Rate of reaction by turbidity and colour – disappearing cross and crystal violet

What it shows

Two classic ways to follow a reaction without collecting gas. In the disappearing-cross method, Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O produces colloidal sulfur until a cross under the flask can no longer be seen. Each run stops at the same amount of sulfur, so 1/t measures the average rate: it is proportional to the thiosulfate concentration and roughly doubles for every 10 °C rise. In the colorimeter method, crystal violet reacts with excess hydroxide; absorbance follows A = εlc, and only the ln A against time graph is straight, showing first order in the dye with k' = −gradient.

How to use

Choose the Method. For the disappearing cross, set Na₂S₂O₃ 0.15 M, Temperature and Time speed, press Add HCl, start timer, then press Cross gone – stop when the cross vanishes, or use Auto measure. Switch the graph between 1/t – concentration and 1/t – temperature. For the colorimeter, set [OH⁻] and Temperature, press Start measuring and compare A – t, ln A – t and 1/A – t. Undo last, Clear table and Reset manage the data.

Parameters you can change

• Measurement method Disappearing cross (Na₂S₂O₃ + HCl), Colorimeter (crystal violet + NaOH)
• Volume of 0.15 M Na₂S₂O₃ (made up to 50 mL with water) 10–50 mL
• Temperature 10–50 °C
• OH⁻ concentration (NaOH, colorimeter) 0.01–0.1 M

Questions to explore

1. Why can 1/t be used as a measure of the rate in the disappearing-cross experiment?
2. How does the time for the cross to disappear change when the temperature rises by 10 °C?
3. Why does a straight ln A against time graph show that the reaction is first order in crystal violet?