RU_KLO
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Ammonium hydroxide titration
Made some Ammonium hydroxide, and want to titrate to know its concentration.
after reading, its stated that ammonium hydroxide is a weak base and if titrated with a stong acid, the solution remains acid (thus interpreting that
cannot find the end point - but could be totally wrong)
Also ph paper turned violet so "not so weak base", maybe because concentration is high.
(or weak and strong does not rely on concetration?)
so:
1) could I use phenolphtalein as indicator?
2) which acid should I use (I have only strong mineral acids - HCL, H2SO4, HNO3)
thanks
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Texium
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Are your acids at a standard, known concentration? If not, you will need to first standardize one of them to get an accurate acid standard for
titration. HCl is the preferred acid for titration because unlike sulfuric, it is monoprotic, so it has a clean transition from acidic to neutral pH,
and unlike nitric, it is non-oxidizing. HCl may be most easily standardized by titration with a solution of pure, oven-dried sodium carbonate with
phenolphthalein indicator. You have to swirl and gently heat your solution as you do the titration though, because dissolved carbon dioxide will lower
the pH and cause overshoot.
Phenolphthalein will be a suitable indicator for an HCl/ammonia titration (ammonia into HCl for suitable color change detection). Although ammonia is
a weak base, phenolphthalein’s indicator range is sufficient that as soon as an excess of ammonia is present, there should be a detectable color
change.
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Fery
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I would use HCl too. As an indicator, I would use methyl orange because solution of NH4Cl is weakly acidic (phenolphthalein has color change in
alkaline range).
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RU_KLO
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Thanks,
ordered Methyl Orange for standarization (read somewhere) of HCL. Did not know that also can be used for ammonia/hcl titration.
if not misunderstood, ammonia (titrant -unknown molarity) should be poured into HCL (analyte)?
(in this case known volume/molariy is the analyte?)
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Bedlasky
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Fery, when you titrate ammonia with acid, you create a buffer, this doesn't give you clear end point. Texium suggestion is better option. Adding known
amount of acid in to volumetric flask and adding ammonia solution in to the burrete and then titrating acid with ammonia using phenolphtalein (first
excess of ammonia will create basic environment).
There is another method, adding ammonia in to the solution of 5% boric acid and titrating with HCl using methyl orange as indicator. In this case you
need to perform blank titration - just titrating boric acid with HCl until indicator change its colour, this volume needs to be subtracted from volume
from ammonia titration. But this method is usually used when ammonia is distilled from solution of ammonium salt upon addition of NaOH, ammonia/water
mixture is distilled directly in to boric acid.
RU_KLO do you have rough idea how much concentrated is your ammonia (10%, 20%, 30%)? If not weigh your ammonia in graduated cylinder and calculate
rough density. You must make dilute ammonia for titration. 0,1-1M HCl is mostly used for acid-base titrations. So if you use 0,1M HCl, you need to
prepare cca 0,1M NH3.
[Edited on 2-11-2022 by Bedlasky]
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Fery
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Hi Bedlasky, titrating that way (HCl into flask, ammonia into burette) could be perhaps favorable for using phenolphthalein. But you also created a
buffer solution, a mixture of NH4Cl + NH3 that would have even less clear titration point. That way you first neutralize all HCl so you have a
solution of NH4Cl in flask with weak acidic pH, then excess of ammonia from burette would change pH only very slowly.
I meant a titration where you put ammonia into flask and HCl into burette. After you neutralize all NH3 then additional HCl from burette would change
pH rapidly. In this titration I would prefer methyl orange.
The method with H3BO3 looks interesting - it's back titration, right?
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Bedlasky
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Fery: Yes, you are right. Texium proposal method would create buffer, not yours. Because you are using methyl orange, you cannot create buffer,
because you haven't any ammonia in the solution, just ammonium cations. I checked my analytical chemistry book, they propose methyl red as indicator
for direct titration or back titration in boric acid environment as I mentioned above.
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Fery
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Hi Bedlasky, methyl red is even better
0,1 mol/l NH4Cl pH = 5,1
color change of methyl red 4,4-6,2 and methyl orange 3,1-4,4
[Edited on 2-11-2022 by Fery]
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RU_KLO
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@ Bedlasky, roughly density 0,95 which corresponds to 15% (from an internet table. No analytical scale. Common 0,01 china scale. expect 10% tolerance.
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woelen
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Even these chinese scales do much better than 10%. Usually their error is 0.0x grams + y% of measured value. Typical values for x and y are x = 2 to 5
and y = 1 to 2. So, only if you measure very small amounts you'll get a lot of uncertainty.
[Edited on 2-11-22 by woelen]
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RU_KLO
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So, rounding up:
Ammonia in flask, diluted to 0.1M (roughly estimated) + methyl orange
HCl in Burette, standarized 0.1M.
and regarding methyl orange. I will get it as salt, for making the indication solution Ive found 2 methods:
which one is right (or better) for the titration Ill do?
1) disolve 0.1g methyl orange in 80ml H2O, complete to 100ml with 95% ethanol.
2) disolve 1g methyl orange in 20 ml methanol (could Ethanol be used?) complete with H2O to 100ml.
(dH2O should be boiled - to remove CO2 or is ok as is?)
[Edited on 2-11-2022 by RU_KLO]
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Bedlasky
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I think that methyl orange will dissolve much quicker in alcohol, so I would make it according to 2). You don't need to boil water for making
indicator, CO2 will dissolve in it anyway during storage. And you add just a few drops in to the titration flask, so it really doesn't matter.
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