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errors in practical

When doing a titration - the usual making up a standard solution with solid and adding the acid to alkali using a burette - there is room for human error(haha). But what other sources of error are there ? - there must be loads.

I was thinking that there may be something to do with exothermic reactions with strong acids - but I dont know the 'whole story' as it were, so if anyone can explain that one I would be greatful!

And any other ideas - usually weak acid and alkalis are used in the lab, so what errors apart from human can there be here?

Reply 1

littleone

errors do do accuracy of measuring instruments, eg a glass pipette of a given grade will measure to the nearest 0.05ml, heat loss to environment not accounted for...

Reply 2

Twiglet

How does heat loss to the envirnoment affect results? Obviously during a titration this would not normally be measured, but how would it lead to wrong/innacurate results?

Reply 3

ChemistBoy

franks

How does heat loss to the envirnoment affect results? Obviously during a titration this would not normally be measured, but how would it lead to wrong/innacurate results?

It wouldn't.

Reply 4

Matthew.M

littleone

errors do do accuracy of measuring instruments, eg a glass pipette of a given grade will measure to the nearest 0.05ml.

These are quantitative errors and can be measured. Lab reagents are also only 99% pure, etc...

Then there's qualitative errors with account for most of the error you will have experienced:
• Poor technique
• Equipment not rinsed correctly
• Consistency lacking
• Side of conical flask coated with acid
(some solution not included in the reaction)
• Extending the number of concordant results to gain a better average

No idea if the loss of heat would make any difference or not though.

Reply 5

ryan750

error is a measure of the uncertainty in any measurement, or in the final answer. In a titration concordant results need to be obtained (that is results within 0.1 cm^3 of one another). The standard solution used is known to have a concentration that is fairly accurate ( usually it is sodium hydroxide).

pipette has error of +-0.1 cm^3

burette has error of +- 0.1 cm^3 - but this is doubled because you measure the value at start of titration then the value at end of titration. This will be true for temperature changes aswell because it is a double measurement.

standard solution can be known to +-0.0005 dm^-3

percentage error = actual error in measurement x 100/ measurement

Percentage error in final value = sum of the percentage errors in each measurement

Reply 6

Twiglet

ok, so it seems like there are 3 sorts of error:
human error - where u just dont rinse equiptment, dont record results right, etc.
apparatus error - where the equiptment used in the experiment is not very accurate, this leads to a percentage error that can be calculated
'chemical error' (for want of a better name) - this maybe includes something to do with exothermic reactions... but it seems that noone can explain

right, when a titration is done, a percentage error can be calculated between your result and the one you were meant to get. This is accountable for by the 'apparatus error' if it is below the total %error for apparatus. If your error is above this, human error played a part.
BUT if human error was to be COMPLETELY eliminated (unlikely, but what if it was) , then the source of that error would have to be 'chemical' . what I dont understand (and I would be very grateful if someone could explain) is what the sources of those 'chemical errors' could be ... (could one reason be something to do with exothermic reactions??)

phew :biggrin:

Reply 7

Twiglet

Quote:
Originally Posted by franks
How does heat loss to the envirnoment affect results? Obviously during a titration this would not normally be measured, but how would it lead to wrong/innacurate results?

It wouldn't

ok, i phrased that question totallly wrong, what i was meaning is ... : if a reaction is exothermic, the whole solution will heat up - will this not affect results ?

Reply 8

oxymoron

It depends what you're measuring.

For example if you are measuring the temperature rise of a reaction to work out the enthalpy change or something like that then heat loss will obviously play a big part as any heat lost to the environment will not go into heating up the solution so you won't take account of that energy in your temperature reading.

However, in an acid/alkali titration heat loss will not affect your results. Here you are meausuring an end-point. If the solution heats up, the rate of the reaction will increase but that does not affect the end-point - the amount of acid needed for neutralisation will always be the same for given concentrations.

Another example where heat given off DOES make a difference is where you are measuring the rate of a reactions because here the increase in temperature would increase the rate.

Reply 9

Twiglet

I'm talking about titrations...
if a reaction is exothermic then will the volume of solutions be affected? and in theory could this not have an affect on volumes and therefore titration results?

Reply 10

oxymoron

franks

I'm talking about titrations...
if a reaction is exothermic then will the volume of solutions be affected? and in theory could this not have an affect on volumes and therefore titration results?

No, an increase in temperature will have no noticeable effect on volumes of solutions so this will not affect results of titrations.

Reply 11

Twiglet

ok, so exothermic reactions dont make a difference, so if all human errors were eliminated, technique was perfect then why might not results be perfect ??
What about solids not being 100% pure??
I'm just thinking that there must be more areas for error in titrations apart from not wrinsing out flasks properly !!

Reply 12

oxymoron

Well you'll still have the equipment error (burette and pipette errors) and they can never be eliminated (although using different equipment which is more accurate will reduce them).

Also if you're using solids to make up solutions then you have to include the errors in weighing the solid (mass balance error) and volumetric flasks (if you used one to make the solution up).