1.2.2 Sample pre-treatment
Cloudy or coloured samples will interfere with many of the tests to be described here - particularly the colourimetric methods in Section 2. Filtration, or perhaps allowing a sample to settle and removing the supernatant, is necessary in these cases - again everything you do to the sample must be logged. Filtration is often used in water analysis to separate dissolved components from those present in suspended matter. Filtration is also recommended when samples are going to be stored for longer periods because it removes biological materials that may react with the analyte. But filtration may present some problems such as adsorption of analytes of interest onto the filter material and/or contamination from the filter material that may dissolve into the sample during filtration.
So, once you have your sample, where is the testing done?
We've used the terms 'on-site' and 'in the field' rather loosely up to now.
To clarify things a little, this doesn't necessarily mean testing outside at, or close to the point of sampling (although it can). In practice analyses are often carried out in nearby (often temporary) accommodation, such as a caravan or Portakabin. Indeed it is possible to introduce significant random errors into your data due to adverse physical and environmental conditions if an analysis is done outside.
In addition, as with any measurement, the competence of the person carrying out the task is crucial; the robustness of many field test methods means the user need not necessarily be a trained laboratory analyst in order to carry them out.
The material you read for Activity 2 serves to illustrate another real advantage of using field kits for water analysis - sample pre-treatment is fairly minimal and preservation steps are rarely required.
Let's take an example.
In Section 6.1 we will be looking at a method of determining the concentration of dissolved oxygen in water samples which produces results in seconds. However, dissolved oxygen is in dynamic equilibrium with the air, so if you do have to transport a sample back to the laboratory for analysis a preservation step is required to lock the oxygen at the concentration it was when sampled.
This isn't a trivial task and involves adding a few millilitres of alkali-iodide-azide reagent (prepared by adding a solution of potassium iodide in sodium hydroxide, to a solution of sodium azide, NaN3, in water) and a few millilitres of manganese (II) sulfate solution in water. This produces a white precipitate of manganese (II) hydroxide, as shown in Equation 1.
The manganese (II) hydroxide reacts with any dissolved oxygen that is present in the water to form an orange-brown precipitate of manganese (III) oxyhydroxide (Equation 2).
The oxygen is now 'fixed'.
However, testing in the field negates the need for this procedure.