OTO Test
OTO (orthotolidine) is an older test reagent that measures total chlorine by producing a yellow colour in chlorinated water.
Definition
OTO (orthotolidine) is an older test reagent that measures total chlorine by producing a yellow colour in chlorinated water.
Typical Values: Measures total chlorine (combined + free); DPD is preferred
In Plain Language
OTO produces a yellow colour in the presence of any chlorine species — both free chlorine and combined chlorine. It cannot distinguish between active and inactive chlorine. Two-way test kits sold inexpensively often use OTO for chlorine testing. Using OTO creates the risk of assuming the water is adequately sanitised when it may have adequate total chlorine but very low free chlorine and high combined chlorine.
Why It Matters
OTO tests are not recommended for accurate pool management because they cannot detect the difference between effective and ineffective chlorine.
Typical Values
Measures total chlorine (combined + free); DPD is preferred
Last reviewed: 2026-06-01