Investigating Reaction Order with Sodium Thiosulfate and a PASCO Colorimeter

The reaction between sodium thiosulfate and hydrochloric acid is a classic way to study rates of reaction. As the reaction proceeds, a yellow sulfur precipitate forms, turning the solution opaque. Using a PASCO colorimeter, students can now measure this change quantitatively and determine the reaction order with precision.

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The Experiment

The reaction is:

$$\text{Na}_2\text{S}_2\text{O}_3(aq) + 2\text{HCl}(aq) \rightarrow 2\text{NaCl}(aq) + \text{SO}_2(g) + \text{S}(s) + \text{H}_2\text{O}(l)$$

Traditionally, students time how long it takes for a cross beneath the flask to disappear. With a PASCO colourimeter, the reaction becomes measurable in real time: the sensor tracks light transmission as the solution becomes cloudy.

Students run several experiments, varying:

• Sodium thiosulfate concentration while keeping acid constant, or

• Acid concentration while keeping thiosulfate constant.

The colorimeter records transmittance vs. time, which can be converted into reaction rate data for analysis.

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The Science

As the sulfur precipitate forms, light transmission decreases. The rate of this change is directly related to how fast the reaction occurs.

By plotting 1/transmittance (or absorbance) against time and comparing runs with different concentrations, students can determine how rate depends on concentration.

If rate ∝ [thiosulfate]¹, the reaction is first order in thiosulfate; if rate ∝ [thiosulfate]², it is second order. The slope of the initial rate graph provides quantitative evidence of reaction order.

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Skills Highlight

• Using a PASCO colorimeter to collect quantitative reaction data

• Calculating initial reaction rates and plotting rate–concentration graphs

• Determining reaction order from experimental evidence

• Understanding how kinetics connects to chemical mechanism

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Why It Works in Teaching

The colorimeter replaces guesswork with real data. Students see how a qualitative “disappearing cross” experiment becomes a precise kinetic analysis, linking visible changes to concentration and time. It’s a perfect example of chemistry moving from observation to quantification.