Experiment: # 1

Name: Leonie Lawrence 0800383

Title: Hardness of Water

Aim: To determine the hardness or softness of a river water sample and that of a controlled sample of CaCo3 solution.

Abstract: This experimental exercise was performed by method of titration. A

standard sample of Ethylenediaminetetraacetic acid (EDTA) solution was

titrated with a 100 ml river water sample and 100 ml CaCo3 standard

solution. Hardness calculated for CaCo3 sample (16.9 ppm) was compared

to the Lenntech Water Solution data table and result indicated this solution

is hard once the value ranges between 121-180 ppm. The river water

sample reading was (174 ppm) thereby indicating this solution was also

hard.

Introduction: Water hardness or softness is an expression for the sum/ amount of

calcium and magnesium cation concentration found in a water sample, as

sited by the E-how article ‘Water Hardness’. These cations may enter a

stream through the weathering of rocks such as limestone (CaCo3).

Calcium occurs in water naturally and is known to give water a better

taste. Calcium and magnesium cations however have the ability to form

insoluble salts with soaps, decreasing soaps cleaning effectiveness, also

forming hard water deposits in hot water heaters. The standard way to

express water hardness is in ppm, and such water quality (harness or

softness) may be calculated using the formula: ml of EDTA used in

Titration /ml of H2O sample * 10 ppm. Another method of

calculation is Volume of Water sample used * 10 ppm (since 1ml of water

sample contains 10 ppm).

The laboratory exercise to determine water hardness or softness

involves titrating the sample of river water and CaCo3 solution with a

standard Ethylenediaminetetraacetic acid solution (ETDA) until the wine

red colour of the Erichrome black T indicator (EBT) turns blue.

Method: Procedure is as seen in the Environmental Chemistry Laboratory Manual

CHY4005. Revised 2010 written by Dr Kerri-Ann Bartley-Hynes

Results: Table 1.1 Demonstrating Titration of a river water sample and a controlled

sample of CaCo3 solution.

Trial 1 River Water

Trial 2 CaCo3 solution

Volume of Sample/ml 100 100

Initial burette reading/ml 49.80 49.80

Volume of EDTA added/ml

32.40 32.90

PH 9.50 9.58

Volume of EDTA used/ml 17.40 16.90

Observation: A colour change was noted from clear to wine red when

Erichrome black T indicator was added to both river water and CaCo3 sample.

Another change noted from wine red to bluish grey after titration had been

complete with EDTA.

Table 1.2 Showing water quality measurements by Lenntech water Quality Table.

Concentration (ppm) Hardness Reading

<61 Soft

61-120 Moderately hard

121-180 Hard

>180 Extremely Hard

Calculations:

1. Calculate the hardness of the water sample: (1 ml of standard EDTA solution contains 10 ppm of CaC03)

Hardness of river water: Volume of EDTA used * 10 ppm

17.40 ml* 10 = 170 ppm

Hardness of CaCo3 solution: Volume of EDTA used * 10 ppm

16.90 ml * 10 = 169 ppm

Discussion: The experimental results obtained demonstrated that the 174 ppm

reading of the river water is an indication of the water sample being hard.

The reference table 1.2 showed that ppm (parts per million) value between

the range of 121-180 is regarded as Hard. 169 ppm was the obtained value

for the CaCo3 sample, thereby demonstrating Hardness as well. Due to the

conductivity of river water its ion concentration is more than that of the

CaCo3 sample and therefore the Hardness measurement is higher.

Calcium as an important determinant of water hardness functioned as a pH

stabilizer in the experiment because of its buffering qualities. The PH

value was calculated for NH3/NH4Cl to be:

At the onset of the experimental exercise, EDTA solution was used

to rinse the burette in order to condition it and prevent errors in the

experiment. The 2 ml buffer solution indicated in the procedure was not

enough to raise the required PH of the titration. Therefore a total of 4 ml

was used for both the River water sample and the CaCo3 solution to

provide a PH of 9.50 and 9.58 respectively. The Erichrome black T

indicator imparted a red color to the solution in the presence of calcium

and magnesium ions, thereby indicating it had not yet complex the EDTA

solution. Once the endpoint had been reached and there was no more

uncomplexed Ca or Mg, the indicator gave a blue colour. The known

solution of CaCo3 was diluted to standard 100ml. Controlling the presence

of calcium and magnesium ions in water is done by an ion exchanger

known as water softeners. These usually contain Na+ ions, which are

released and substituted by Ca2+ and Mg2+ ions. Calcium compounds can

also be applied for wastewater treatment. Drinking water pH and hardness

may be altered by means of calcium carbonate and calcium hydroxide.

Using EDTA is a practical method of demonstrating water

hardness or softness however there is a limitation to this procedure. If the

water sample does not have a sufficient amount of Magnesium it will be

extremely hard to titrate the sample to distinguish the endpoint. Other

errors such as parallax could have been prevented when conducting the

volumetric reading, as the eye must be positioned on the line directly

perpendicular to the measuring rule on the burette and other apparatus.

Conclusion: It may be concluded that the aim was achieved and both samples of river

water and CaCo3 sample were found to be ‘hard’ with water quality

measurements of 174 ppm and 169 ppm respectively.

Answers to questions given:

1. Give reason and supporting equation why the buffer is added to the sample before

titration.

A buffer is added to the reaction solution in order to maintain the PH of the

solution. Eg. The accepted experimental PH value should range 9.50-10.

Equation:

2. Calculate PH of NH3/NH4CL buffer. (Kb of Nh3 is required)

3. Giving suitable examples explain why hardness is a suitable water quality indicator.

A Water Quality Assessment article defines stream water hardness as being the

total concentration of cations, specifically calcium (Ca2+), magnesium (Mg2+), iron (Fe2+),

and manganese (Mn2+) in the water. Water hardness is a specifically important water

quality indicator as it provides a measure of the influence of human activity on the water

systems in specific areas.

An example of this is Acid Mine Drainage which often results in the release of

iron into a stream. The presence of iron produces extraordinarily high hardness readings.

Other examples include foaming agents such as those in soaps and detergents. They do

not work well in hard water. In addition hard water tends to leave hard; scaly calcium

deposits on faucets accounting for the very reason many people install water softener

systems in their homes.

References:

University of Technology Environmental Chemistry Lab manual (CHY4005) 2010, by Dr. Kerri-Ann Bartley-Hynes pg 6.

www.ganon.edu (sited definitions.)

www.enow.com

www.cotf.edu (water quality assessment article)

www.lenntech.com (water quality measurements by The Lenntech water Treatment and Purification Solutions Limited)