Odd silver mirror result

Tollen's reagent plus propanal/propionaldehyde seems to give a grey, waxy blob on top of the aqueous solution. Anyone seen this before?
 

CovTech

COMMITTEE
Have you tried it with a different aldehyde in the same tube? Trying to eliminate the tube itself as a factor before blaming the propanal?
 
Have you tried it with a different aldehyde in the same tube? Trying to eliminate the tube itself as a factor before blaming the propanal?

I can also back up what Mike is saying. I couldn't get it to work, also getting some weird blob. An Edexcel core practical involves obtaining the silver mirror using propanal. I tried new test tubes cleaned with nitric acid, new stock of propanal, different temperatures and no luck.
I ended up replacing it with 20% glucose solution which works a treat and gives the same negatives for the other tests as propanal would have.
 
I can also back up what Mike is saying. I couldn't get it to work, also getting some weird blob. An Edexcel core practical involves obtaining the silver mirror using propanal. I tried new test tubes cleaned with nitric acid, new stock of propanal, different temperatures and no luck.
I ended up replacing it with 20% glucose solution which works a treat and gives the same negatives for the other tests as propanal would have.
The school has struggled for 5 years to get it to work using propanal, but I got it to work using glucose so it will now be given out instead of propanal but it will be labelled up as propanal, its the teachers idea so I will roll with that :D
 
The school has struggled for 5 years to get it to work using propanal, but I got it to work using glucose so it will now be given out instead of propanal but it will be labelled up as propanal, its the teachers idea so I will roll with that :D

It is indeed one of those cases where ignorance truly is bliss ;)
 
Yes, we were getting the same result with other tubes. What's really odd is that classes have been doing it for a week or so now and this is the first instance of this phenomenon.
 

CovTech

COMMITTEE
Sorry guys - no idea
I was curious and tried it just now - perfect silver mirror with Propanal
IMG_20190404_092033[1].jpg
Method I used was:
1) 1cm3 of 0.1M silver nitrate in a tube
2) add one drop of 1M NaOH
3) add 1M NH3 dropwise until the precipitate just dissolved
4) add 10 drops of propanal
5) put in freshly boiled water bath for 5 mins

Came out like in the picture first time
 
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CovTech

COMMITTEE
Perhaps it's the age of the Propanal? I know it'll oxidise to the acid and mine is admittedly an almost new bottle....
 
Perhaps it's the age of the Propanal? I know it'll oxidise to the acid and mine is admittedly an almost new bottle....

I used a new bottle after my old stock stopped working thinking it might have been that, but still didn't work. The new (sealed) bottle was a few months old and unopened so it could be that was long enough for it to oxidize?
 

CovTech

COMMITTEE
I used a new bottle after my old stock stopped working thinking it might have been that, but still didn't work. The new (sealed) bottle was a few months old and unopened so it could be that was long enough for it to oxidize?
I can't see how it would have in that amount of time (unless your store has a worrying issue with ventilation and a very weird combination of fumeso_O)

Afraid I don't know what else to suggest
 

007

COMMITTEE
I used a new bottle after my old stock stopped working thinking it might have been that, but still didn't work. The new (sealed) bottle was a few months old and unopened so it could be that was long enough for it to oxidize?
That's a strange result - we have a bottle of ethanal that's 4 years old and I trialled it last week and it still provides a positive test for silver mirror. The reason why I rarely use it now aside from the unpleasant fumes it exudes is it doesn't stay in a pipette so whilst extracting it from the bottle it spills everywhere, glucose like others have suggested is the way I go now and it works everytime, and if you add solid sodium carbonate to the reaction mixture after the silver mirror test it proves the presence of a carboxylic acid (fizzes) as the aldehyde has been oxidised by the Tollens'.
 
That's a strange result - we have a bottle of ethanal that's 4 years old and I trialled it last week and it still provides a positive test for silver mirror. The reason why I rarely use it now aside from the unpleasant fumes it exudes is it doesn't stay in a pipette so whilst extracting it from the bottle it spills everywhere, glucose like others have suggested is the way I go now and it works everytime, and if you add solid sodium carbonate to the reaction mixture after the silver mirror test it proves the presence of a carboxylic acid (fizzes) as the aldehyde has been oxidised by the Tollens'.
I agree it is an odd result. Propanal vapour is unpleasent and it is very volatile so that it can be gone before you know it. I see nothing wrong with using Glucose as it has an aldehydic group.:);)
 
I agree it is an odd result. Propanal vapour is unpleasent and it is very volatile so that it can be gone before you know it. I see nothing wrong with using Glucose as it has an aldehydic group.:);)
But if you draw the structure of glucose that has an aldeydic group the kids get upset as its not a cyclic structure :D
 
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