Sciencemadness Discussion Board

How to accurately determine NH3 concentration?

fusso - 23-5-2019 at 05:54

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?

Ubya - 23-5-2019 at 06:16

titration is the keyword

fusso - 23-5-2019 at 07:05

What indicator should I use?

Ubya - 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]

Pumukli - 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]

MrHomeScientist - 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]

+1 for the not sucking on pipettes recommendation

Sulaiman - 23-5-2019 at 10:46

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.

Tsjerk - 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.

Ubya - 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

AJKOER - 7-9-2019 at 04:40

NH3 presence in an excess of HCl vapor produces a NH4Cl cloud formation. As NH4Cl is not soluble in acetone, add acetone and collect NH4Cl. In air, the acetone evaporates leaving NH4Cl.

Interestingly, one may be able to replace the HCl with more friendly CO2 and water vapor, and conduct the experiment at 5 C. The major product is reportedly solid NH4HCO3 (see https://link.springer.com/article/10.1007%2Fs11814-011-0203-... ).
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A more theoretically interesting non-chemical based path would be assume higher concentrations of ammonia gas (again assuming excess of the selected acid gas) increases the per unit particle presence of say NH4Cl. Then, we may have a suspended sediment concentration determination problem.

Modern technology to the rescue, for example, to quote a source ('Laser Diffraction Sensors measure Concentration and Size Distribution of Suspended Sediment' at http://www.comm-tec.com/Library/Technical_Papers/Various/coo... ):

"New Developments, LISST-25: As a precursor to the newest developments, we note first the development of the LISST-25 sensor. The principle of the LISST-25 is based on ideas from laser diffraction, as follows. According to diffraction, the scattered light energy falls at larger angles on the ring-detector plane for finer particles, and vice versa. To measure suspended sediment concentration (SSC), the sensed scattered light energy per unit sediment concentration should be identical for any size."

Or, likely with much less precision, one could note the diffraction of a light laser for varying concentration of NH3 in excess acid gas and interpolate the concentration.

[Edited on 7-9-2019 by AJKOER]

Heptylene - 7-9-2019 at 07:10

Quote: Originally posted by Ubya  
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


I think Sulaiman was talking about disposable vs glass Pasteur pipette not volumetric ones. I'm sure nobody in their right mind would use a disposable pipette for volumetry (at least I hope)...

@fusso: If you have a good quality volumetric flask, and a calibrated scale, you could measure the density of your ammonia solution and figure out the concentration from density tables. Of course your ammonia must be pure.
You'll need to determine the density with an accuracy better than 0.0005 g/ml to achieve the 0.1 M accuracy you want.