When 0.15M sodium thiosulphate and 2M hydrochloric acid are mixed together they react to form a precipitate of sulphur and the mixture goes cloudy. In this investigation a Light Level Sensor is used with a datalogger to monitor the change in light level through the reaction mixture as the turbidity increases. Different concentrations of sodium thiosulphate are used to see how this affects the rate of production of the precipitate.

A simple fully-enclosed DIY turbidimeter with light source and a Light Sensor can be constructed to hold a boiling tube.

Na2S203(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + SO2(aq) + S(s)


Hydrochloric acid (HCI)
10% to 25% (Solutions 1M to 6.5M)
Xi; R36/37/38
Wear nitrile gloves and eye protection.
Refer to Hazard Sheets for First Aid.

  1. Assemble the apparatus as shown in the diagram. Connect the terminals of the light source in the turbidimeter to a suitable low voltage power supply and switch on.

  2. In four separate boiling tubes make up the following solutions:

    Tube 1 40cm3 Na2S2O3 + 0cm3 water Label as 0.15M
    Tube 2 30cm3 Na2S2O3 + 10cm3 water Label as 0.113M
    Tube 3 20cm3 Na2S2O3 + 20cm3 water Label as 0.075M
    Tube 4 10cm3 Na2S2O3 + 30cm3water Label as 0.038M

    Place the label at the top of the boiling tube so it does not interfere with the measurements. Note the temperature of the solutions.

  3. Click the Launch button to load the logging software.

  4. Click on the Run Icon to begin logging. 

  5. Put 5cm3 of 2M hydrochloric acid into a boiling tube and place in the turbidimeter box. Add the 0.15M sodium thiosulphate solution from Tube 1, stir and immediately start logging the data. Seal the box from ambient light.

  6. Repeat the experiment with the other concentrations of thiosulphate (use an Overlay facility if available).
  1. Identify the set of data that corresponds to each concentration of thiosulphate.

  2. The rate of reaction in this experiment may be inferred in a number of ways:-

    a. The average gradient of the trace during e.g. the first minute.
    b. The time taken for the light level to fall to an arbitrary graph figure.
    c. The time taken for the light level to fall to half the initial value.

  3. Use the appropriate software to create a graph showing the rate of reaction against concentration of sodium thiosulphate.

  4. The results can be saved, printed or copied into your report document as required.
  1. How does the rate of reaction vary with the initial concentration of sodium thiosulphate? Suggest a reason.

  2. All the reactions are virtually over after 5 minutes. How do the shape of the graphs tell you that this is the case? Is there a relationship between the final turbidity and the initial concentration of sodium thiosulphate?

    In another variation of this experiment try also varying the molarity of the acid (0.5 and 1M) or the starting temperature (30, 40, 50 and 60°C) of the reactants.

  3. What factors increase the rate of this reaction?

  4. The reaction is followed here by changes in turbidity as colloidal sulphur forms. How does this differ from colorimetry which largely uses the same experimental set-up? Why is it important to use coloured filters in colorimetry?
Similar procedures to those shown here can be used in the following colorimetric experimental investigations:-

Iodine Clock reaction

Autocatalysis - reaction between manganate(VII) and ethanedioic acid

Propanone-iodine reaction

©1999 Data Harvest
Education		 Chemical Reactions Sensing Science