Overview
This lecture explains how to use a colorimeter with Benedict's test to more precisely determine glucose concentration in a solution.
Recap of Benedict's Test
- Benedict's reagent is blue due to copper (II) ions (Cu²⁺).
- Reducing sugars like glucose donate electrons, reducing Cu²⁺ to Cu¹⁺, forming a red precipitate.
- The color change from blue to red indicates the presence and approximate amount of reducing sugar.
- The Benedict's test is semi-quantitative; brick red means more glucose than green.
Principle of Colorimetry
- After the reaction, the remaining solution is less blue because some Cu²⁺ ions have reacted.
- The higher the glucose concentration, the less blue the solution remains.
- Subtle changes in the blueness may be hard to detect by eye, so a colorimeter is used.
- Before using a colorimeter, filter off the red precipitate to leave only the solution.
How a Colorimeter Works
- A colorimeter shines light (usually red) through the filtered Benedict's solution.
- Benedict's solution absorbs red light the most, as red is the complementary color to blue.
- A red filter is used so mainly red light passes through the sample.
- Less blue (more reacted glucose) means less red light is absorbed and more is transmitted.
- The transmitted red light is detected by a photoelectric cell, indicating glucose concentration.
Key Terms & Definitions
- Benedict's reagent — a chemical solution used to test for reducing sugars.
- Reducing sugar — a sugar that can donate electrons, e.g., glucose.
- Colorimeter — a device that measures the absorption of specific wavelengths of light by a solution.
- Photoelectric cell — a sensor that detects the amount of transmitted light.
- Complementary color — a color that is opposite to another on the color spectrum; red is complementary to blue.
Action Items / Next Steps
- Watch the next video for the detailed procedure of using a colorimeter to measure glucose concentration.