systematic errors in titrations (1 Viewer)

[stickyricebuns]

if anyone is awake rn
what r the effects of miscalibrating the volumetric stuff in titrations? im talking about rinsing the burettes/pipettes/conical flask with the wrong substance which dilutes the solutions that go in them. what effect do they have on the concentration and number of moles of both the initial solutions and hence what effect does that have on the final calculated concentrations of the titrant/titrand/whatever ur trying to find?

i only have a vague idea of it, eg. rinsing the burette with water decreases the concentration of the titrant...so what does this mean for the final concentration that u calculate for the titrand?

 

[Eagle Mum]

The burette should be rinsed with the same titrant (known solution) solution only. Water would dilute the titrant so more volume would be required to react with the titrand, giving a falsely elevated titre measurement.

Rinsing the flask with water wouldn’t change the amount of analyte in the flask, as long as the titrand (unknown) volume is accurately delivered by a pipette.

The pipette can be rinsed with the titrand but care must be taken that this doesn’t change the volume of titrand delivered by the pipette by drip from the sides (ie. only draw & eject titrand from the same end +/- reverse pipetting technique). Rinsing the pipette with water may dilute the titrand so less analyte would be delivered to the flask, giving a falsely decreased titre measurement. If the titrand is an acid, rinsing the pipette with a stronger more acidic solution will falsely increase the titre measurement, whilst rinsing with an alkali would falsely decrease the titre measurement. Conversely, if the titrand is a base, rinsing with a stronger more alkaline solution would increase the titre measurement and rinsing with an acid would decrease the titre measurement.

If the titrand is an acid, any additional acid in the flask will increase the titre measurement and any base will decrease the titre. The converse applies for a titrand which is a base.

A falsely elevated titre measurement produces an overestimate of the concentration of the titrand (number of moles of the unknown analyte) and a falsely decreased titre produces an underestimate.

 

[CM_Tutor]

I agree with @Eagle Mum, except for the red part of the quoted section...

If the titrand is an acid, rinsing the pipette with a stronger acid will falsely increase the titre measurement, whilst rinsing with an alkali would falsely decrease the titre measurement. Conversely, if the titrand is a base, rinsing with a stronger base would increase the titre measurement and rinsing with an acid would decrease the titre measurement.

Rinsing the pipette with an acid or base (as appropriate) that is not the one used in the titration is obviously bad practice, unnecessarily adding an additional substance to the mixture. If measuring an acid with a pipette that was rinsed with a base, obviously the residual base will react with the measured acid, so that the concentration of the acid added from the pipette will have been lowered, leading to a measured titre that is lower than it should be. Similarly, if measuring a base with a pipette that was rinsed with an acid, obviously the residual acid will react with the measured base, so that the concentration of the base added from the pipette will have been lowered, leading to a measured titre that is lower than it should be.

In the case of an acid being measured in a pipette washed with a different acid, it is not the difference in strength between the acid used in the washing and that being measured that introduces the error, it is their difference in concentration. Remember that the calculation of the unknown concentration (following the assumption that the equivalence point is well approximated by the measured end point) does not involve the strengths of the species involved, merely their concentrations. In other words, if my acid is 0.1000 mol L^-1 and I measure 25.00 mL by pipette but have washed the pipette with 0.2000 mol L^-1 acid, the residue will cause the overall concentration of acid present to increase. Similarly, if I have washed with 0.05000 mol ^-1 acid, this will dilute my aliquot and lead me to have in my conical flask less acid than I thought was the case.

The strength of the washing acid or base will, however, have an effect on the pH of the solution when the reagents are present in their stoichiometric ratio, and thus the choice of appropriate indicator could be effected, in turn reducing the accuracy of the results.

So, if I was working with vinegar (acetic acid, a weak acid) of 0.1000 mol L^-1 and rinsed the pipette with 0.01000 mol L^-1 hydrochloric acid, the vinegar would be diluted and the result effected in a similar way to if the pipette was washed with water - even though HCl is the stronger acid, because it is more dilute and it is the dilution that alters the accuracy. Alternatively, if the rinsing was done with 0.2000 mol L^-1 acid, the effect would be similar to adding more than 25.00 mL because extra acid is being added to the conical flask, and it would not matter whether the rinsing was with HCl, an acid stronger than acetic acid, or propanoic acid, which is weaker than acetic acid.

 

[Eagle Mum]

Oops, yes - I meant ‘more concentrated’, not stronger.