Sodium Acetate pH - 0.2M Solution, No Acetic Acid Added

[philip041]

I have 0.2M solution of Sodium Acetate, (100ml with 1.6g NaAc)

I want to work out the pH it should be. THERE IS NO ACETIC ACID ADDED.

This is causing me nightmares! I am measuring ph 6.5 but apparently that is wrong. Why would that be so?

Cheers!

 

[Borek]

Do you know what pH should it have?

How are you measuring pH?

 

[philip041]

Well I was told by someone in my lab they remember it should be pH8. I found some botch eqn but worked out that maybe it should be pH9?

I am using some super fancy pH meter, the glass tube ones. I double checked against another one and it turns out its not so fancy after all. I measured pH7 with a different one, however this is still low. Especially shouldn't the pH go UP when you NaAc?

I think the eqn was, (sorry no Latex):

pH =0.5 * (14 +pKa + log(Ci)) where Ci corresponded to the molar something, o.2

Cheers

 

[Borek]

Compare calculation of pH of weak acid or base. What you have is a solution of a weak base (acetate).

Closer to 9 if you ask me.

Have you calibrated your ph meter?


& Buffer Maker - the ultimate buffer calculator

 

[philip041]

yeah I worked out 9.03 or something. pH meter was recently calibrated yet seems to still give 0.5 pH below another pH meter in dept. I think it highly unlikely that that one is also wrong as it is used constantly by a research group and they would have to be pretty careful about things like that.

We were careful with our measurements, we wash everything, we use really pure water, what is going on!

 

[chemisttree]

Calibrate your electrode and leave the probe in the sodium acetate solution for about 10 minutes. Prepare a fresh solution of sodium acetate and measure that solution.

What solution do you store the electrode in? What is the slope of your electrode?

 

[philip041]

I don;t know what we store it though I will find out tomorrow. it is stored vertically.

 

[Borek]

pH meter was recently calibrated

What do you mean by "recently"? Calibrate it now to be sure.

 

[philip041]

Will calibrate now, it is in saturated KCl, as normal I think for these things

 

[chemisttree]

I don;t know what we store it though I will find out tomorrow. it is stored vertically.

Very funny! Of course I was referring to the http://biology.bard.edu/ferguson/course/bio141/Lab/Lab_Appendix_1.pdf" procedure...

Saturated KCl is not a standard storage solution (its what is used in the filling solution in many electrodes, though). It will likely salt up the bridge. It is more common to store the electrode in a pH 4 or pH 7 buffer solution.

 

[philip041]

Hmm I will ask my supervisor again, as I also thought it odd thwe ownat it would be stored in that. I think maybe we have solved the problem, the set of standards , (fluka analytical ph 4,7,10) were chosen from a list and ours was probably set to the wrong one so we changed it to the set which seemed most appropriate(fisher).

however still doens;t explain why we measure pH 7.5 for something which should be 9.

We used 1.6g of NaAc for 100 ml water, to make o.2M soln?

 

[philip041]

I think maybe the elctorde is wrong actually, when we make a measeurement it takes absolutly for ever to stabablise. It will keep climbing really slowly until it reaches a plateau but this can take 10-15 minutes, why the delay?

 

[Borek]

That's one of the possible effects of electrode aging.

Check out information from this site:

http://www.ph-meter.info/pH-electrode

 

[soot]

Does sound like a cooky electrode.

What kind of sodium acetate are you using? Is it anhydrous or hydrated? That could effect your expected concentration by almost half. Check the reagent bottle as well or you could try a quick and dirty titration to see if that changes anything.

 

[Borek]

Won't hurt to check the concentration, but to get below pH 8.0 you need to dilute sodium acetate to 2e-3M, this doesn't sound likely.

 

[chemisttree]

I think maybe the elctorde is wrong actually, when we make a measeurement it takes absolutly for ever to stabablise. It will keep climbing really slowly until it reaches a plateau but this can take 10-15 minutes, why the delay?

Did you record the slope for the electrode during your calibration procedure?