With that idea of too much indicator being a source of error, it's legitimate.
In using an indicator the aim is to make sure the endpoint and the equivalence point are the same or are as close to each other as possible - this maximises accuracy. We can look at the endpoint and the equivalence point in terms of pH, as the endpoint of a particular indicator is dependent on the pH of the entire solution only, and the equivalence point would occur at a specific pH.
I'm going to give a really detailed explanation of this just to clearly show what happens
Suppose that we are titrating 20.00 mL of 0.1000 M NaOH with 0.1000 M HCl. The equivalence point would be at a titre of 20.00 mL. A chemist has a hypothetical indicator with pKa = 7 (at pH 7 of solution the indicator is in the middle of changing colour). Suppose (s)he is overenthusiastic with the indicator and adds too much of it.
So we dump in 20.00 mL of HCl, and let's assume that all the HCl reacts with the OH- ions (let's ignore any reaction with the indicator and with water). This is the equivalence point. But is it the endpoint? The species present are Cl-, Na+, H2O, OH- (from self-dissociation of water, can be neglected), HIn, In- and H+. BUT since the chemist put too much indicator in, [H+] >>> 10^(-7). So pH (at equivalence point) ~ 4 (for instance). BUT the colour change starts to occur when pH ~ 8; this is the endpoint. Inaccurate.
However, if only like 1 drop of indicator is added, then pH (endpoint) ~ pH (equivalence point), so no problem, provided you choose the correct type of indicator.
A good way to investigate if too much indicator would cause inaccuracies is to draw a titration curve with equivalence point in the middle of the steep bit, drawing where the endpoint would be in curve, and then varying the pH of the equivalence point.
Anyway to other possible sources of error - i usually have:
- parallax
- in transferring solution into conical flask, not wiping outside of pipette after sucking up liquid (this causes more than the accurate amount of liquid to be transferred)
- not washing down conical flask after transferring solution from pipette and touching to side of flask to remove correct amount of liquid
- not leaving the correct amount of liquid in the pipette after transferring into flask
- not washing down flask with wash bottle just before endpoint (need to do this because you're swirling flask during titration)
- endpoint/equivalence point discrepancies (eg. incorrect indicator used, endpoint occurring only after excess reagent is added, etc.)
- not using half-drops in titrations
- careless errors (eg. washing volumetric glassware with wrong reagent, washing conical flasks with reagent, leaving funnel in burette during titration, etc.)
- washing storage beakers with water immediately before storing reagent in it (dilutes reagent)
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