solution colour and ion concentration investigation

Investigation - Solution Colour and Ion Concentration

Aim: To investigate whether solution colour can be used to reliably determine the concentration of coloured ions in a solution.
Hypothesis: the concentration of permanganate ions in the solution is inversely proportional to the percentage transmission of light through the solution.

Dependent variable: concentration of permanganate ions.
Independent variable: percentage transmission of light through the solution.

Equipment:
20 ml of 0.00100 molL-1 potassium permanganate
20 ml distilled water
6 identical test tubes
1 Test tube rack
1 plastic pipette
10 ml measuring cylinder
Photometer

Method:
1. Measure 5 ml of potassium permanganate with the measuring cylinder, place this into the 1st test tube. Then measure 2 ml of distilled water with a plastic pipette, also place in the 1st test tube. Stir the solution gently with the pipette to ensure that the potassium permanganate is evenly diffused through the solution.
2. Repeat step 1, but this time put 4 ml of potassium permanganate and 3 ml of distilled water instead into the 2nd test tube. Keep repeating step 1, with 4ml(KMnO4) and 3ml(H2O), 3ml(KMnO4) and 4ml(H2O), 2ml(KMnO4) and 5ml(H2O), 1ml(KMnO4) and 6ml(H2O), 0.5ml(KMnO4) and 6.5ml(H2O). Place these solutions in separate test tubes and arrange the solutions in chronological order with descending concentration.
3. Label each test tube so they don’t get mixed up.
4. Measure 7 ml of distilled water with measuring cylinder and place into the 7th test tube, this is used for calibration.
5. Calibrate the photometer by placing the test tube with distilled water in it, adjust the scale to 100% by pressing then calibrate button.
6. Place each of the 6 test tubes one by one into the photometer, record down the percentage transmission of light readings in a table, and repeat this step 3 times to complete 3 trials.
7. Finally, measure 7ml of the potassium permanganate with unknown concentration using a measuring cylinder, place this solution into the photometer and record down its percentage transmission of light.

Data: 0.00100 molL-1 KMnO4 (ml)
Distilled water (ml)
Concentration of permanganate ions (molL-1)
Transmission of light (%)

Trial 1
Trial 2
Trial 3
Average
5.0
2.0
7.14 * 10-4
1
1
1
1.00
4.0
3.0
5.71 * 10-4
1
1
2
1.33
3.0
4.0
4.29 * 10-4
2
3
5
3.33
2.0
5.0
2.86* 10-4
2
4
7
4.33
1.0
6.0
1.43 * 10-4
10
12
21
14.3
0.5
6.5
7.14 * 10-5
28
16
30
24.7
Transmission of light (%) of unknown solution: 34

Data Processing and Analysis:

Conclusion:
According to the results and processing/analysis of data, we can conclude that the concentration of permanganate ions in the solution is proportional to the percentage transmission of light through the solution. Because from the graph - concentration of permanganate ions against the transmission of light through the solution, we can see that as the concentration of permanganate ions increases in the solution, the transmission of light increases as well.
However, there are some sources of error present in this investigation. For example, the measuring equipment we have such as the plastic pipette and the measuring cylinder all contain uncertainty in the measurement, for the measuring cylinder it is ±0.05ml. This could to some degree affect our investigation result, making our actual concentration deviate from the theoretical one. Also, the photometer makes two light transmission readings of the same solution with rather large differences, such as the light transmission of 0.00100 molL-1 KMnO4 , trial 2 showed a percentage of light transmission of 16% where as trial 3 showed a percentage of light transmission of 30%, that is 14% difference in measuring light transmission on the exact same solution. Thus also contributing to uncertainty of results.
In spite of the uncertainties, we can still draw the conclusion that the concentration of ions in the solution is inversely proportional to percentage of light transmission through the solution, light transmission decreases as ion concentration increases, thus it is reliable to use solution colour to determine the concentration of coloured ions in solution.

Working out unknown concentration of potassium permanganate:
Since the solution of the lowest concentration we have, gotten only 24.7% of light transmission, extrapolation is required to work out the unknown concentration of potassium permanganate as the percentage of light transmission for unknown solution is 34%, this may contribute to the inaccuracy of the answer.