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Author: Subject: How to accurately determine NH3 concentration?
fusso
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[*] posted on 23-5-2019 at 05:54
How to accurately determine NH3 concentration?


I have some conc NH3(aq) which is >25% but I want to determine the molarity to the nearest 0.1M. What should I do?
What's the rate of change of NH3 concentration when said soln is exposed to air?




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Ubya
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[*] posted on 23-5-2019 at 06:16


titration is the keyword




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fusso
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[*] posted on 23-5-2019 at 07:05


What indicator should I use?



Useful sites:
Balance Chemical Equation: http://www.webqc.org/balance.php
Molecular mass and elemental composition calculator: https://www.webqc.org/mmcalc.php
Solubility table: https://en.wikipedia.org/wiki/Solubility_table
Azeotrope table: https://en.wikipedia.org/wiki/Azeotrope_tablesIt's not crime if noone finds out - Nyaruko
List of materials made by ScienceMadness users: https://docs.google.com/spreadsheets/d/1nmJ8uq-h4IkXPxD5svnT...
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Ubya
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[*] posted on 23-5-2019 at 09:18


Quote: Originally posted by fusso  
What indicator should I use?


Probably methyl red or methyl orange

Remember to use dilute solutions, you want concentration not activity

[Edited on 23-5-2019 by Ubya]





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Pumukli
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[*] posted on 23-5-2019 at 09:41


A while ago I had to titrate fairly concentrated dimethylamine in water. I don't have the laboratory workbook at hand but as I remember I pipetted 1 ml of such solution into a 250 cm3 measuring flask and topped it up with distilled water.
Then I took 10 (or 5?) cm3 of this diluted solution and titrated against 0.1 N HCl, using methylorange as indicator.
The 0.1 N HCl was made by approximate dilution from store bought muriatic acid and was standardized by titration against precisely measured 80 - 100 mg portions of the primary standard KHCO3.

Don't pipette the strong NH3 solution by mouth! Doing so with dimethylamine was bad enough, it extremely fumed! NH3 has even higher volatility so I assume it would be even worse! :-)
Maybe a pipette ball would do the trick.

[Edited on 23-5-2019 by Pumukli]
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MrHomeScientist
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[*] posted on 23-5-2019 at 10:27


Don't pipette anything by mouth! Why would anyone do this anymore?


Edit to be more on topic: Agree with the other posters. Take a 1mL sample of your solution, dilute it with distilled water (maybe 10x?), then titrate against a good standard acid (maybe sulfuric, since it doesn't fume) using methyl red/orange indicator.
Here's an interesting description of one process: http://people.bu.edu/straub/courses/demomaster/weaktitrate.h...

[Edited on 5-23-2019 by MrHomeScientist]
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Sulaiman
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[*] posted on 23-5-2019 at 10:46
+1 for the not sucking on pipettes recommendation


Disposable pipettes at about one pence each are cheaper than the cost of washing a reusable pipette,
and the risks of poisoning or cross-contamination make disposable transfer pipettes an essential part of my lab,
keep a (re-used, rinsed) pipette with each of your presently-in-use liquids for convenience, safety and cleanliness.





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Tsjerk
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[*] posted on 23-5-2019 at 12:01


Just remember NH3 is a weak base, so the equivalent of any strong acid won't give a neutral solution. When using methyl red it will indicate about the equilibrium, when using an indicator that turns at more neutral pH you could use a back titration.

This involves adding a known excess of a strong acid and to titrate that with a strong base.
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Ubya
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[*] posted on 23-5-2019 at 12:07


Quote: Originally posted by Sulaiman  
Disposable pipettes at about one pence each are cheaper than the cost of washing a reusable pipette,
and the risks of poisoning or cross-contamination make disposable transfer pipettes an essential part of my lab,
keep a (re-used, rinsed) pipette with each of your presently-in-use liquids for convenience, safety and cleanliness.



Reliable and accurate measurement is more important, disposable pipettes are convenient but not as precise. The negative part of course is taking care of them





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