The calibration curves on all three occasions give me good reason to say that the experiment is very much reproducable and the quality of the graphs linearity on all occasions that the serial dilution method of obtaining the 1.27ppm is an adequate one.
Analysis of the water samples shows that in all cases, the levels of Cr(VI) are below the allowed amount. The exception was the Brook sample (from Haydock) and LJMU tap water. These will have had a large amount of Iron in. Iron as well as Mercury both interact with the DiPC. This will have masked the overall result of the Cr(VI) reaction.
This practical has been designed (as stated) to be performed by BSc II/HNC II groups as well as the Environmental chemists. As it stands, the practical can be carried out in roughly 2.5 to 3 hours. The questions set at the end should form the basis of a homework or assignment.
It is vital that the chemicals used (especially the DiPC) are fresh and of AR grade. As a side test, I also used the dichromate at SLR and an unspecified quality. Both gave a result for the calibration curve, but towards the top end (the 4 and notably the 10ppm standards) the line tended to become of the true pattern (i.e. not straight). This was not the case with fresh AR grade material. It is not important to the grade of the conc. sulphuric acid. All water MUST only be deionised.
Sources of errors will be in the preparation of the standard solutions by serial dilution and the fact that the DiPC is light sensitive. Any laxness on the part of the student will show in the calibration curve and the least means results.