pH in Strong Chlorate (Electrolytic Cooler)

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Sodium chlorate is an inorganic compound with the chemical formula NaClO3. It is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 300 °C to release oxygen and leave sodium chloride. Several hundred million tons are produced annually, mainly for applications in bleaching paper.


Commercial Use

The main commercial use for sodium chlorate is for makingchlorine dioxide(ClO2). The largest application of ClO2, which accounts for about 95% of the use of chlorate, is in bleaching of pulp. All perchlorate compounds are produced industrially by the oxidation of solutions of sodium chlorate by electrolysis. Sodium chlorate may be used to control a variety of plants including morning glory, canada thistle, johnson grass, bamboo, Ragwort, and St John's wort. The herbicide is mainly used on non-crop land for spot treatment and for total vegetation control on areas including roadsides, fenceways, and ditches. Sodium chlorate is also used as a defoliant and desiccant.

Application Overview

Industrially, sodium chlorate is produced by the electrolysis of a hot sodium chloride solution:

NaCl+3H2O→NaClO3 +3H2

In chemistry and manufacturing, electrolysis is a method of using a direct electric current (DC) to drive an otherwise non- spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential.

This reaction progresses in heat (at least 70 degrees Celsius), and controlled pH. In lower temperature or with high pH another reaction progresses:

2 NaCl + H2O → NaClO + NaCl + H2

The Chloralkali process is an industrial process for the electrolytic production of sodium chlorate. It can also be synthesized by passing chlorine gas through a solution of sodium hydroxide. It is then purified by crystallization.

Typical Process Details: 


Strong Chlorate (Composition: NaCl 8%, NaClO3 36%, Na2Cr2O7 0.3%, Water Balance. NaOCl 2 to 5 gpl, pH 5 to 7) 

Press Nor/Max 

1.1 / 1.8 kg/cm2 (g) 

Temp Nor/Max/Min 

85 / 90 / 25 deg.C 

Design Parameters: 

Pressure - FV/6.11 kg/cm2 (g) & Design temperature- 117 deg.C 


1330 kg/m3 @ N.O.C 

pH Nor/Max/Min 

(5.8-6.2) / 6.8 / 4.5 


Insertion type 

Insertion Length 

Minimum 60 mm 

Typical Problems:

  • Frequent cleaning


  • Use of differential sensor

Typical problems

  • Frequent cleaning


  • Use of differential sensor

Product Recommendation

Measurement System

Process Liquid Analyzer: 

  • 2-wire FLEXA pH/ORP Analyzer


  • Dual sensor measurement on 2-wire type analyser
  • Indication of sensor wellness


  • 4-wire PH450G pH/ORP Analyzer



  • Easy touchscreen operation
  • Trending display up to 2 weeks
  • Advanced Process Temperature Compensation

Sensor Selection:

FU20-FTS sensor from Yokogawa is the perfect sensor for this application.  Cleaning is probably not necessary for FU20-FTS sensor because of its design. pH electronics is required with two high impedance inputs like FLEXA series or PH450 series.

FU20-FTS is possible if the salt content is stable during the pH control step.


The FU20-FTS is a differential pH sensor. This means that the reference is not a (liquid) junction but a glass sensor which does not respond to pH changes (within the applicable range of the sensor). Therefore the sensor is truly maintenance free and the output voltage of the sensor depends only on the salt concentration of the process.

The sensor responds to pH changes rather than analyzes the accurate pH value. In that sense it is best to describe the sensor as pH control sensor rather than pH measuring sensor.

A rule of thumb is that a change in salt concentration of +/- 25% has an effect of less than 0.1pH on the pH reading.

Tangible benefit

Save down time in cleaning, repeated calibration, improve end product quality.

Note: For additional information on this application contact the local Yokogawa Process Liquid Analyzer Department

Related Products & Solutions

2-Wire Transmitter/Analyzer FLXA202/21

FLEXA ™ 시리즈 분석기는 산업용 설비의 연속 온라인 측정에 사용됩니다. 단일 또는 이중 센서 측정 옵션이 있어 가장 유연한 2 선식 분석기입니다.

4-Wire pH/ORP Analyzer PH450

PH450 pH와 ORP 분석계인 PH450은 pH와 온도, ORP(Redox) 측정을 결합한 다변형 분석계로, mA 전류 출력 2개, 독립적인 SPDT 접점 출력 및 HART 등 다양한 출력기능을 통해 활용할 수 있습니다. 

Differential pH/ORP Sensors

양이온 차 pH 및 ORP 센서는 기존 센서가 비효율적인 어려운 어플리케이션을 위해 설계되었습니다. 여기에는 전해액 공정 및 치즈 제조와 같은 다양한 어플리케이션에서 염수 용액과 같은 측정이 포함됩니다.

pH and ORP Analyzers

pH와 ORP 미터, 분석계 및 전송기는 수질/제품 품질을 보장하고 유출물 배출량, 배치 중화, 펄프 스톡, 세정기, 냉각 타워, 화학, 폐수 처리 및 기타 여러 어플리케이션의 연속 공정 모니터링에 사용합니다.


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