Colorimetry Experiment: Determining Copper Sulfate Concentration

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Introduction

The objective of our colourimetry experiment is to determine the unknown concentration of copper sulfate (CuSO₄).

Method for Colourimetry

Ensure you have the correct filter set for the substance you are testing and fit it into the colourimeter.
Start with pure water and set the scale.
Utilize a series of standard reference solutions and record their absorbance values to create a calibration graph.
Introduce the solution being tested and record its absorbance value.
Use the previously plotted graph to determine the concentration of the tested solution.

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Making Dilutions

For each of our five dilutions, the total volume remained at 10 mL. The dilutions were prepared as follows:

0.00 concentration: 10 mL of water
0.20 concentration: 8 mL of water + 2 mL of copper sulfate
0.40 concentration: 6 mL of water + 4 mL of copper sulfate
0.60 concentration: 4 mL of water + 6 mL of copper sulfate
0.80 concentration: 2 mL of water + 8 mL of copper sulfate
1.00 concentration: 10 mL of pure copper sulfate

We measured these solutions meticulously using measuring cylinders and pipettes to ensure accuracy, which is essential for obtaining reliable results.

The molar mass of CuSO₄ is:

Molar mass (Mr) = Molar mass of crystals (solid) + (5 × Molar mass of H₂O)

Molar mass (Mr) = 159.5 + (5 × 18) = 249.5 g/mol (in aqueous form)

For 1000 cm³, 249.5 g corresponds to a 0.1 M solution (since 0.1 mol × 0.1 L = 0.01 mol).

For 100 cm³, 2.495 g corresponds to a 0.1 M solution.

A colourimeter is an instrument used to measure the amount of light passing through a substance, which is indicative of its purity. It allows for the recording of both absorbance and transmission values. Different substances absorb light at different rates due to their pigments, necessitating the use of appropriate filters to obtain accurate results.

In the case of copper sulfate, which is blue-green in color, an orange-red filter with a wavelength of 600 nm is used (Saps.org.uk, 2018).

Regarding health and safety considerations, it is essential to handle glass beakers with care to avoid breakage and potential injuries. When handling liquids, precautions should be taken to prevent spillage, as spilled liquids can create slip hazards. Additionally, care should be exercised when introducing liquids into the colorimeter to avoid spills and ensure safety.

Absorbance and Transmission Results

Concentration (mol/dm³)	Absorbance 1	Absorbance 2	Average Absorbance	Transmission 1 (%)	Transmission 2 (%)	Average Transmission (%)
0.00	0.00	0.00	0.00	100%	100%	100%
0.02	0.52	0.53	0.525	31%	32%	31.5%
0.04	0.70	0.71	0.705	22%	21%	21.5%
0.06	0.58	0.57	0.575	28%	27%	27.5%
0.08	0.92	0.93	0.925	13%	14%	13.5%
0.10	0.90	0.91	0.905	14%	15%	14.5%

Unknown Samples A, B, C, D:

Sample	Absorbance	Concentration (mol/dm³)	Transmission (%)
A	0.14	0.004	95%
B	0.28	0.008	45%
C	0.49	0.018	30%
D	0.82	0.060	10%

Analysis of Results

Our results demonstrate a general trend of increasing absorbance as the concentration of the copper sulfate solution rises, as indicated by our graph with a best-fit line. However, it's important to note that we encountered two anomalous absorbance results at concentrations of 0.06 and 0.10. These anomalies could be attributed to inaccuracies in measuring and pouring the solutions, resulting in deviations from the expected values.

Regarding transmission, we observed a corresponding decrease as concentration increased. Again, two anomalous transmission results were recorded, likely due to the same issues with accurate solution handling.

The potential inaccuracies in our results could also be linked to the age of the colorimeter, as we were working with an older model that may not have been as precise as modern ones.

Evaluation of the Experiment

This experiment marked our first experience with a colorimeter for most of the group members, including handling and preparing solutions for it. This lack of familiarity and training likely contributed to some inaccuracies in both solution preparation and result recording.

Additionally, the use of an older colorimeter may have introduced inaccuracies into our measurements due to its age and potential calibration issues.

We noticed that our values often fluctuated within a 0.1 range of each other, necessitating recording two values and calculating an average. These fluctuations were also reflected in the anomalous results we observed in our graphs.

To improve the accuracy of our experiment, we could have worn gloves when handling the cuvette to prevent fingerprints from affecting the absorption readings. Fingerprints contain oils and dirt that can interfere with the passage of light, leading to variations in the results.

Furthermore, the use of computer software to create electronic graphs could have enhanced the accuracy and readability of our results. It would also have been beneficial to implement more precise techniques for solution measurement and handling to minimize errors.

While using a colorimeter provides quantitative data, an alternative qualitative approach using visual observation would likely be less accurate due to differences in individual color perception.