Reducing sugars and starch: interpreting colour standards

What you’ll learn

• Use standards to estimate the concentration of a biological molecule.
• Apply dilution factors and explain limits of a semi-quantitative assay.

Understand the idea

Core practical 1 uses colour standards to estimate reducing sugar with Benedict's reagent and starch with iodine. A qualitative result indicates presence or absence within a test's detection limits. A semi-quantitative result places a sample near, or between, known standards. If a sample lies between two reference colours, report a supported interval rather than inventing a highly precise concentration. A blue-black iodine response supports starch; a suitable Benedict's colour or precipitate response supports reducing sugar.

Comparison is valid only when the standards and unknown have been treated consistently. Relevant factors include sample and reagent volumes, timing and, for Benedict's comparisons, the controlled heating conditions. A known positive sample checks that the test can respond, and a negative control checks background colour or contamination. Strongly coloured or cloudy biological extracts can interfere, so interpret an appropriate sample blank and recognise that visual matching is subjective.

Dilution brings an unknown into the useful range of standards. The dilution factor is final volume divided by the volume of original sample used. Multiply the estimated diluted concentration by that factor to recover the original concentration. These assays are interpreted within supervised laboratory methods, particularly where heated reagent or chemical splashes are involved.

Key formulae

$$\mathrm{dilution\ factor}=\frac{V_{\mathrm{final}}}{V_{\mathrm{sample}}}$$

$$c_{\mathrm{original}}=c_{\mathrm{diluted}}\times\mathrm{dilution\ factor}$$

Worked example

A food extract is diluted using one volume of extract and three volumes of water. Its valid colour comparison lies between standards of $1.0$ and $1.5\ \mathrm{g\,dm^{-3}}$. Estimate a concentration interval for the original extract.

1. The final mixture contains four volumes for each original sample volume, so the dilution factor is $4$.
2. Multiply both ends of the estimated interval by $4$.
3. The original concentration lies between $4.0$ and $6.0\ \mathrm{g\,dm^{-3}}$. The visual evidence does not justify a single exact value within this interval.

Answer: Approximately $4.0$–$6.0\ \mathrm{g\,dm^{-3}}$.