II51 Hypochlorite

By Yasuo Seto


Hypochlorite a (HClO) is a weak acid, which exerts oxidative action; it is being widely used in the salt forms for bleaching and other purposes. Sodium hypochlorite (NaClO) aqueous solution is usually used as a disinfectant, bleaching agent, fungicide and ink eraser, and is thus usually included in detergents for laundry. Calcium hypochlorite [Ca(ClO)2, bleaching powder] is also used as a bleaching agent and disinfectant. The kinds of household utensils containing hypochlorite are numerous; therefore the poisoning cases, due to accidental ingestion of hypochlorite and due to inhalation of chlorine gas produced upon mixing a hypochlorite salt with an acidic solution, are relatively many. The toxicity of the aqueous solution of a hypochlorite salt is due to injuries of the mucous membranes caused by its strong alkali and oxidizing actions, and due to disturbances of the respiratory organs caused by inhalation of chlorine gas produced under acidic conditions. The oral LD50 value of hypochlorite for rats is 850 mg/kg.

To detect hypochlorite, a preliminary qualitative test using the iodine-starch paper is being usually used. However, this method is not specific for hypochlorite, but is generally responsive to every oxidizing compound; it is not suitable for specifying hypochlorite. For analysis of hypochlorite, the detection of a hypochlorite ion is most preferable. In this chapter, a capillary electrophoresis (CE) method for analysis of hypochlorite is presented.

Reagents and their preparation

Sodium hypochlorite solution commercially available is diluted with 0.05 M NaOH solution to prepare 500 |g/mL (in the form of a hypochlorite ion) stock solution. Various concentrations of the solution are prepared by diluting the stock solution with 0.05 M NaOH solution for constructing a calibration curve.

CE conditions

CE column: a fused silica capillary column (104 cm x 50 |m i. d.)

CE conditions: an HP3DCE system (Yokogawa Analytical Systems, Tokyo, Japan); buffer solution: HP Basic Anion Buffer; column temperature: 30 °C; impressed voltage: -30 kV; detection: indirect absorption detection (signal 350/20 nm, reference 275/10 nm); sample injection: pressure injection (50 mbar, 6 s).

Procedure i. A specimen solution is appropriately diluted with distilled water, and passed through a membrane filter of 0.25 pm cellulose acetate.

ii. The above solution is placed in a sample vial to be set for CE.

iii. Calculation: known concentrations of hypochlorite solutions are analyzed by the above CE to construct a calibration curve (concentration of a hypochlorite ion in a specimen vs. peak area). A peak area obtained from a specimen is applied to the calibration curve to obtain its concentration.

Assessment of the method

> Figure 1.1 shows an electropherogram for the standard solution of 500 pg/mL hypochlorite ion. The hypochlorite ion (ClO-) was eluted at 8.2 min. Peaks observable at 6.0, 6.9 and 7.1 min were due to chloride ion (Cl-), chlorate ion (ClO-) and carbonate ion (CO2-), respectively. Even in the undiluted sodium hypochlorite solution, decomposition product chloride and chlorate ions are contained. During preparation of the solutions and also during analysis, there is a possibility of decomposition of the hypochlorite ion. As shown in > Figure 1.1, the peaks of such decomposition products are remarkably observed even in the electropherogram for the standard sodium hypochlorite solution. The carbonate ion observed in the chromatogram is due to atmospheric carbon dioxide, which is easily dissolved in alkaline solution.

The present CE conditions can be used for general analysis of negative ions, including cyanide, azide and arsenate [1, 2]; the conditions can be also applied to chlorate and analogous anions. The pH of the buffer is about 12; under this condition, the decomposition of hypochlorite ion is suppressed. Since it is difficult to prepare the standard hypochlorite solution without any decomposition, the true hypochlorite concentration in a specimen is lower than the value calculated using the calibration curve.

Electropherogram for the hypochlorite ion (data donated by Yokogawa Analytical Systems).

Toxic and fatal concentrations

The hypochlorite is decomposed very easily. Especially in biomedical specimens, it reacts with proteins and other components and is decomposed very rapidly. Because of such reasons, there are no data available on the fatal blood levels of hypochlorite. There is a report describing the difficulty for detecting hypochlorite from blood of a victim, who committed suicide by ingesting a bleaching agent [3].

Notes a) Hypochlorite is very unstable in acidic aqueous solution. It is decomposed to produce chlorine gas. At neutral pHs, the compound produces both chlorate and chloride ions by disproportionation reaction; at alkaline pHs, it is relatively stable. Hypochlorite is decomposed under weakly alkaline or weakly acidic conditions to produce chloride ion together with oxygen in the activated state, resulting in its bleaching action.


1) Yokogawa Analytical Systems (1999) Instruction Manual for the Kit To Be Used for Capillary Electrophoresis Analysis of Toxic Anions, HP5964-3029, Tokyo (in Japanese)

2) Soga T, Ross GA (1999) Simultaneous determination of inorganic anions, organic acids, amino acids and carbohydrates by capillary electrophoresis. J Chromatogr A 837:231-239

3) Kamiyama S, Beppu K (1979) A fatal suicidal case by drinking of liquid bleach. Jpn J Legal Med 33:13-19 (in Japanese with an English abstract)

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