Practical investigations
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At the moment I am beginning to start a project on the cost effectiveness of household bleaches by determining the concentration of hypochlorite ion using iodine, potassium iodide and sodium thiosulphate, but I dont have a clue about what reactions are taking place or basically anything about it.
Igloo writes
The majority of bleaches contain chlorate(I) [hypochlorite] ions, ClO. These are oxidising agents, so iodide ions, I, for example, will be oxidised to iodine molecules,I2, and the latter can be estimated using a titration with sodium thiosulphate. The relevant reactions are:
ClO(aq) + 2I(aq) + 2H+(aq) -> Cl(aq) + I2(aq) + H2O(l)
I2(aq) + 2S2O32(aq) -> 2I(aq) + S4O62(aq)
What you need to do is to measure out a known volume of the bleach* into a flask, and to add an excess of acidified potassium iodide solution. The resulting mixture which contains iodine will probably be brown in colour, and can be titrated with sodium thiosulphate of a known concentration until the colour of the mixture becomes pale yellow. A few drops of starch can then be added and the titration can be taken to completion, represented by a colour change from intense blue-black to colourless.
* You will almost certainly need to dilute the bleach by a known factor before you start, because it is likely to be rather concentrated and therefore somewhat hazardous to use in this form. Some trial experiments will be needed in order to dilute the bleach appropriately.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Incidentally if you are a Salters student you will need to use other analytical techniques to gain full credit. We have already I am currently carrying out my A2 individual investigation. I have carried out a bleach cost analysis on four different brands of bleaches using a titration with sodium thiosulphate. For the Salters chemistry course, I must also carry out more experiments. What other experiments can I carry out on the brands of bleaches and how can I fit this into the investigation?|answered a question on this].
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 26 January 2006
