess of 25 ppm interferes by precipitation as silver phosphate. Iron in excess of 10 ppm interferes by masking the endpoint. The chloride test is unsuitable for blowdown calculation only in those cases where the feed water chloride is quite low, such as 0.5 ppm or 1.0 ppm. In this range, a slight analytical error in determined feed water chloride will cause an appreciable error in the calculated rate of blowdown.
For greater accuracy in low chloride concentrations, a microburet for silver nitrate delivery is recommended. Accurate determination of chloride above 1000 ppm is not feasible with this method; for such high concentrations it is recommended that the sample be diluted with distilled water to lower the chloride concentration below 1000 ppm.
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