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Author: Subject: Acidimetry issues
Bedlasky
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[*] posted on 27-4-2021 at 11:16
Acidimetry issues


Hi.

I made 1l of 1M HCl. I standardized it with NaHCO3 (normal baking soda, this should be good source of quite pure NaHCO3), methyl red was used as indicator. NaHCO3 was weight to 4 decimal places. I titrated NaHCO3 until orange-red colour was observed, than solution was bring to boil (releasing CO2 from solution), cooled down and than titrate until pink colour was observed. Exact concetration of HCl was determined as c = 0,1015 mol/l.

With this HCl I tried today titrate Na3PO4.12H2O and mixture of NaOH/Na2CO3.

Na3PO4: Solution was titrate to thymolphtalein (Na2HPO4), after that methyl red was added and solution was titrate until pink colour was observed (NaH2PO4). Sample contained 111,82% Na3PO4.12H2O. First titration: V(tph) = 12,6 ml, V(mr) = 25,4 ml; second titration: V(tph) = 12,1 ml, V(mr) = 24,3 ml.

NaOH/Na2CO3 mixture: Solution was titrate to phenolphtalein and than to methyl red. Solution was bring to boil at the end of the titration to methyl red to release CO2 from the solution. Sample contained 27,27% Na2CO3 and 103,89% NaOH (I made mixture containing 17,59% Na2CO3 and 82,41% NaOH from pure chemicals). First titration: V(phph) = 16,6 ml, V(mr) = 14,9 ml; second titration: V(pph) = 16,9 ml, V(mr) = 14,9 ml.

So where is mistake? 103% NaOH? 112% Na3PO4? Really? Wtf... I really don't see any mistake.




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CharlieA
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[*] posted on 27-4-2021 at 14:36


There may not be any mistake. Are the results within your experimental error? For example, even if you know the concentration of HCl to 4 significant figures, are the rest of your measurements good to at least 4 significant figures? Did you dry your samples before the titrations? For example, a mixture of X.12 H2O with x.10 H20, will not give the same result as X.12 H20 only.
Just random thoughts before my dinner is ready!:D
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Bedlasky
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[*] posted on 28-4-2021 at 03:07


Yes, I weighed it to 4 decimal places. No, I didn't dry it. Yeah, Na3PO4 is maybe mixture of hydrates. But what about NaOH/Na2CO3 mixture? This mixture is hygroscopic, so it should contain some water (which lowers the percentage of Na2CO3 and NaOH and not rising it).

[Edited on 28-4-2021 by Bedlasky]




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Oxy
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[*] posted on 28-4-2021 at 06:25


NaHCO3 can't be used for making standardized solution. You should heat it to 270-300 deg. C for about 1 hour and convert it all to anhydrous Na2CO3.
Ratio of NaOH/Na2CO3 you got from the titration is quite close to the ratio of prepared sample so I suspect it's the result of wrong HCl concentration
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Bedlasky
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[*] posted on 28-4-2021 at 06:33


NaHCO3 is primary standard. Why shouldn't I use it?

Quote: Originally posted by Oxy  
NaHCO3 can't be used for making standardized solution. You should heat it to 270-300 deg. C for about 1 hour and convert it all to anhydrous Na2CO3.
Ratio of NaOH/Na2CO3 you got from the titration is quite close to the ratio of prepared sample so I suspect it's the result of wrong HCl concentration




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[*] posted on 28-4-2021 at 06:42


It can be contaminated with Na2CO3 and water
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Bedlasky
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[*] posted on 28-4-2021 at 07:05


I will try standardization with tris.



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[*] posted on 28-4-2021 at 08:28


Since you have NaHCO3, you might as well bake some to give high purity Na2CO3 and see what result you get with that.
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Bedlasky
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[*] posted on 28-4-2021 at 08:54


Yes, I can try that, but I want comparison with standard indenpendent on NaHCO3 (for the case that NaHCO3 have some impurity, but I doubt that).



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[*] posted on 28-4-2021 at 09:07


It's not a great answer (because some may be lost as "spray"), but weigh the bicarbonate before and after heating.
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[*] posted on 28-4-2021 at 09:26


Better is to heat bicarbonate, put in to dessicator and than weigh.

[Edited on 28-4-2021 by Bedlasky]




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