The progress of some chemical reactions can be assessed by monitoring the rate of formation of a precipitate that makes the solution turbid, or where there is a perceptible change in colour.

Look at this video – it shows the reaction between sodium thiosulfate and hydrochloric acid.

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What do you think is the main pitfall with measuring the rate of this reaction?

Deciding when the cross has disappeared, as this represents a fixed amount of sulfur that has been created and will be sufficient to obscure the cross. Typically students have difficulty deciding when the cross has disappeared. Also this method only measures the average rate over the course of the practical. 

The other issue here is one of safety because the reaction produces toxic sulfur dioxide and whilst observing the change students could inhale the gas evolved.

For students, the key to the successful monitoring of rate of reaction is an awareness of the equipment that is available to allow accurate measurements to be made.

To what equipment could they be introduced in order to make it easier for them to monitor rate?

A data logger with a light sensor is particularly useful here. Not only can the end of the reaction be detected but also the progress of the reaction can be tracked by providing a continuous monitoring method.

Colour change can also be monitored using a colorimeter.