Group 7: Halogens
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A. I need to investigate the percentage sodium hypochlorite in commercial bleach using gravimetric analysis. I know I need to dilute the bleach, treat it with SO2, and add silver nitrate to produce silver chloride which I will then weigh ... but I don't really understand this and don't know where to start.
B. In the gravimetric method I'm going to use SO2 gas, but what is the purpose of adding nitric acid? And do you know roughly how much silver nitrate I would need? (I'm going to add 5 ml of bleach to 100 ml flask, and fill up to mark with deionised water)
130408.
Jenkin writes
A. Commercial bleach is a mixture of sodium hypochlorite, NaClO, and sodium chloride, NaCl. It is made by reacting chlorine with sodium hydroxide solution:
Cl2 + 2NaOH -> NaClO + NaCl + H2O
The NaClO is the active bleaching agent; the NaCl is not removed as it does not affect the bleaching effect and removing it would be difficult and expensive.
Gravimetric analysis is a difficult technique that requires practice and good technical skill experience if it is to be give accurate results. In the gravimetric method, all the chlorine in the bleach will be converted to silver chloride. Since only half of is present as hypochlorite (OCl-), you will assume that half of the mass of the silver chloride will have originated from the hypochlorite.
Start with a measured sample of the bleach. If possible use a ‘simple’ brand of bleach, not a thickened or otherwise ‘super’ version. You can probably assume the % of NaClO is not more than 5% and therefore that a 25cm3 sample will be sufficient.
You are using SO2 to reduce the ClO- to Cl-, the reaction being:
SO2 + ClO- + H2O -> SO4 2- + Cl- + 2H+
I don’t know a particularly easy way of doing this; Are you going to use SO2 gas, or add sodium sulphite, Na2SO3 ? Don’t add hydrochloric acid, which obviously contains Cl- ions.
The reduction complete, you would then add an excess of silver nitrate solution and filter off the precipitated silver chloride. You will need to use a quantitative filter paper for this and then heat to constant weight. The filter paper will burn away as you do this, but the weight of its ash will be stated on the box.
The calculation is quite simple. Convert the mass of AgCl to moles, and then divide by 2 to get the moles of NaOCl. Convert to mass and express it as a % of your original sample.
B. Don’t add nitric acid before the SO2, as any acid will release chlorine gas from OCl- ! As well as being dangerous you would lose chlorine to the atmosphere. The point of adding nitric acid before silver nitrate is to prevent precipitation of silver carbonate from any carbonate (or dissolved CO2) present.
Without knowing the strength of the bleach (and, of course, that’s the point of the experiment), it’s difficult to estimate the quantity of silver nitrate. I would guess that there will be no more than 10% total Cl in the bleach, which would be 0.5 g in a 5 ml sample. 1 mole of Cl- reacts with 1 mole of AgNO3, so you could use around 0.02 mol of silver nitrate to ensure you have an excess. This would be 200 cm3 of a 0.1 M solution.
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updated: 13 April 2008
