Chemistry Titration - evaluation

hey crazy dudes.
doing an evalution coursework on a titration. Trying to think of all the possible errors and improvements. anybody got any ideas? and also do you think i should add all the errors to give one final maxium possible error?

cheers

Reply 1

Katie J

10

Is this AS chemistry? I did an acid-base titration last year for my coursework, so will do my best to remember!

Firstly, unlike most other investigations its not good to say you have anomalies, with titrations you repeat until you get successive results the same or very close! You can however have lots of preliminary rough ones to comment on! An inaccurate titration is not much use to an A-Level chemist!

You will need percentage errors of all the equipment you used to assess the effect on your result/conclusion. (error / reading x 100 = % error) The error should be marked on the apparatus (eg burette) or in text book, if not a simple internet search should find them. It's best to comment on these errors on there own as well as collectively, eg the biggest error will have biggest effect etc.

Experimental errors;

Need lots of practise for good titrations. (my friend got away nicely with using 'human error' as the main source!)

When filling the volumetric flask making sure the meniscus (dip on surface of solution) rests on the calibration line, and again with the pipette. (Obviously expalin the effect, eg weaker/stronger alkali, therefore more/less acid to neutralise it, therefore titre not accurate etc.)

Determining end point is also always good to chuck in (for exam questions I always use this one, colour change isn't always obvious, our preparation ones were lovely purple to colourless, the actual ones were dark yellow to faint orange, not easy! But this does depend on what you're titrating & indicator)

Another classic, though could risk losing marks as its a bit silly, another friend forgot to add indicator so never saw a colour change on her rough one!

Residues in glasswear. Washings procedure should already be mentioned in plan I think but still good to cover this.

As I said its not wise to include anomalies, you could mention what you did to prevent error & this should give you the same mark allocations. (worked for me!)
These include things like inverting volumetric flask, swirling conical flask after each drop, wind guard on balance, white tile (for colour change), systematic washing of glasswear, residue procedures, homogenising solution by stirring/swirling etc.

Yes, total all % errors to one big one then find that % of your concentration (conclusion value). This % value of concentration is therefore the +/- value for within which your concentration will be.

Hope that's the kind of thing you were after.
There you go, thats saved me about an hours revision!

Reply 2

thatsthebadger

OP

thanks. yeah its as-level chemistry, you guessed right! i should have said. useful information. thanks again

Reply 3

thatsthebadger

OP

soz for double posting, but i have another question.
the precision errors ( i have four) , if you add them up, and get 1% for example, does that mean that your titre could be +/- 1%. or could it mean that your titre is already wrong, and could be +2% for the real value. +/- 1% means you have a 2% margin for error does it not? i am confused!

Reply 4

GH

13

thatsthebadger

soz for double posting, but i have another question.
the precision errors ( i have four) , if you add them up, and get 1% for example, does that mean that your titre could be +/- 1%. or could it mean that your titre is already wrong, and could be +2% for the real value. +/- 1% means you have a 2% margin for error does it not? i am confused!

It means the maximum error is +/- whatever total percentage you got.

Reply 5

Katie J

10

thatsthebadger

soz for double posting, but i have another question.
the precision errors ( i have four) , if you add them up, and get 1% for example, does that mean that your titre could be +/- 1%. or could it mean that your titre is already wrong, and could be +2% for the real value. +/- 1% means you have a 2% margin for error does it not? i am confused!

errm....I'm always a bit hazey when it comes to the maths side of chemistry...I think it means the concentration you calculate (in answer to a task I assume) is +/- 1%, I think thats what I did, you're right its a margin of error thing.
My CW task (set by teacher last year) gave me the degree of accuracy we had to reach, if were more than that much out we had to repeat it, but I don't think anyone did have to. Acid rain would cause more damage than the acid we used but for the sake of 'saftey in procedure' marks we all wore goggles & lab coats! Be very careful with the number of significant figures you use, they love to pick up on that!

EDIT:
Decided rather than try & work it out I'd copy and paste the very last bit of my CW, please ignore bad wording & cheesy explanations; I have a pedantic chem teacher!

The percentage errors calculated above and areas of potential errors are only minimal. The overall percentage error is 0.7089 %. To be able to calculate the range of accuracy for my results I need to find 0.7089 % of my concentration (0.1002).
0.7089 % of 0.1002 = range
0.7089 % x 0.1002 = +/-0.0007103178 moldm-3
100
My result therefore could be +/- 0.0007103178 of the 0.1002 moldm-3 calculated in my analysis.

I still don't think that's made it any clearer!

Reply 6

thatsthebadger

OP

it kinda helps. iam just a bit confused. well i got to finish it now (in for tommorrow) so i'll just go with what i got. thanks again

Chemistry Titration - evaluation

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