Practical investigations
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I'm currently doing an analysis of the acid content of vinegars and seeing whether or not they meet the 4% (w/v) law specification. I've worked out the concentration of the sodium hydroxide after I standardised it with sulfamic acid, but when working out the minimum and maximum concentrations of the sodium hydroxide, do I replace the average titre value (from number of moles of sulfamic used in the reaction with NaOH = cv and v = titre value x 10-3) with the higher range of the titre value (i.e. with the error). If the latter is the case, do I use the higher or lower range to work out the maximum NaOH concentration?
Igloo writes ...
I don’t entirely understand what you mean by “higher or lower range”, but I’ll briefly run through the error calculations and hopefully this will cover what you want to know.
The sulfamic acid is the standard so its purity can be taken to be virtually 100%. In weighing out the mass used for your standard solution there will be an error involved in weighing it out, so, for example, if you weighed out 5.00 g on a balance weighing to 0.05 g, the actual mass used could be as little as 4.95 g and as much as 5.05 g. This gives a percentage error of about 1%.
Your titre using a conventional burette is correct to about 0.1 cm3, so, for example, if your median titre was 11.4 cm3, then the actual titre can be taken to lie between 11.3 cm3 and 11.5 cm3, giving a percentage error of about 0.9%.
In a similar manner you can estimate the error involved in using the pipette.
Finally all of the errors can be added together to give a total percentage error for the instrumentation, let’s say 3%. You need to realise that we have not yet taken into account all the errors involved in practical work, through spillage, imprecise transference of solids and liquids from one vessel to another, and of course these errors can only be guessed at. You may like to make an inspired estimation as to what these are.
Finally, when you state the concentration of your sodium hydroxide solution, having carried out all the necessary calculations, you need to increase (and decrease) the calculated figure by the total percentage error in order to arrive at a value to quote. If the error you have estimated is 5% and the calculated concentration is 0.58 mol dm-3, then this value should be given as 0.58 plus or minus 0.03 mol dm-3. In this case the maximum would be 0.61 mol dm-3.
Do realise that in practice there is a very high probability that most of these errors will “cancel out” so that the actual concentration is unlikely to be anywhere near the extremes.
Always carry out a risk assessment and check with your teacher before starting any practical work.
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: 17 April 2007
