Test kits for water analysis
Test kits for water analysis

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Test kits for water analysis

5 Titrimetric methods

A titration is a procedure whereby an unknown quantity of a particular substance is measured by adding it to a standard reagent with which it reacts in a definite and known proportion. If you refer to a balanced chemical equation for the reaction, the point when the reactants have reacted together stoichiometrically is called the equivalence point, and this is what we determine by titration.

Whereas it is possible to measure an equivalence point instrumentally, observing a colour change using a chemical indicator is more convenient. The volume of standard added when the colour change occurs is called the end-point. Ideally the end-point should be the same as the equivalence point but these do not always match exactly so there is generally an in-built error when titrating using an indicator. In practice though, this is extremely small and tends to be ignored.

Now watch Video 5 Doing titrations in which a titration is demonstrated.

Download this video clip.Video player: Video 5
Skip transcript: Video 5 Doing titrations.

Transcript: Video 5 Doing titrations.

Narrator
A titration is a classical analytical technique where an unknown quantity of a particular substance is determined by adding it to a standard solution - that's one that we know the concentration of to a high degree of accuracy. We can calculate the concentration of the unknown by referring to a balanced chemical equation for the reaction.
Modern analytical laboratories have auto-titrators, which largely do the job for us, and allow a large throughput of samples; but if you are asked to carry out a titration manually - on the bench, using standard laboratory glassware - then this is the apparatus you'll need.
We're going to demonstrate an acid-base titration. Our unknown is a solution of sodium hydroxide and our standard is a 0.1 mol l-1 solution of hydrochloric acid. We prepare this by diluting a commercially available standard. The label tells us we need to dilute to 500 ml. Notice we wash the bottle out with deionised water and make up to the mark in a volumetric flask; again with deionised water.
Now we need to prepare our burette. First, a quick check of the tap to make sure it turns freely - if it doesn't it'll need to be taken apart, cleaned and possibly greased. We then wash the burette with distilled/deionised water. Now we condition the burette using titrant solution, in other words our standard. Notice we're using a funnel to fill the burette - sometimes you have to jiggle it a little to get the liquid to flow freely so all the surfaces are coated with standard solution. Notice we don't run the solution out completely, to prevent air bubble formation in the tip. If bubbles do form they can be removed by gentle tapping. If you find bubbles are difficult to remove it means your burette is dirty and needs to be washed with detergent.
After re-filling with fresh standard, we run it out until we reach the zero mark - or whatever point we can see easily. We record this value remembering to read off the volume from the bottom of the meniscus, which must be at eye-level. Next, we pipette a known volume of our unknown solution into a conical flask. We add a couple of drops of indicator - in this case phenolphthalein - which is pink in alkali and colourless in acid. The white card below the flask will help us see the colour change.
We'll start by doing a rough titration. The next step is rather like patting your head and rubbing your tummy; notice the arrangement of the analyst's hands and how he manages to multi-task, swirling and adding the standard at the same time. For our rough titration we add the standard quite quickly, slowing a little as we start to see a colour change, but we don't worry too much about hitting the end-point exactly. We record our rough value. A good tip is to use a piece of card, or filter paper to help you read the meniscus position. So our titre is about 25 ml.
Now we repeat the titration accurately. Because we know the rough end-point, we can add our standard fairly quickly until we approach this value. Let's now fast forward to the point where we're about 3 to 4 ml away from the end-point. Now it's time to slow right down and add our standard drop by drop: closing the tap; touching the droplet to the side of the flask; and washing titrant into the flask with distilled/deionised water. Washing ensures any splashes of reagent are transferred to the solution in the flask; this will have no effect on the titration result.
The pink colour is fading - we're almost at the end-point. Notice how the indicator changes colour, in the region around the drop, when the titrant hits the solution. It then reverts to colourless as titrant disperses through the solution to react with the analyte. The end-point for phenolphthalein is when the pink solution just turns colourless. Finally, we read off the new meniscus position. The titre is the difference between the initial and final readings.
Generally, we do a rough and two accurate titrations, but if you find the difference between titres for your two accurate ones are more than about 1 ml apart, a further repetition is recommended.
End transcript: Video 5 Doing titrations.
Video 5 Doing titrations.
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The challenge for test kit developers is to take the apparatus shown in Video 5 and convert it into a form that can be used in the field.

Activity 3  Titrations in the field

Timing: Time allowed: 2 hours

A number of kits incorporating portable titration apparatus have been developed to allow titrations to be carried out on-site: hand-held titrators and drop-count bottles to name but two.

Look again at the websites of some of the major manufacturers listed in Activity 1, or any other appropriate resource you have available, and write a brief summary of two marketed kits for field titration. As far as possible include such information as detection limits, the accuracy claimed and how reagents are disposed of.

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