measuring visible change in liquids

Measuring the appearance of cloudy sulfur when we mix acid and sodium thiosulfate is a popular science teaching activity. We can change the temperature or concentration of the reagents, and see whether the mixture goes cloudy any faster.

Often we put the beaker of mixture on a piece of paper and see how fast the writing on the paper disappears. Don’t laugh we do this a lot.

More recently we have used data loggers to measure the cloudiness of the mixture. For example:

A light sensor here monitors the cloudiness of the mixture. The beaker is surrounded by a black paper tube. The light sensor probe is inside the tube too. This seemed more scientific – except that a beaker is poor optical glass. What’s more you cannot guarantee the depth of the solution. A cuvette would be better – so would something that required less effort to set up.

A compromise is this arrangement – unarguably quick, cheap and easy. You place a plastic box or cuvette in front of the sensor. If you like, Blu-tak helps stop it falling over too easily. The measured light levels are affected by stray light but at least pupils can see the reaction taking place – this is essential because seeing the reaction vary under different conditions is what the lesson is about.

Here’s the idea – deftly modified with Blu-tak by Bellemoore School

Using a colorimeter like this is the next step along – it is key to doing better chemistry. This particular PASCO model automatically zeros and selects filters.

When you mix the liquid and put it in the colorimeter, remember to keep some back in the beaker. You can watch it change visibly as the graph on the computer changes.

Here is a typical result – collected by Laurence Rogers, author of Insight data logging software:

The graph shows four different graphs produced under four different conditions. You can join in the fun and answer these questions*:

a) if the results were taken at different temperatures, which graph is which?

b) how will you measure the rate of change from the graph?

• will you measure the gradient? If so where on the graph?
• will you measure the net change in light level?
• will you measure the overall rate of change (the average gradient)?
• will you measure the intercept with the x-axis?

After enjoying a marked improvement in results using a colorimeter it’s hard to go back to Blutack-style science. There are several opportunities to use one in advanced chemistry and a few more in biology and biochemistry work.

Use food dye and bleach and try to find the order of the reaction with respect to each reactant. Depending on whether the reaction is zero, first or second order you obtain a graph where absorbance, Ln absorbance or 1/absorbance is a straight line. Don’t quote me on that but the person who told me sounded confident.

Find the order of the reaction with respect to the reactants in the propanone – iodine reaction.
Measure the rate of breakdown of starch by amylase under different conditions. This sometimes works, but it’s not the most reliable reaction.