Thursday, November 17, 2011

Determination of total hardness


Determination of total hardness (EDTA titrimetric method)


All natural waters contain dissolved cations and anions. Water dissolves many ions as it flows through minerals. Although water hardness is defined as the quantity of cations with a +2 or +3 charge, calcium ion and magnesium ion are the most common of such ions in natural water. The formation of solid calcium carbonate is an endothermic process. Thus, when water containing both carbonate and calcium ions is heated, calcium carbonate can precipitate out onto the walls of pipes, boilers, and household items such as

tea pots. This can shorten the life-time of some of these items.

In addition, an insoluble scum develops when hard water comes into contact with soap. Both calcium and magnesium ions are responsible for this precipitate. This scum can be very difficult to clean. However, there is some evidence that hard water has beneficial health effects. Selenium, for example, may help prevent cancer. Soft water drinking supplies have been associated with an increased heart attack risk.

Total hardness (TH) is defined as the sum of the multivalent cations in the water. Calcium (Ca2+) and Magnesium (Mg2+) tend to be the largest components of hardness, so
TH is typically approximated as these two components.

TH = E(Multivalent cations) = Ca2+ + Mg2+

Water is classifies as soft or hard depending on the amount of hardness ions present. Surface water is generally soft fewer minerals dissolve in it. However surface water can be hard. Total hardness can be divided into two components which is carbonate hardness (CH), also known as temporary hardness and non carbonate hardness (NCH), also known as permanent hardness: TH = CH + NCH

When the hardness is numerically greater than the sum of the carbonate alkalinity and the bicarbonate alkalinity, that amount of hardness which is equivalent to the total alkalinity is called “carbonate hardness”; the amount of hardness in excess of this is called “noncarbonated hardness”. Carbonate hardness cannot be greater than total hardness. When the hardness is numerically equal to or less than the sum of carbonate and bicarbonate alkalinity, all of the hardness is “carbonate hardness” and there is no “noncarbonated hardness”. The hardness may range from 0 to 100 of mg CaCO3/L, depending on the source and treatment to which the water has been subjected.


To determine the total hardness (in mg CaCO3/L) in the samples given



1. Pipette

4. Measuring cylinder (50ml, 100ml)

2. Pipette filler

5. Burette with retort stand

3. Beaker

6. Conical flask (250ml)


1. Eriochrome Black T indicator

2. Buffer solution

3. Standard EDTA titrant (0.01M)

4. Standard Calcium solution


1. Chemical such as buffer solution, standard EDTA titrant (0.01M) and standard Calcium solution are prepared by the lab assistance.

2. A 25ml sample is diluted to about 50ml with distilled water in a 250ml conical flask.

3. 1ml buffer solution is added.

4. A very little of Eriochrome Black T powder is added into conical flask and the sample is mixed well.

5. The sample is titrate with standard EDTA titrant slowly, with continuous stirring until the initial red tinge color is changed into light blue color (end point).

6. The tittration is completed within 5 minutes, and is measure from time of buffer addition.

7. The volume of standard EDTA titrant used is recorded.

8. The above steps are repeated with other samples given.


Hardness (EDTA) as mg CaCO3/L = (AxBx1000)/ ml Sample

A = ml titration for sample end

B = mg CaCO3 equivalent to 1.00 ml EDTA titrant



B = 1.0; ml sample = 25ml




Volume of standard EDTA titrant used (ml)



Hardness (mg CaCO3/L)




From the lab result, we can see that different volume of standard EDTA titrant is needed to change sample from the initial red tinge color into light blue color. Both sample A and B required 1.1ml and 1.9ml respectively to perform the changes. After perform the calculation, the total hardness of sample A is 44 mg CaCO3/L whereas the total hardness of sample B is 76 mg CaCO3/L.

Softening is the process of removing hardness which is one of the operations inside a water treatment plant. Ion exchange and precipitation are typical methods used. Effluent of water treatment plant always contains lower value of total hardness than influent. Therefore, sample A which contains less total hardness is effluent and sample B is the influent.

As we know, the indicator turns red when added to a solution containing Ca and Mg ions. EDTA, the titrant, complexes with Mg and Ca cations, removing them from association with the indicator. When all the Mg and Ca are complexed with EDTA, the indicator will turn blue. According to the lab result, sample A need less EDTA to change from the initial red tinge color into light blue color and its total hardness is lower than sample B. That means sample A containing less Ca and Mg ions if compared to sample B.

In our experiment, the chemical is prepared by lab assistance. During the preparation of standard calcium solution, the acidification and boiling step is needed to remove carbonates, which if present, will precipitate CaCO3 when the solution is made basic. The precipitate obscures the end point. The excess HCl must be neutralized carefully. If too much NaOH is added, Mg(OH)2 precipitates, giving a poor end point.

It is necessary to keep the pH of sample solution at about 10 for two reasons: (a) all reactions between metal ions and EDTA are pH dependent, and for divalent ions, solutions must be kept basic (and buffered) for the reaction to go to completion; (b) the eriochrome black T indicator requires a pH of 8 to 10 for the desired color change. If the pH of the sample solution is raised to about 13, magnesium will precipitate as the very insoluble hydroxide which will not react with EDTA.



The total hardness of sample can be determined from this experiment. Influent and effluent from sewage treatment plant shows different value of total hardness. Both sample A and B recorded 44 mg CaCO3/L and 76 mg CaCO3/L respectively. Total hardness of sample A is lower than sample B. From this result, we can conclude that sample A is the effluent and sample B is the influent.


1. Complete titration within 5 minutes, measured from time of buffer addition.




3. “Introduction to Environmental Engineering” (Second Edition), 2004, P. Aarne Vesilind & Susan M. Morgan, Thomson Brooks/Cole.

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