2.1 Test strips
Arguably field testing in its most convenient form is realised using test strips.
These establish the presence of a particular analyte (although you need to keep a weather-eye open for possible interferences) and provide you with a rapid semi-quantitative determination of a wide range of chemical species - typically in the milligrams per litre (mg l-1) range.
It's easy to take the humble test strip for granted, but there is a lot more to it than meets the eye.
Test strips are virtually laboratories at your fingertips, with the reagents located in reaction zones (small pads) at one end. Typically these pads will contain not only the reagents required to effect a chemical reaction, which will produce a coloured product (some of this chemistry is considered in Section 3), but also additional reagents such as buffers and sequestering agents. Test strips have a finite shelf life and must be stored at a temperature specified by the manufacturer.
Suggest why most reagents used in the reaction zone of a test strip tend to be insoluble or sparingly soluble in water.
This minimises the loss of reagent into the sample being tested (a process aptly referred to as 'bleeding').
To carry out a test, a strip is dipped into the test solution and after a short while colour development occurs, although for some tests the sample has to be treated in some way, such as by adjustment of pH. The reaction time is specified by the manufacturer and this is important as the colour may continue to change after this period has elapsed. The concentration of analyte in the sample is determined by visual comparison with a printed reference chart showing a series of colours corresponding to specific concentrations.
Figure 3 shows a sample of tap water being tested for nitrate (NO32−) levels using a commercial test strip.
Looking more closely at the test shown in Figure 3, the scale on the reference chart shows:
0-10-25-50-100-250-500 mg l-1 NO3-.
This indicates how accurately we can expect to be able to measure NO3- concentration.
What NO3− concentration is indicated by the strip in Figure 3?
The NO3− concentration is somewhere between 50 and 100 mg l−1.
If you have an NO3− concentration greater than 500 mg l−1 a colour more intense than the darkest colour on the reference chart will be observed.
If the nitrate concentration of your sample is greater than the maximum on the reference chart, what course of action would you take to enable you to measure the concentration of this ion using the test strip?
Dilute the sample until the concentration is within range; you would then multiply the measured concentration by the appropriate dilution factor.
As with analytical measurements in general, test strips are susceptible to problems due to interferences. Manufacturers supply data indicating the concentrations of foreign substances below which interference is negligible (an example is shown in Table 1 for the nitrate test demonstrated here).
Table 1 Interference limits for nitrate test strips. Concentration of foreign substances in mg l−1.