Sodium Thiosulfate (Laboratory Grade)

Laboratory-grade sodium thiosulfate (sodium thiosulphate pentahydrate, Na2S2O3·5H2O, CAS 7772-98-7) is a colourless crystalline reagent at 99.5% minimum purity to ACS specification, supplied by Global Chemicals LLC from stock in Muscat in 500 g, 1 kg and 5 kg containers, with standardised 0.1 N and 0.01 N solutions on request. Every lot ships with a certificate of analysis and safety data sheet; the technical liquid grade for plant dechlorination is listed under oilfield chemicals.

The titrant of iodometry. Thiosulfate reduces iodine to iodide quantitatively, so every oxidant that liberates iodine from potassium iodide is measured with it: free and total chlorine and hypochlorite assay, bromine, hydrogen peroxide and persulphate, copper, and dissolved oxygen by the Winkler method, which water, aquaculture and environmental laboratories across Oman still run beside their electrodes. The end point is the disappearance of the blue starch-iodine colour. Thiosulfate is not a primary standard; standardise the solution against potassium iodate or potassium dichromate, and make it up in freshly boiled, cooled water with a little sodium carbonate or borax as preservative, kept in an amber bottle, because carbon dioxide, bacteria and light slowly decompose it.

Sample preservation. Microbiology laboratories put thiosulfate in sample bottles before sterilising them, about 0.1 mL of a 3% solution per 120 mL bottle, so the chlorine residual in a treated-water sample is destroyed at the moment of sampling and does not keep killing bacteria on the way to the laboratory, which would give false-negative coliform results. Analytical laboratories use it the same way to quench chlorine before organic analysis, and photographic and silver-recovery work uses it as the fixing salt.

Handling and storage: sodium thiosulfate is not hazardous. The pentahydrate effloresces in dry air and cakes in humid air, so keep containers tightly closed; it reacts with acids to release sulphur dioxide, so store it apart from them. Weigh it with clean spatulas and dissolve in distilled water.

The laboratory chemicals supplier guide linked on this page lists the full reagent range, grades and documentation. Delivery is same day within Muscat, next day to Sohar, Nizwa, Salalah and Duqm, and by road across the GCC with a certificate of origin. Request a quote with the pack size and quantity, or the normality and volume of solution you need.