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ENGI 9628

Environmental Laboratory

Lab #3-1

Water hardness Determination

Faculty of Engineering & Applied Science

HARDNESS OF WATER

PURPOSE

Using the Standard Methods to determine the calcium, magnesium and total hardness.

THEORY

Water hardness has been traditionally defined as the capacity of water to precipitate soap. Water hardness is a property caused by the presence of polyvalent cations, mainly Mg2+ and Ca2+, but may also include iron (ferrous), strontium and manganese. These ions react with soap and certain anions, producing precipitates, which appear as scale in pipes, boilers, humidifiers and kitchen appliances.

Hardness is classified with respect to the metallic cation (eg. Ca or Mg hardness) and with respect to the associated anion (noncarbonate and carbonate hardness). The concentrations of Ca, Mg or other hardness-causing cations are converted to the equivalent amount of calcium carbonate so that the degree of hardness is expressed as mg/L of CaCO3

The anions that are involved with precipitate formation include: sulfate, carbonate, bicarbonate, bypochlorite, chloride, nitrate and silicate. Carbonate (temporary) hardness is defined as the amount of hardness equivalent to the total alkalinity. Non-carbonate (permanent) hardness is defined as the amount of hardness in excess of carbonate hardness.

ATPLICATIONS OF HARDNESS DATA IN ENVIRONMENTAL ENGINEERING PRACTICE (Ref 2)

Hardness is an important consideration in determining the suitability of water for domestic and industrial uses. The engineer uses it as a basis for recommending the need for softening processes. The relative amounts of calcium and magnesium hardness and of carbonate and non carbonate hardness present in a water are factors in determining the most economical type of softening process to use, and become important considerations in design. Determinations of hardness serve as a basis for routine control of softening processes.

Hard waters are satisfactory for human consumption as soft waters. Because of their adverse action with soap, however, their use for cleansing purposes is quite unsatisfactory, unless soap costs are disregarded.

INTERFERENCES

Some metal ions interfere with this procedure by causing fading or indistinct end points. A delay in titration (>5min) may allow calcium carbonate to precipitate and result in a low value.

Suspended or colloidal matter also may interfere with the end point.

APPARATUS

1. pH meter

2. Magnetic stirrer and stir bars 3. Buret, 25ml 4. Volumetric pipets 5. Miscellaneous glassware

REAGENTS

Total Hardness

1. Buffer solution

• Disolve 16.9g ammonium chloride in 143ml concentrated ammonium hydroxide.

• Add 1.25g of Disodium EDTA and dilute to 250 ml with distilled water.

2. Standard EDTA titrant, 0.01M (WILL BE PROVIDED TO STUDENTS) 3. Indicator Calmagite

• Dissolve 0.10g Calmagite in l00ml in 100ml distilled water.

Calcium Hardness

1. Sodium Hydroxide 1.0 N

• Dissolve 40g NaOH in distilled water and dilute to 1000 ml.

2. Murexide (Ammonium Purpurate) Indicator

• Mix 0.2g of Murexide with 100g of solid NaCl and grind the mixture.

PROCEDURE

Total Hardness

The Standard Method 2340 C. known as the EDTA Titrimetric Method will be used to determine Ca

and Mg hardness using colorimetric titration.

Ethylenediaminetetraacetic acid (EDTA) functions as a chelating agent on certain cations. Chelation occurs when EDTA shares a free electron with a cation to form a stable complex. The number of coordination bonds that are formed between the chelation agent and the cation determines the stability of the complex, EDTA has four ionizable hydrogen atoms and a total of six coordination bond sites available.

When a solution of EDTA is added to a water sample, it combines with calcium (or magnesium) to form a soluble-chelated stable complex.

EDTA and the complexes it forms are all colorless. As a result, the addition of colour-forming indicators is used in the colorimetric titration. When a small amount of a dye such as Eriochrome Black T or Calmagite is added to an aqueous solution containing calcium and magnesium ions at a ph of 10.0, the solution becomes wine red.

During titration with EDTA: all free hardness ions are complex according to

Finally, when all the free hardness ions have been exhausted, the EDTA disrupts the red complex [M. Eriochrome Black T] because it is capable of forming a more stable complex with the hardness ions. This action frees the Eriochrome Black T indicator and the wine red color changes to a distinct blue color, heralding the end of the titration (Ref 2)

There are several competing equilibria involved when determining hardness using the EDTA method. In order to overcome this, the sample solution is buffered to a pH of 10.0 ± 0 . 1 as a compromise between chelate stability and the prevention of precipitate formation.

Procedure

1. Select a sample volume which requires less than 15 ml EDTA titrant. Normal sample volume is 25ml. Do not extent duration of titration beyond 5 minutes measured from the time of buffer addition.

2. Dilute 25 ml sample to 50 ml with distilled water. 3. Add 1 to 2ml of buffer solution to give pH of l0.0 to 10.1. 4. Add approximately 0.2 g of Eriochrome black indicator or 1 to 2 drops

indicator solution and swirl to mix. 5. Add the EDTA trirant slowly, with continuous stirring until the last reddish tinge

disappears from the solution, adding the last few drops at 3-5 second intervals. The color of the solution at the end point is blue under normal daylight conditions.

Calcium Hardness

This is a modification of the standard method used previously to determine the total hardness. Standard methods lists no titrimetric method for determining either calcium or magnesium hardness.

1. Use 50-ml sample or a smaller aliquot diluted to 50 ml so that the calcium content is approximately 5-10 mg. Analyze waters with alkalinity higher than 300 mg/1 by taking a smaller aliquot and diluting to 50mL.

2. Add 2.0 ml NaOH solution or greater volume sufficient to produce a pH of 12-13. 3. Add approximately 0.2g of Murexide indicator and swirl to mix. 4. Add the EDTA titrant slowly, with continuous stirring until the color changes from

pink to purple. CALCULATION

For both methods the calculation method is the same

Where,

A = mL EDTA required to titrate sample to end point

B = mg CaCO3, when 1 mL of EDTA (0.0100 M) = 1 mg CaCO3

Magnesium hardness is determined by subtracting calcium hardness from total hardness.

REFERENCES

(1) Standard Methods for the Examination of Water and Wastewater, 19th Edition, 1995

(2) Sawyer & McCarty, Chemistry for Environmental Engineer, 3rd Edition, New York: McGraw-Hill. 1978. PP 377-384.