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
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
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
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. He used a home-made colorimeter - which
doesn't help my point
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
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 science. There are several opportunities to use
one in advanced chemistry and a few more in biology and biochemistry
- 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
Activities in this section adapted from The IT in Science book of Data logging and Control. © IT in Science and may be reproduced as needed for use within your school.