4 : RESULTS

4.i : The solutions.

All of the solutions were made up as in the revised method sheet before the practical commenced. The DiPC was originally stored in a volumetric flask, but being light sensitive, this had to be disposed of after a few hours. The next DiPC solution was stored in an amber bottle and in a cupboard when not in use.

The solutions were the same for the entire run of practicals to ensure reproducibility of results (except for the DiPC used. This was fresh each time. Any variation would be shown in the calibration curves.)

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4.ii : Calibration curve from the standards.

As this practical has been carried out over a number of practical sessions, there are 3 calibration curves. These will be denoted as (i), (ii) and (iii).

(i) This was really just a test run to ensure that the practical was fine, the instrumentation was working and that the basic theory obeyed the Beer law.

(ii) The main one. Using this calibration curve, the interfering ions were assessed and the water samples tested.

(iii) This was performed at LJMU using a bottled water as the test subject. It was carried out on a different machine to the above.

The graphs for (i) and (ii) are absorbance vs. concn (in ppm).

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4.iii : The water samples - collection points

The water samples were collected from five parts of Haydock, St. Helens, one from the mains water tap, one from the water still at the University of Salford, LJMU tap water and deionised water and finally a bottled water (commercially available).

The four types were : (1) Rain water pond.

(2) Fishing lake.

(3) Sewerage outlet.

(4) Brook.

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4.iv : Preparing the water samples

This was carried out as per revised method. After filtration, a 10cm3 aliquot was placed into a test tube and treated with the DiPC. There was no need for acidification as the pH would be low enough from the addition of conc. sulphuric acid.

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4.v : Results for calibration curve with Cr(VI)-DiPC & acid.

The following three tables refer to the absorbance values recorded on the three different occasions when a calibration curve was constructed.

All graphs for the calibration curves and least squares analysis follow the results of the water samples.

Conc Abs
0 0
0.400000 0.029800
1 0.073350
2 0.172960
4 0.360260
10 0.950740

Table 4a. Calibration curve data, week 1

Conc Abs
0 0
0.400000 0.041390
1 0.097890
2 0.191600
4 0.397510
10 0.951790

Table 4b. Calibration curve data. week 2

Conc Abs
0 0
0.400000 0.031000
1 0.094000
2 0.177000
4 0.372000
10 0.937000

Table 4c. Calibration curve data, week 3. (reduction in accuracy)

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4.vi : Results from ion interferences.

These results prove the value of the DiPC for determination of Cr(VI).They can all be discounted from the tests at the concentrations used (1.27ppm per ion)

Metal ion Absorbance
Co2+ -0.08858
Hg2+ -0.06404
Fe2+ -0.09653
Fe3+ -0.09653
Ba2+ -0.07724
Cr3+ -0.07063

Table 5. Interference from other ions.

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4.vii : Results from water samples

Samples 1-4 are the pond waters, 5 is from University of Salford Physical Chemistry teaching lab, 6 is for the bottled water, 7 and 8 are tap water & deionised water respectively from LJMU. The final three were performed at LJMU.

Sample Absorbance Location Conc. ppm
1 0.08978 Rain water pond 0.095
2 0.05449 Fishing lake 0.068
3 0.01073 Sewerage outlet 0.021
4 0.21443 Brook Unknown
5 0.00611 Phys Chem tap Too low
6 0.004 Bottled water 0.5
7 0.152 Tap, LJMU Unknown
8 0.000 Deionised water 0.0

Table 6. Water sample absorbance values.

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4.viii : Cr(VI) reduction by l-Ascorbic acid

This has two results; result 1 from addition of ascorbic acid before complexation with DiPC, result 2 from addition after complexation. The result clearly shows that for this method to be effective, the Cr(VI) has to be reduced by the ascorbic acid prior to complexation.

Result number Absorbance
1 0.062
2 0.86105

Table 7. Reduction by ascorbic acid.

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4.ix : Graphs

A further graph (the original from LJMU instrumentation) is in the appendices. The calibration graphs are on the following pages.

Graph 1. Calibration curve from week 1

Graph 2. Calibration curve from week 2

Graph 3. Calibration curve from week 3

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4.x: Environmental question

A question is asked in the new method regarding the environmental impact of the three methods and which will have the worst impact. The answer will be a simple one - the Ascorbic acid.

If the Cr(VI) is precipitated out, the solid can be filtered and the waste water run to waste. This may cause some Mg2+ ions to be released. The effect of this will be negligible due to the streams having their own natural buffers, dissolved CO2 in the water and rocks.

Ascorbic acid is completely soluble and any excess will not be compensated using this buffering system in the stream. The ascorbic acid will radically alter the plant growth (l-Ascorbic acid is better known as Vitamin C) as well as an overall increase of water pH.

The addition of Vitamin C supplements to plant and animal life will radically upset the ecosystem of the water stream resulting in a loss of life (quite possibly) from over population, over acidic water and de-oxygenation of the water by increase activity in the stream.

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