[naissessom]

Hi,
I'm taking my second year of chemistry (AP) in high school, and this is a question pertaining to a lab in which we first synthesized copper sulfate pentahydrate, and then analyzed it.
The process for this part of the lab is as follows:
1. Weigh out .10 - .15 grams of the copper sulfate hydrate (CSH) in a 250 mL Erlenmeyer flask.
2. Dissolve in 10-20 mL of water.
3. Add 10 mL of 1M NH4Cl.
4. Add 3M of NH4OH until solution is clear blue.
5. Dilute in 50-100 mL of water.
6. Add 1.31 g of Murexide indicator.
7. Titrate with standard 0.020 M ethylenediamine-tetracetic acid (EDTA) until solution is violet.
8. Repeat.
9. Calculate percent of copper in CSH, and take the average of the two experimental values.

My question is about the error analysis. The experimental value I got (Average of two trials) turned out to be 23.1%, and the actual value is 25.5%, so the percent error is only 9.41%.
In my error analysis, I proposed that the titration does not fully go through because the free-floating Sulfate (SO4(2-)) ions in solution act as bronsted-lowry acids, and therefore less EDTA is used. But it has since then dawned on me that all Sulfates (Except Pb, Ba, and Hg, of course) are soluble, and that the sulfate ions have no effect on the titration.

My second thought was that either the copper does not fully combine with the ammonia (coming from the ammonium hydroxide, which decomposes into ammonia and water), or that not all of the murexide replaces the ammonia and complexes with copper, or a combination of the two. Except that there's no reason for either of these deficiencies, and so they don't work.

So in conclusion, I'd greatly appreciate it if someone could suggest a better explanation for the error ("human error" doesn't count). I've given this much thought, and just can't think of it. Thanks!

[mike]

Where did your percent error of 9.41% come from, this seems a little high.

Your procedure involves a few mass, concentration and volume steps. Have you thought about the errors associated with the equipment you used? WHat is the error in the values read from say the balance, the burette, the pipette etc. Are there errors quoted with the "known" concentrations?

[Borek]

1. Weigh out .10 - .15 grams of the copper sulfate hydrate (CSH) in a 250 mL Erlenmeyer flask.

Depending on the temperature, humidity of air, and the history of their changes, substance weighted can be CuSO₄^.5H₂O, CuSO₄^.3H₂O or mixture of both.

[naissessom]

Where did your percent error of 9.41% come from, this seems a little high.

(25.5-23.1)/25.5 = .0941 - 9.41%  .

Borek, thank you. I was not aware that there existed an alternate form of the hydrate.

I appreciate the help.

[naissessom]

On second thought, Borek, that would give me more copper than there should be, whereas I have less.

I give up, I'll just blame it on the meniscus.

[Borek]

Borek, thank you. I was not aware that there existed an alternate form of the hydrate.

CuSO₄^.5H₂O
CuSO₄^.3H₂O
CuSO₄^.2H₂O
CuSO₄^.1H₂O
CuSO₄

Note lack of 4.

How do you know you have less - if the sample was contaminated with some other salt your result can be correct.

One more reason for the titration error can be of kinetic nature - some heavy metals react with EDTA slowly. They are better determined with reverse titration - known excess amount of EDTA is added to sample, left for 15/30 minutes, and then EDTA is titrated with Mg or Ca. If the copper is complexed slowly, that can be the reason of negative error. I have no idea how fast copper reacts with EDTA; it should be described in some analytical chemistry books.

[mike]

I give up, I'll just blame it on the meniscus.

What about my suggestions? Did you look at all of the other errors involved (including the meniscus)?

You can't compare the two values without quoting their errors?