The Lovibond comparator is an example of a colorimeter made in Britain by The Tintometer Ltd.
It was invented in the 19th century by Joseph Williams Lovibond and updated versions are still available.
Given those assumptions, the concentration will be indicated by the disc which best matches the color of the solution.
If a more exact measurement is required other tests can be conducted in a laboratory.
It is used in a chemistry laboratory, where the pH of the sample can be measured (approximately) using the Lovibond comparator.