Background

What is hard water?

Hard water is caused by the presence of naturally occurring calcium and magnesium salts in water. Water hardness is usually noticed because of difficulty in lathering soap and the formation of a scum when washing. Calcium and magnesium ions (Ca²⁺ and Mg²⁺) form insoluble salts with soaps causing precipitation of this soap scum. Also, hard water can react with dissolved carbonates in water to form a precipitate of insoluble calcium carbonate (CaCO₃). This “scale” (also called “limescale”) can build up on the inside of water pipes to such a degree that the pipes become almost completely blocked. Hard water is considered a problem to industries, agriculture and households that have to spend more money on extra soap and the replacement of blocked pipes.

Different geographic areas have varying amounts of calcium and magnesium salts in the water, caused by soft rainwater filtering through different rocks dissolving these molecules on the way. Magnesium is found in rocks, such as dolomite and magnesite, whilst calcium is more abundant in limestone and some basalts.

The unit of measurement for hardness is milligrams per litre of calcium carbonate
(mg l^-1 CaCO₃).

Water with less than 50 mg l⁻¹ of calcium carbonate is regarded as soft, with quantities between 50 and 200 mg l^–1 as intermediate, and with quantities greater than 200 mg l^–1 as hard.

The effect of hard water on health is monitored by the World Health Organisation (WHO). It is found to potentially cause eczema in young children and there may also be a relationship between water hardness and cardiovascular health, so it is recommended to maintain a minimum total hardness of drinking water of 150 mg l⁻¹ CaCO₃.

What is a titration?

Titration is a chemical analysis where a standard solution of known concentration is used to determine the unknown concentration of a particular chemical solution. The known standard solution is added from a burette to the sample containing an unknown quantity of the chemical of interest, which it reacts with in a definite and known proportion. An indicator is often used to indicate the end point of the reaction.

In this experiment the concentration of Ca²⁺ and Mg²⁺ ions is determined using a known concentration of EDTA (ethylenediaminetetraacetic acid). EDTA is a large molecule that captures metal ions by forming several bonds to a single ion. The metal–EDTA complex is stable, and its formation takes place in a controlled 1:1 mole ratio (one mole of metal ions reacts with one mole of EDTA, forming one mole of the metal–EDTA complex).

Volumetric glassware used in titration

Glass is used to produce glassware for use in the chemistry laboratory because it is relatively unreactive, highly durable, easily customisable, and inexpensive (see Figure 1).

Choosing the correct volumetric glassware for an experiment is important. Some glassware is used to precisely measure the volume of solutions as it has been calibrated to a high level of accuracy, for example volumetric pipettes and burettes. Although much glassware is graduated, in some cases these graduations are just approximate (for example in conical flasks) and they should not be used if an accurate volumetric measurement is required.

Figure 1. Images of glassware used in a titration experiment.

Volumetric measurements using narrow tubular glassware such as a burette, can be challenging. This is because the surface of the water has a pronounced curvature, the meniscus, as shown in Figure 2. Volumetric pipettes and burettes are calibrated to take this into account.

Figure 2. The water meniscus in a burette (note that it is curved).

A complexometric titration

Water hardness can be measured using a procedure known as complexometric titration by adding a known concentration of the chelating agent EDTA through a burette to a sample containing an unknown amount of calcium and magnesium ions. EDTA reacts and captures these metal ions creating a larger metal complex. So, whilst there are Ca²⁺ and Mg²⁺ ions in the solution the EDTA will react with them until all metal ions are used up.

Figure 3. (a) EDTA molecule; (b) EDTA metal complex (the complex has a negative charge which is not shown in this representation).

The end point of this titration is indicated by a change in the colour of the solution, aided by the addition of an indicator. An indicator is a substance that has a marked colour change on completion of a particular chemical reaction, which makes it easier to identify when the end point has been reached.

Mg²⁺ and Ca ²⁺ ions are colourless at pH 10 but the solution will turn pink to red in colour when adding the indicator Eriochrome Black T (ErioT). On titration with EDTA, the solution will eventually turn blue when sufficient EDTA has been added and reacted with all the Ca²⁺ and Mg²⁺ ions present in the sample.

Figure 4. Water sample containing Ca2+ and Mg2+ ions and the indicator ErioT: (a) before titration (colour pink), (b) at the end point (colour blue).

An ammonia buffer solution is used to make sure that the sample stays at pH 10. This is important because at this pH, a marked colour change is observed at the end point and the EDTA will not react with any other metal ions in the solution (such as Fe ²⁺) that may be present in the water sample.

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