The amount of residual chlorine left in the chlorinated water after the required contact period, can be experimentally determined by using any of the following tests:
In this test, 10 ml of chlorinated sample of water is taken after the required contact period, in a glass tube. To this 0.1 ml of orthotolidine solution is added. The color formed is observed after 5 minutes. The formation of yellow color normally indicates the presence of chlorine (either combined or free) in the water. The more yellow the color, the greater, is the chlorine residual. The amount of residual chlorine can be ascertained by comparing the colour developed in the glass tube with the standard colors already kept in the laboratory. This test, is therefore, very simple and does not require much technique or time. Under normal conditions, a lemon yellow color is satisfactory for public water supply. The orthotolidine test will normally gives the total residual chlorine present in water. However, it may be adjusted so as to give separately the quantities of free residual as well as combined residual of chlorine. The free residual chlorine forms the yellow color during the first 5 seconds of the addition of orthotolidine, while the combined residual chlorine goes on forming the colour for about 5 minutes. Hence, the colour after 5 seconds will give the quantity of free residual chlorine, and the colour after 5 minutes will give the free and combined chlorine. The difference in value between the two values is the combined chlorine. The orthotolidine test, however, is not accurate, because the impurities such as iron, manganese, nitrate etc., are likely to cause a false yellow colour, and indicating wrong and increased chlorine residual.
Orthotolidine Arsenite test
To overcome this problem Orthotolidine – Arsenite (OT A) test is carried out. Sodium arsenite is added to the chlorinated sample of water. This will dechlorine the sample and orthodolidine is then added. The colour formed (Xl) now is only the intensity of colour caused by interfering agents like nitrates, iron, manganese etc. Now another sample is taken in another test tube and orthotolidine solution is added first, and just after 5 seconds, sodium arsenite is added. The sodium arsenite will arrest the colour to be formed by the combined chlorine. Hence, the colour observed at the end of the experiment (X2) will be due to the free residual plus due to the interfering colour causing compounds of iron, manganese etc. Then, a third test is conducted on a third sample of water. In this, only the orthotoldine solution is added to the given sample of water and the colour is noted after 5 minutes. The noted colour (X3) will evidently be due to the free and combined chloeine plus due to interfering colour causing copounds.
Now the different chlorine residuals can be easily determined as follows:
Free residual chlorine = (X2) - (X1)
Combine residual chlorine = (X3) - (X2)
In the orthotolidine test as well as in the OT A test, the temperature of water is to be controlled to room temperature.
Diethly-p-Phenylene-Diamine (DPD) Test
This test is widely used nowadays. The testing is carried out in a colour comparator, so as to easily compare the developed colour with the standard colours. The comparator is provided with a special dulling screen with a disc which must be fitted to the right hand viewing window of comparator. The comparator discs will be available in two different chlorine concentration ranges;
0.1 to 1.0 mg/L of chlorine and 0.2 to 4.0 mg/L of chlorine.
In order to carry out the test, 10 mL of water sample is taken in a tube, and it is placed in the left compartment of the comparator. A reagent tablet (DPD chlorine tablets) is placed in another tube to which 1 cm depth of water is added and the tablet is allowed to dissolve. More water is now added to this until the tube contains 10 mL. Now the comparator disc is rotated till the colours match. The residual chlorine amount can be directly read from the window in the lower right corner of the instrument.
This is similar to DPD test with comparator instrument, called chloroscope. Orthotolidine is added to the sample of water to produce colour and that colour is matched with standard colours by rotating the disc through the viewing window.