risk assessment (thiosulfate)

Risk assessment (thiosulfate): Do you have a RA for the reaction of 0.1M sodium thiosulfate with 2M hydrochloric acid, especially the cleaning up and disposal.

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Publication Date: 30 July 2015
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risk assessment

For anyone who wishes to carry out their own site-specific risk assessment for any science activity, we have published an ASSIST information sheet AIS: Risk management and risk assessment which offers an instructional guide on how to carry out and document a risk assessment using the Science ASSIST risk assessment template. We do not carry ready-made risk assessments for specific activities carried out in schools.

Prior to conducting any science activity it is good practice to go through the four-step process of:

  1. Identifying the hazard
  2. Assessing the risk
  3. Implementing the control measures
  4. Monitoring and reviewing the effectiveness of the control measures

For the simplicity of answering the question, we shall deal here with the chemicals used and produced in this activity.



2.Assess the risk

3.Implement control measures

4.Review effectiveness of control measures Ø

0.1 mol/L sodium thiosulphate

body tissue irritant


Wear safety glasses and lab coat


2 mol/L hydrochloric acid

corrosive to eyes and skin


Wear safety glasses and lab coat


sulphur dioxide gas

skin and eye irritant


Carry out this reaction in a well-ventilated area and wear safety glasses and lab coat


Ø Ask this question: ‘Are the risks effectively controlled?’


Left over reaction mixtures from this activity consist of solid sulfur and other sulfur species and some unreacted reactants, such as excess hydrochloric acid in the waste mixture. It is suggested that used glassware is cleaned using a bottle brush and detergent and then rinsed (or wash in a dish washer) as soon as possible to reduce the likelihood of a film of sulfur drying on the glassware.

The best practice for dealing with this waste is to:

  1. Neutralise the collected waste with a carbonate, such as a solution of sodium carbonate or solid sodium carbonate or calcium carbonate (marble chips) in portions, until all the acid has been depleted by bubbling off as carbon dioxide in a fume hood. This can be determined by using an indicator such as universal indicator to achieve a pH of between 6–8.
  2. Once neutralised, the solids in the waste mixture should be filtered out before flushing the filtrate down the sink. A funnel with a wad of cotton wool is adequate for this purpose. This way all the sulfur species present in solution are removed together with other insoluble matter.
  3. The separated insoluble matter with sulfur products can be disposed of together with general waste given that the quantities produced in this activity are relatively small. Please refer to our previous response on sulphur disposal for further information.

Disposal of relatively small quantities:

If the class size is very small and the quantity of waste is small, then the amount of sulfur residue would be negligible and there would be only a small amount of excess hydrochloric acid in the waste mixture, then we advise that waste from this activity could be diluted further with plenty of water before disposing of down the sink. 


Flinn Scientific Inc. 2009. ‘Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid’ CHEM FAX publication, no: 91860 https://www.flinnsci.com/media/622120/91860.pdf  

‘Safety Data Sheet: sulfur’, August 2014. Chem-Supply website https://www.chemsupply.com.au/documents/ST0061CH71.pdf

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