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Calibration, standards and serial dilutions

Edexcel IAL Biology · WBI16 Practical Skills in Biology II

Unit 6 adds 'discuss calibration of instruments'. Calibration checks an instrument against known values so readings can be trusted or converted to real quantities.

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Calibrating instruments

Calibration curves

A calibration curve converts a reading (absorbance, colour, time) into a concentration.

1. Make a series of standard solutions of known concentration.

2. Measure each with the method (colorimeter absorbance).

3. Plot reading against known concentration and draw a line or curve of best fit.

4. Measure the unknown and read its concentration from the curve.

Used in CP1 (glucose), CP2 (vitamin C, C₁V₁ = C₂V₂ also credited in the October 2023 mark scheme), CP18 (starch remaining, amylase activity).

Serial dilutions

Each step dilutes by the same factor. A 10-fold serial dilution: 1 cm³ of solution + 9 cm³ of water, mix, take 1 cm³ of that + 9 cm³ water, and so on: 10⁻¹, 10⁻², 10⁻³...

C₁V₁ = C₂V₂ for a single dilution: to make 20 cm³ of 0.25% from a 1% stock, V₁ = (0.25 x 20) / 1 = 5 cm³ stock + 15 cm³ water.

Serial dilutions are used to make standards and to dilute bacterial cultures to countable plates (CP13).

Worked example

How would you make 50 cm³ of 0.2 mol dm⁻³ sucrose from a 1.0 mol dm⁻³ stock?

  1. C₁V₁ = C₂V₂: 1.0 x V₁ = 0.2 x 50
  2. V₁ = 10 cm³ of stock
  3. Add 40 cm³ distilled water

Answer: 10 cm³ stock + 40 cm³ distilled water.

Key idea: Calibrate: colorimeter with a blank and standards, pH probe with buffers, graticule with a stage micrometer. Calibration curve = readings of known standards used to convert unknowns. Serial dilution by a constant factor; C₁V₁ = C₂V₂.

Check yourself

  1. What volume of a 2.0% stock is needed to make 40 cm³ of a 0.5% solution?
    Show answer

    10 cm³. V₁ = 0.5 x 40 / 2.0 = 10 cm³.

  2. A culture is diluted 10-fold three times in series. What is the overall dilution factor?
    Show answer

    1000. 10 x 10 x 10 = 1000 (a 10⁻³ dilution).

  3. A 1.0 mol dm⁻³ stock is diluted 10-fold twice. What is the final concentration in mol dm⁻³?
    Show answer

    0.01 mol dm⁻³. 1.0 ÷ 100 = 0.01 mol dm⁻³.

  4. Why is a colorimeter zeroed with a cuvette of distilled water or reagent only?
    • So readings measure only the absorbance due to the substance being tested
    • To heat the cuvette
    • To make the filter work
    • To increase absorbance
    Show answer

    So readings measure only the absorbance due to the substance being tested. The blank removes the effect of the solvent and cuvette.

  5. How is a pH probe calibrated?
    • By measuring distilled water only
    • By leaving it in the sample overnight
    • By placing it in buffer solutions of known pH and adjusting it
    • By heating it
    Show answer

    By placing it in buffer solutions of known pH and adjusting it. Known standards.

  6. What is the purpose of a calibration curve?
    • To convert a measured reading into a concentration using standards of known concentration
    • To show the trend of the independent variable
    • To calculate the standard deviation
    • To find the anomalous results
    Show answer

    To convert a measured reading into a concentration using standards of known concentration. Readings of known standards give the conversion.

  7. In CP11, why should the same light meter be used for every quadrat?
    • Light meters wear out after one reading
    • It is cheaper
    • Different meters may be calibrated differently, adding systematic differences
    • Light intensity does not vary
    Show answer

    Different meters may be calibrated differently, adding systematic differences. Consistency between readings.

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Last updated 8 November 2026. Spotted a mistake? Tell us.