we have an assignment where we have to test water quality in a bunch of samples, things like dissolved oxygen, phosphates, pH etc. does anyone know what the reagents are for testing amounts of dissolved O2, phosphates and chlorine ions? we were using a stormwatch kit for it. thanks in advance to anyone who can help.
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An extract from Dreamerish*~'s Guide to Water Monitoring and Management:
For determining the prescence of phosphate ions in a sample, a good test is acidifying the sample and adding ammonium molybdate solution and stirring the mixture gently while heating it for a few minutes. The appearance of a yellow precipitate indicates the prescence of phosphate.
Note that you can't measure the exact amount of ions in solution using these qualitative tests, instead you would use quantitative tests like Atomic Absorption Spectroscopy to measure the exact concentrations of a specific ion in a sample.
Hope that helps.
You can qualitatively test for chloride ions in a sample by adding a drop of silver nitrate solution. The sample will go milky due to the formation of silver chloride, which is insoluble.Dreamerish*~ said:TESTING FOR DISSOLVED OXYGEN (DO)
Oxygen is usually present in concentrations of 6-9 ppm. At concentrations of lower than 5 ppm, aquatic organisms begin to suffocate.
Oxygen-sensitive Electrode: Dissolved oxygen meters are based on electrochemical cells. The most common type of probe uses a gold or platinum electrode and a silver electrode in a KCl electrolyte solution. The voltage applied between the gold/platinum and silver electrodes does not cause electrolysis until oxygen reaches the electrolyte solution. The electrodes and electrolyte are separated from the water being sampled by a plastic membrane that oxygen can diffuse through. The amount of electrolysis is proportional to the amount of oxygen. The DO level is measured by a milliammeter.
Winkler Method: (This may look a little confusing, and I still can't write the equations from memory. Don't worry, we don't need it.As long as we know the oxygen-sensitive electrode method, who gives a toss about Winkler.) The Winkler method is a way of fixing the amount of dissolved oxygen in a sample and determining the DO by titration at a later time.
Manganese (II) ions and hydroxide ions are added to the water sample. The amount of insoluble brown manganese (IV) oxide produced depends on the amount of DO:
2Mn2+ + 4OH- + O2(aq) --> 2MnO2(s) + 2H2O(l)
Acidified iodide solution reacts with the MnO2, producing a yellow iodine solution.
2MnO2(s) + 8H+ + 4I- --> 2Mn2+ + 4H2O(l) + 2I2(aq)
The iodine released is titrated against a standard sodium thiosulphate solution from a burette containing starch indicator. The starch indicator forms a blue colour with iodine, and the blue colour disappears at the end point.
2I2(aq) + 4S2O32-(aq) --> 4I- + 4S2O32-(aq)
It can be seen that each dissolved O2 molecule gives 2MnO2 which gives 2I2 which reacts with 4S2O32-(aq). Thus for each molecule of thiosulphate used at the end in the titration there was 1/4 mole of dissolved oxygen in the original sample.
When water samples are collected from different locations, identical containers, identical sealing systems and the same collection procedures must be used. It is important that the containers are completely filled with water so there is no air space, and that they are kept out of light so algae present cannot add to the oxygen level by photosynthesis.
For determining the prescence of phosphate ions in a sample, a good test is acidifying the sample and adding ammonium molybdate solution and stirring the mixture gently while heating it for a few minutes. The appearance of a yellow precipitate indicates the prescence of phosphate.
Note that you can't measure the exact amount of ions in solution using these qualitative tests, instead you would use quantitative tests like Atomic Absorption Spectroscopy to measure the exact concentrations of a specific ion in a sample.
Hope that helps.
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thanks for trying, but unfortunately i need the quantitative tests which is starting to look like isnt even in the syllabus, the textbook or any of the study guides ive looked at. guess will have to try to get in touch with someone in the class and hope they remember what reagents we used for each. ill post the stuff here after i find it so that if anyone else ends up needing it itll be easy to find.
That's alright, I suspected you needed the quantitative tests, which are definitely beyond the scope of the syllabus. The only thing I can suggest to you is have a look through some textbooks/reference books on more advanced chemistry, probably at uni level. Good luck with the assignment.