colorimeter for acid reacting with thiosulfate

A colorimeter for the reaction of acid and thiosulfate – rates of reaction

The reaction of acid with thiosulfate produces a precipitate in a beaker, and we normally measured how quickly a cross drawn on paper disappears. Electronic colorimeters can measure this but the following alternative may make what you’re measuring clear. That’s a pun. My alternative uses a light sensor to record the turbidity, an LED torch to act as a light source and a box used to keep plastic mints or pH paper as a cuvette. A proper cuvette needed small volumes.
The reagents are 0.1M HCl and 0.2 M sodium thiosulfate. Start with a 5ml syringe full of thiosulfate placed in the cuvette. Start recording light levels and squirt in 1ml acid and stir with anything to hand. A plastic coffee stirrer stick works well.
After recording wash up and start over without moving your setup. This time use 4ml thiosulfate plus 1ml water, start recording, squirt in 1ml acid and stir. Repeat with 3ml thiosulfate plus 2ml water and squirt in 1ml acid and stir.

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