The main use of the Tollens' reagent is to detect an aldehyde group, or sometimes an alpha-hydroxyketone.
However, the use of the Tollens' reagent has fallen off as more modern techniques have been developed.
The modern methods like NMR are generally quite expensive and not very accessible to the average person.
For people who are hobbyists, or those who are working on a budget, the classic methods are still pretty useful.
If a chemical containing an aldehyde group or an alpha-hydroxyketone is added to Tollens' reagent, will form a silver mirror.
As the video progresses, I'll explain exactly why the silver mirror forms, and the chemical reactions that are involved.
Because the formation of the silver mirror on its own was pretty cool, people developed an entire demonstration surrounding this effect.
I figured it only made sense to do both the silver mirror and the Tollens' test in the same video,
so that's what I did.
I'll make the Tollens' reagent first, then I'll play around with it a bit making some silver mirrors,
and then I'll move on to doing actual proper tests.
To make the Tollens' reagent, I need silver nitrate, sodium hydroxide, and ammonium hydroxide
which is also known as aqueous ammonia.
The sugar on the left is dextrose, and it's used in the silver mirror demonstration
but it's not used for the actual Tollens' reagent.
The sugar and the sodium hydroxide were purchased locally, but the ammonium hydroxide had to be purchased online.
Ammonium hydroxide is pretty much just super strong ammonia cleaner.
It's likely that it can be replaced by the more dilute stuff you find in stores,
but I've never tried it personally so I can't say that it works for sure.
It might be possible to buy the silver nitrate online
but I just made it myself by reacting silver metal with concentrated nitric acid.
I covered this preparation in a previous video and if you're interested in checking it out, I've provided a link in the description.
The first thing that we need to do is make 1L of 0.1M silver nitrate solution.
This sounds a little bit complicated, but it just means we have 0.1 moles of silver nitrate dissolved in 1L of water.
One mole of silver nitrate is about 170g, so 0.1 moles is about 17g.
I then manually mix it to get the silver nitrate to dissolve.
Silver nitrate is very soluble in water so after mixing for several seconds it should all dissolve.
My solution here is still cloudy and this is because the silver nitrate that I used wasn't super pure.
I mean, I did make it myself, so I assumed it would be super pure and amazing quality,
but you know, apparently I'm not as good as I thought.
On a serious note, silver nitrate tends to decompose back into metallic silver and this is probably what's floating around.
After letting it settle for a bit, I take a look at the bottom and we can actually see some silver there.
To clean up the solution, it's pretty simple and all we need to do is a quick gravity filtration.
The solution was filtered through three or four coffee filters and what came out was nice and crystal clear.
It should be noted that because some of the silver nitrate decomposed,
we probably have less than a 0.1M solution, but that's not really a problem.
Making the silver mirror isn't a super exact process, so if the silver nitrate solution is a little less concentrated,
it's really not a big deal at all.
Anyway, after several minutes everything eventually filters through and I go ahead and slap on a sticker that says silver nitrate.
Now it's time to make the Tollen's reagent and to start things off,
I measure out a 150mL of the silver nitrate solution.
I then pour in 9mL of concentrated ammonium hydroxide.
This leads to the formation of brown silver oxide.
However, in the presence of ammonia, silver oxide will react with it and form a soluble complex.
Once we have a crystal clear solution again, I go ahead and dump in about 2.4g of sodium hydroxide.
Some of the sodium hydroxide will react with the silver nitrate to form more silver oxide.
But since we have an excess of ammonia present, just like before the silver oxide will form a complex and dissolve into solution.
The Tollens' reagent is ready to be used as soon as all of the sodium hydroxide dissolves.
The Tollens' reagent should be used immediately after making it because it quickly degrades.
As it degrades, it forms something called silver nitride which is a contact sensitive explosive.
So, it really needs to be disposed of properly.
The typical way is to neutralize the Tollens' reagent using a little bit of dilute acid.
Anyway, after making the Tollens' reagent it only took me several minutes to set things up,
but by the time I went to use it it was already discolored a little.
I decided to start with the silver mirror, so I went ahead and poured the Tollens' reagent into a flask.
As you can see here the solution has already turned a little bit brown.
When making the silver mirror though, it really doesn't matter if a little bit of it has broken down.
With the Tollens' reagent in the flask, I then add about 1g of dextrose.
I cap the flask and start shaking it and initially, it doesn't seem like too much is happening.
Then very quickly silver metal starts to precipitate out and the solution becomes nearly black.
As I continue to shake the flask, it will appear to be more and more reflective
until eventually it's very clear that a silver mirror is forming.
Ok, so now we should talk about what exactly is going on here.
The dextrose that I added is known as a reducing sugar, and it contains a group known as an aldehyde.
Then, we have the Tollens' reagent, which is actually an oxidizing agent.
When the dextrose and the Tollens' reagent are mixed, our sugar will be oxidized and our Tollens' reagent will be reduced.
When the Tollens' reagent is reduced, the complex will fall apart and the silver will be reverted back to elemental silver.
As the silver is formed, it precipitates as a very fine particulate which slowly settles on the walls of the glass.
It took quite a bit of time for my mirror to start forming and
I had to put down the flask a few times to take a break.
It took a while because I'm doing it at room temperature,
but if I wanted to speed things up, I could have put it into a hot water bath.
The final mirror that formed was honestly a lot better than I expected it to be.
Once I was satisfied with the mirror, I could dump out the contents of the flask.
The waste here contains a little bit of silver which can be recycled.
It would only be something like a dollar or two worth of silver, so I don't really think it's worth the time.
As I said before though, the waste does need to be disposed of properly and to do this we need to acidify it.
That was a little bit out of the scope of the video though, so I don't show it here.
Now there's the option where you can either keep the silver mirror or clean the flask.
If the flask is washed and capped, the mirror should remain untarnished for a decent amount of time.
I wanted to keep my flasks though, so I quickly cleaned it with some concentrated nitric acid.
The nitric acid reacts with the silver to form silver nitrate which dissolves into the water.
After just several seconds of shaking, the flask is pretty clean.
The brownish color that you see is not silver, and it's actually nitrogen dioxide gas that's been let off by the nitric acid.
I wanted to see the mirror form without the flask moving, so I decided to do it in a beaker.
With stirring, I added the dextrose and I waited for the mirror to form.
You guys might notice a random glass stopper that's there and I placed it in there to see if it would get plated with silver.
I forgot to film taking it out to show you guys what it looked like,
but it wasn't too interesting because little to no silver deposited on it.
In the silver mirror video by NurdRage, he was able to cover a glass slide with silver and make a small mirror out of it.
I'm not exactly sure here why it didn't work for my glass stopper, but maybe I just didn't leave it in the solution long enough.
Also, NurdRage used heat which increases the rate that the silver mirror forms, so maybe that helped out too.
Anyway, what you're seeing here is sped up and it took about three minutes in real time to fully form the mirror on the beaker.
After the mirror had formed, I dumped out the contents and washed out the beaker and took a look inside.
When looking at it from the outside, the mirror is very nice but when we look at the inside, it's kind of gross.
The silver that we see in the beaker is exposed to air and it gets oxidized,
but the silver that we see from the outside is protected from this and remain shiny.
Some of you might have been wondering how well the silver stuck to the beaker so using my fingers, I tried to scratch them off.
I was honestly surprised how well it stuck and to scrape it off, I would probably have to use some metal tools.
Just like before, I decided to kill the mirror so I poured in a little bit of concentrated nitric acid.
With a little bit of swirling, the beautiful silver mirror slowly dies.
Anyway, that's about it for the silver mirror demo and now I want to do some real Tollens' test.
The most common use of the Tollen's test is to determine whether an unknown contains either an aldehyde or an alpha-hydroxyketone.
If the unknown does contain either of these groups, we'll get the same reaction that we just saw and a mirror will form.
To get things started, I prepare some fresh Tollens' reagent.
By the time I set up this little apparatus here, you can see that the solution has become a little bit discolored.
If the unknown that we use is very dilute, this in theory could produce a false positive.
The unknowns that I'm using are pure and undiluted though, so it really shouldn't be an issue.
With the Tollens' reagent in the test tube, I go ahead and add a few drops of the first unknown.
Before I even take it off the stand, we can already see some silver forming.
When I shake the test tube, a lot of silver starts to fall out of solution
and we can see a mirror forming on the sides.
This is a very strong positive, and it's highly suggestive that we have an aldehyde present.
It's important to note that I said it only *suggests* that an aldehyde is present and that it's not for certain
and this is because the Tollens' reagent isn't a hundred percent specific for aldehydes.
The Tollens' test will also give a positive result for things like alpha-hydroxyketones
and even some odd false positives for things like formic acid.
A lot of these old-school methods used to identify functional groups need to be paired with other techniques.
Anyway, now we can move on to the second unknown.
It took me a few minutes between the tests and you can see the Tollens' reagent got even a little darker.
Anyway, just like before I pour in a bunch of the unknown and I swirled around the test tube.
This time however, we don't form any colloidal silver.
Some aldehydes won't react right away, and it's sometimes necessary to heat the solution.
A negative result is confirmed if no silver mirror forms after five minutes in a hot water bath.
This means that whatever was added is not an aldehyde, an alpha-hydroxyketone, or any of the other compounds that would give a positive.
Just for fun, I pour the remains of the other unknown in and we very quickly get a positive result.
Anyway, that's about it for the Tollens' reagent.
In the future, I might make a video where an unknown is identified using multiple old classic techniques.
One of the classic things that was done is starting with an unknown sugar, using various different techniques,
you can actually identify exactly which sugar it is.
In the last video, you guys used the Strawpoll to vote for dibenzalacetone
but I ran into a little bit of a problem, so I had to post this one first.
I was halfway through editing the dibenzalacetone video,
but then I realised I did no test at the end of the video to show the sun-blocking capabilities.
I had to order some things online to do the demonstration, and I was debating whether or not to post the video anyway,
but I decided it made a lot more sense to just post everything all at once and to not separate it between experiment and demonstration.
With that being said, I received what I needed yesterday,
so I'm going to go ahead and edit things as fast as I can and try to put the video out by the end of the week.
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