Test kits for water analysis
Test kits for water analysis

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Test kits for water analysis

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.

S230_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371