Eduqas Biology for A Level Book 1: 2nd Edition

Practical exercise To determine glucose concentration Rationale The Benedict’s test detects the presence or absence of reducing sugars so it is qualitative. It can indicate relative concentrations of different solutions so it is semi-quantitative. It can be made quantitative using a calibration curve: the absorption of red copper (I) oxide is measured in a colorimeter for glucose solutions of known concentration. A plot of absorbance against concentration produces a calibration or standard curve. The test is repeated with a solution of unknown concentration, and from its absorbance, the glucose concentration can be read from the graph. Design for plotting standard curve Experimental factor Description Value Independent variable concentration of reducing sugar glucose 0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm –3 Dependent variable absorbance of light at 440 nm AU Controlled variables volume of Benedict’s solution 4 cm 3 concentrations of Benedict’s solution as prepared commercially incubation time 8 minutes temperature 80ºC Reliability calculate mean for three readings at each glucose concentration; see Study point on p90 and the Key term on p65 Hazard temperatures above 60ºC can scald; Benedict’s solution can irritate skin and eyes Apparatus ▪ Glucose solution of unknown concentration ▪ Benedict’s solution ▪ 80ºC water bath ▪ Glucose solutions at the following concentrations: 0.2, 0.4, 0.6, 0.8, 1.0 mol dm –3 ▪ Distilled water ▪ Colorimeter ▪ Test tubes ▪ Cuvettes ▪ Syringes ▪ Timer Method 1. In each of 7 test tubes, mix: ▪ 4 cm 3 of the five solutions of known concentration, distilled water or the test solution ▪ 4 cm 3 Benedict’s solution. 2. Place each tube in the 80ºC water bath for 8 minutes. 3. With a blue filter (440 nm) in the colorimeter, zero the colorimeter using water. 4. For each tube, resuspend the precipitate and place the solution in a cuvette. 5. Read the absorption, as a measure of how much copper (I) oxide is present. Study point Remember the three parts of a risk assessment: • Hazard – why an object or chemical is potentially harmful • Risk – the action in the experiment that could cause harm • Control measure – how to minimise or prevent harm. Key terms Independent variable: The variable that the experimenter purposely changes in order to test the dependent variable. Dependent variable: Experimental reading, count, measurement or calculation from them, the value of which depends on the value of the independent variable. Controlled variable: Factor that is kept constant throughout an experiment, to avoid affecting the dependent variable. Eduqas Year 1/AS Biology 28