Experimental Procedure

Overview

This experiment involves preparing solutions of potassium dichromate (K₂Cr₂O₇) at different concentrations and measuring their absorbance using a spectrophotometer to verify the Beer-Lambert Law.

Materials Required

• Potassium dichromate (K₂Cr₂O₇)
• Distilled water
• Volumetric flasks
• Beakers
• Micropipettes
• Quartz cuvettes (1 cm path length)
• Spectrophotometer
• Computer with spectrophotometer software

Step-by-Step Procedure

Step 1: Solution Preparation

1. Prepare Stock Solution: Create a standard K₂Cr₂O₇ solution with concentration approximately 3.16 × 10⁻³ M.
2. Prepare Dilutions: From the stock solution, prepare six different concentrations of K₂Cr₂O₇ using the following volume ratios:
   • Solution 1: 0.5 mL K₂Cr₂O₇ : 9.5 mL water
   • Solution 2: 1.0 mL K₂Cr₂O₇ : 9.0 mL water
   • Solution 3: 1.5 mL K₂Cr₂O₇ : 8.5 mL water
   • Solution 4: 2.0 mL K₂Cr₂O₇ : 8.0 mL water
   • Solution 5: 2.5 mL K₂Cr₂O₇ : 7.5 mL water
   Note: Calculate the molar concentrations of these solutions before proceeding with measurements.

Step 2: Instrument Setup

3. Power On: Turn on the spectrophotometer by clicking the power button.
4. Initialization: Wait for 30 minutes for proper instrument initialization.

Step 3: Concentration Selection

5. Select Concentration: Click and drag on the concentration bar to choose the appropriate concentration.
   • Important: Start with the lowest concentration solution first.
   • Why start with lowest concentration? This helps establish a baseline and ensures proper instrument calibration.

Step 4: Sample Preparation

6. Obtain Beaker: Click on the beaker to take a clean, dry beaker.
7. Transfer Solution: Click on the volumetric flask to pour the solution into the clean, dry beaker.
8. Collect Sample: Click on the micropipette to collect the appropriate quantity of solution from the beaker.
9. Prepare Cuvette: Take a quartz cuvette (1 cm path length) by clicking on it.
10. Fill Cuvette: Pour the solution from the micropipette into the cuvette by clicking on the micropipette.
    • Note: In real measurements, fill the cuvette to two-thirds of its volume.

Step 5: Spectrophotometer Operation

11. Open Instrument: Click on the spectrophotometer lid to open it.
12. Place Sample: Click on the cuvette to place it in the sample holder.
    • Reference Setup: Use water as the sample blank/reference in an identical cuvette.
    • Double Beam Setup: For double beam spectrophotometers, place the sample in the front holder and reference in the back holder simultaneously.

Step 6: Wavelength Scan

13. Initiate Scan:
    • Click on the computer monitor first
    • Then click on the Scan button to observe the wavelength scan
14. Scan Parameters:
    • Choose appropriate wavelength range for incident light
    • Run scan in absorbance or transmittance mode
    • Data is automatically stored in the computer
15. Single Beam Considerations: If using a single beam instrument:
    • First run scan with sample blank/reference
    • Then run scan with sample
    • Subtract reference data from sample data for respective wavelengths

Step 7: Data Collection

16. Reset: Click on Reset button to start new measurement.
17. Repeat Measurements:
    • Select next higher concentration
    • Repeat the measurement process
    • Important: Rinse the cuvette with a small portion of the next solution before each measurement
    • Continue for all concentrations sequentially
18. Collect Data: Click on the Data tab to collect all experimental data.

Step 8: Data Analysis

19. Plot Absorbance Spectra: Plot absorbance vs. wavelength for different concentrations to determine spectral peak positions.
20. Identify λ_max: Find the wavelength of maximum absorbance (λ_max) for all concentrations.
21. Create Data Table: Compile absorbance data at λ_max and at another wavelength (e.g., 335 nm) for all concentrations.

Experimental Data

Table 1: Absorbance Values for Different Concentrations of K₂Cr₂O₇

S.No	Concentration K₂Cr₂O₇ (M)	λ_max (nm)	Absorbance at λ=350 nm	Absorbance at λ=335 nm
1	1.58 × 10⁻⁴	350	0.152	0.128
2	3.16 × 10⁻⁴	350	0.304	0.256
3	4.74 × 10⁻⁴	350	0.456	0.384
4	6.32 × 10⁻⁴	350	0.608	0.512
5	7.90 × 10⁻⁴	350	0.760	0.640
6	9.48 × 10⁻⁴	350	0.912	0.768

Note: The values in the table are theoretical and may vary slightly in actual measurements due to experimental conditions and instrument calibration. The absorbance values follow the Beer-Lambert law, showing a linear relationship with concentration at both wavelengths.

Step 9: Verification and Analysis

22. Plot Calibration Curve: Plot absorbance vs. concentration.
    • Connect points with line segments
    • Perform linear regression analysis
    • Calculate correlation coefficient
23. Verify Beer-Lambert Law:
    • Observe whether absorbance values show linear correlation with concentration
    • Analyze transmittance values
    • Discuss results and deviations from linearity

Best Practices

Experimental Considerations

1. Solution Preparation: Use dilute solutions for better linearity
2. Wavelength Selection: Measure at absorbance maxima (λ_max)
3. Light Source: Use monochromatic light for accurate measurements
4. Temperature Control: Maintain constant temperature throughout measurements
5. Cuvette Care: Use clean, matched cuvettes
6. Instrument Calibration: Properly zero the instrument with blank solution

Safety Precautions

• Handle K₂Cr₂O₇ with care as it is toxic
• Wear appropriate personal protective equipment
• Dispose of solutions according to institutional guidelines
• Clean all glassware thoroughly after use

Data Quality

• Take multiple readings for each concentration
• Ensure proper instrument calibration
• Check for any systematic errors
• Validate results with known standards