How to Test Residual Chlorine in Water Using O-Toluidine – A Standard Guide for Laboratories and Water Plants
1. What is O-Tolidine?
· Chemical name: Ortho-Tolidine
· Molecular formula: C14H16N2
· CAS Number: 119-93-7.
· Properties: Colorless to pale yellow liquid, distinctive odor, volatile and toxic.
This is a color indicator used in chlorine residual quantification by spectrophotometry.
2. Why test residual chlorine in water?
Chlorine is a common disinfectant used in:
· Drinking water
· Swimming pools
· Treated wastewater
However, excess residual chlorine can cause skin irritation, respiratory and digestive issues, while too little chlorine fails to disinfect effectively.
WHO allowable residual chlorine levels:
· Drinking water: 0.2 – 0.5 mg/L
· Wastewater discharge: ≤ 1.0 mg/L (depending on standards)
3. Principle of residual chlorine testing using O-Tolidine
When O-Toluidine reacts with free chlorine, it forms a yellow-colored compound (chlorinated aniline):
The deeper the yellow color → The higher the chlorine concentration.
· Color can be assessed visually against a standard chart
· Or measured by spectrophotometer at 430 nm wavelength
4. Safety precautions when using O-Toluidine
O-Toluidine is highly toxic and possibly carcinogenic (IARC Group 2B), so you must:
· Work in a fume hood
· Wear full gloves, masks, and safety goggles
· Avoid inhalation and direct contact with skin or eyes
· Dispose of chemical waste properly according to environmental regulations
5. Comparing O-Toluidine and DPD methods
|
Criteria |
O-Tolidine |
DPD |
|
Sensitivity |
Medium |
High |
|
Safety |
More hazardous |
Safer |
|
Reaction time |
1–2 minutes |
30 seconds |
|
Color produced |
Yellow |
Pink-purple |
|
Usage popularity |
Traditional |
WHO/ISO standard currently |
Recommendation: Use DPD for higher safety environments; O-Toluidine remains economical and common for small to medium water treatment facilities.
6. Purchase high-quality O-Tolidine at D&H
D&H is a chemical product offering:
· Analytical grade product – purity >99.5%
· Ready-to-use convenient solutions in safe packaging
· Technical support and MSDS provided
· Available in 100 mL, 500 mL, 1 L bottles , delivery and 24/7 consultation
7. Contact for advice, pricing & samples
Email: [email protected]
Zalo/Hotline: +84 913390054
Website: https://dhchemical.com.vn/