Solving Titration Problems: NaOH + Oxalic Acid

[ASidd]

I am doing titration of NaOH with oxalic acid at school and none of my results are close together at all.

I got
1. 26.6 mL
2. 27 mL (although I want to discount this since I wasn't very careful with this)
3. 25.5 mL
4. 26 mL
5. 25.3 mL (For this titration I realized that there was some insoluble material at the bottom of my conical flask. Thus I wanted to discount this as well for possible risk of contamination)

Can someone explain any possible reasons for such large discrepancies? I have been very careful washed all my equipment etc. but still got bad results.

 

[Borek]

Hard to say anything. At least please describe the procedure you are following.

 

[ASidd]

It's just a simple titration. Measure 25 mL of NaOH solution (with unknown concentration) using a pipette and titrate it using 0.05M oxalic acid.

 

[Borek]

OK, judging from your description you didn't use any indicator to determine the end point. In such a case I am quite surprised your results are that consistent.

 

[ASidd]

No I used phenolphthalein indicator. I thought in titration's it's kind of expected that an indicator will be used;hence didn't mention it in my previous post.

 

[Borek]

Different indicators will yield different results, so it is a crucial information.

Was this procedure suggested to you? Titrating oxalic acid with NaOH against phenolphthalein makes sense, but the reverse titration is a rather unorthodox idea, especially taking into account possible contamination of NaOH with carbonates. Effectively that would mean titration of a weak base with a weak acid, which never yields sharp end points.