Light Absorbance and Transmission: A colorimeter measures how much light of a specific wavelength passes through a solution (transmission) or is absorbed by it (absorbance). As a colored substrate (like starch-iodine) is broken down, the solution becomes lighter, increasing transmission and decreasing absorbance.
Filter Selection: A color filter (often red for blue-black starch-iodine complexes) is used to maximize the sensitivity of the device. The filter should be the complementary color to the solution to ensure maximum light absorption by the target molecules.
Calibration: Before measuring unknown samples, the colorimeter must be calibrated using a 'blank' (usually distilled water or the starting reagent) to set a baseline of transmission or absorbance.
Purpose of Serial Dilution: This technique is used to create a range of known concentrations from a single stock solution. It is essential for constructing a calibration curve which allows researchers to convert colorimeter readings into actual concentration values.
The Process: A specific volume of stock solution is added to a volume of solvent (e.g., into for a -fold dilution). This process is repeated using the new solution as the 'stock' for the next step, resulting in an exponential decrease in concentration (e.g., , , ).
Temperature Regulation: Since enzyme activity is highly sensitive to thermal changes, a thermostatically controlled water bath should be used. All reactants should be pre-incubated in the water bath to reach the target temperature before mixing.
pH Maintenance: Enzymes have specific optimum pH levels where their tertiary structure is most stable. Buffer solutions are added to the reaction mixture to resist changes in pH that might occur as products are formed or as the reaction progresses.
| Feature | Spotting Tile Method | Colorimetry Method |
|---|---|---|
| Data Type | Qualitative / Semi-quantitative | Quantitative |
| Endpoint | Subjective visual color change | Objective light measurement |
| Precision | Lower (limited by sampling intervals) | Higher (continuous monitoring possible) |
| Equipment | Pipettes, spotting tile, iodine | Colorimeter, cuvettes, filters |
Initial Rate of Reaction: Always focus on the initial rate (the gradient of the curve at ). This is when the substrate concentration is not yet a limiting factor and the enzyme is working at its maximum capacity for those conditions.
Common Misconception: Students often state that enzymes are 'killed' by high temperatures. In exams, always use the term denatured, explaining that the heat breaks bonds in the tertiary structure, changing the active site shape.
Verification: When calculating rates from a graph, ensure the units on the axes match your final answer. If a graph plateaus, explain that the substrate has been exhausted or the enzyme has become the limiting factor.
