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Test kits for water analysis
Test kits for water analysis

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2.2 Colour comparators

Another device for rapidly determining the concentration of an analyte by eye is the colour comparator. In a nutshell, a reagent is added to the sample and a colour develops. This is visually matched to a series of standard solutions or a reference chart. In Activity 1 you probably came across a number of variations on this theme from different manufacturers.

A schematic of a common form of comparator is shown in Figure 4. Rather than standard solutions, this uses a disc which incorporates a colour scale as a series of glass filters around the circumference simulating the colour of the standards continuously. This is inserted into a viewer which also accommodates the sample (coloured by addition of reagent) and a sample blank. The latter is lined up with the disc, the coloured filters are rotated into position next to the test sample and the colours compared by looking through a viewer.

Figure 4  Colour comparators (Thomas and Ando, 1996)

Question 6

Suggest what factors dictate the accuracy and precision of analysis by visual comparison methods (using a test strip or colour comparator).

Answer

You may have considered factors such as:

  • the effectiveness of the analyst's eyesight - gradations in colour may be quite subtle on the reference chart
  • variations depending on the ambient lighting, e.g. strong sunlight
  • limitations on how small the concentration increments are represented by distinguishable colours on the reference chart
  • if using a reference colour chart, the effectiveness of the printing process used to produce it
  • reference charts may fade in the light.

The concerns raised in the final three bullet points are avoided if coloured glass standards are used.