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Orthotolidine (OT) test and Orthotolidine – Arsenite (OTA) test

Free chlorine combines with ammonia and nitrogen of organic matter to form ‘Chloramines’
Chloramines are not efficient residual disinfectants
Yet these get included in the Total chlorine content of the water to be tested
We intend to measure only the ‘free residual chlorine ’ as this only will provide protection from subsequent contamination
We can measure ‘free chlorine ’ using OT test 10 seconds reading
We can measure ‘total chlorine ’ using OT test 5 minutes reading
‘Combined chlorine ’ = total chlorine - free chlorine
Orthotolidine (OT) test
Enables Total, free and combined chlorine in water to be determined with speed and accuracy
The reagent is ‘analytical grade Orthotolidine’, dissolved in 10% solution of HCl
When this reagent is added to the water containing chlorine , it turns yellow
The intensity of yellow colour is proportional to the concentration of chlorine
This yellow colour is produced by both free and combined 〖Cl〗_2 residuals
However, OT reacts with free chlorine instantaneously but reacts more slowly with combined chlorine
Add 0.1 ml of reagent to I ml of water
Yellow color is produced and matched against charts within 10 sec, this gives the amount of ‘free chlorine ’
The yellow color appearing after a few minutes, gives the amount of ‘Total chlorine ’
The orthotolidine test, however, is not accurate, because the impurities such as iron, manganese, nitrate etc., are likely to cause a false yellow color, and indicating wrong and increased chlorine residual
Orthotolidine – Arsenite (OTA) test
This is an improvement of the OT test
This also determines the free and combined chlorine residuals
In addition, the yellow color caused by the presence of other interfering substances such as nitrites, iron and manganese is noted AFTER DE-CHLORINATING the water sample
Hence the error is overcome
Park’s Textbook of Preventive and Social Medicine
Howard W. Adams, A. M. Buswell and L. H. Enslow. THE ORTHO-TOLIDINE TEST FOR CHLORINE [with DISCUSSION]; Journal (American Water Works Association), Vol. 25, No. 8 (AUGUST, 1933), pp. 1118-1139

OT and OTA test:

Membrane processes in water purification:

Super Chlorination and Break Point Chlorination:

Slow sand filter:

Rapid sand filter:

Purification of water at a large scale:

Criteria for a Chemical to be Suitable as Water Disinfectant in Water Supplies:

Water Supplies Disinfection with Chlorination:

Principles of Chlorination:

Methods of Chlorination in Water Supplies:

Horrock's Apparatus:

Disinfection of well during emergency:

Double Pot method:

Disinfection of Wells:

OT and OTA test:

Domestic or Household Purification of Water: