A tale of chemistry, biochemistry, physics and astronomy – and shiny, silver balls

A new school term has started here, and for me this year that’s meant more chemistry experiments – hurrah!

Okay, actually round-bottomed flasks

The other day it was time for the famous Tollens’ reaction. For those that don’t know, this involves a mixture of silver nitrate, sodium hydroxide and ammonia (which has to be freshly made every time as it doesn’t keep). Combine this concoction with an aldehyde in a glass container and warm it up a bit and it forms a beautiful silver layer on the glass. Check out my lovely silver balls!

This reaction is handy for chemists because the silver mirror only appears with aldehydes and not with other, similar molecules (such as ketones). It works because aldehydes are readily oxidised or, looking at it the other way round, the silver ions (Ag+) are readily reduced by the aldehyde to form silver metal (Ag) – check out this Compound Interest graphic for a bit more detail.

But this is not just the story of an interesting little experiment for chemists. No, this is a story of chemistry, biochemistry, physics, astronomy, and artisan glass bauble producers. Ready? Let’s get started!

Bernhard Tollens (click for link to image source)

The reaction is named after Bernhard Tollens, a German chemist who was born in the mid-19th century. It’s one of those odd situations where everyone – well, everyone who’s studied A level Chemistry anyway – knows the name, but hardly anyone seems to have any idea who the person was.

Tollens went to school in Hamburg, Germany, and his science teacher was Karl Möbius. No, not the Möbius strip inventor (that was August Möbius): Karl Möbius was a zoologist and a pioneer in the field of ecology. He must have inspired the young Tollens to pursue a scientific career, because after he graduated Tollens first completed an apprenticeship at a pharmacy before going on to study chemistry at Friedrich Wöhler’s laboratory in Göttingen. If Wöhler’s name seems familiar it’s because he was the co-discoverer of  beryllium and silicon – without which the electronics I’m using to write this article probably wouldn’t exist.

After he obtained his PhD Tollens worked at a bronze factory, but it wasn’t long before he left to begin working with none other than Emil Erlenmeyer – yes, he of the Erlenmeyer flask, otherwise known as… the conical flask. (I’ve finally managed to get around to mentioning the piece of glassware from which this blog takes its name!)

It seems though that Tollens had itchy feet, as he didn’t stay with Erlenmeyer for long, either. He worked in Paris and Portugal before eventually returning to Göttingen in 1872 to work on carbohydrates, going on to discover the structures of several sugars.

Table sugar is sucrose, which doesn’t produce a silver mirror with Tollens’ reagent

As readers of this blog will know, the term “sugar” often gets horribly misused by, well, almost everyone. It’s a broad term which very generally refers to carbon-based molecules containing groups of O-H and C=O atoms. Most significant to this story are the sugars called monosaccharides and disaccharides. The two most famous monosaccharides are fructose, or “fruit sugar”, and glucose. On the other hand sucrose, or “table sugar”, is a disaccharide.

All of the monosaccharides will produce a positive result with Tollens’ reagent (even when their structures don’t appear to contain an aldehyde group – this gets a bit complicated but check out this link if you’re interested). However, sucrose does not. Which means that Tollens’ reagent is quick and easy test that can be used to distinguish between glucose and sucrose.

Laboratory Dewar flask with silver mirror surface

And it’s not just useful for identifying sugars. Tollens’ reagent, or a variant of it, can also be used to create a high-quality mirror surface. Until the 1900s, if you wanted to make a mirror you had to apply a thin foil of an alloy – called “tain” – to the back of a piece of glass. It’s difficult to get a really good finish with this method, especially if you’re trying to create a mirror on anything other than a perfectly flat surface. If you wanted a mirrored flask, say to reduce heat radiation, this was tricky. Plus it required quite a lot of silver, which was expensive and made the finished item quite heavy.

Which was why the German chemist Justus von Liebig (yep, the one behind the Liebig condenser) developed a process for depositing a thin layer of pure silver on glass in 1835. After some tweaking and refining this was perfected into a method which bears a lot of resemblance to the Tollens’ reaction: a diamminesilver(I) solution is mixed with glucose and sprayed onto the surface of the glass, where the silver ions are reduced to elemental silver. This process ticked a lot of boxes: not only did it produce a high-quality finish, but it also used such a tiny quantity of silver that it was really cheap.

And it turned out to be useful for more than just laboratory glassware. The German astronomer Carl August von Steinheil and French doctor Leon Foucault soon began to use it to make telescope mirrors: for the first time astronomers had cheap, lightweight mirrors that reflected far more light than their old mirrors had ever done.

People also noticed how pretty the effect was: German artisans began to make Christmas tree decorations by pouring silver nitrate into glass spheres, followed by ammonia and finally a glucose solution – producing beautiful silver baubles which were exported all over the world, including to Britain.

These days, silvering is done by vacuum deposition, which produces an even more perfect surface, but you just can’t beat the magic of watching the inside of a test tube or a flask turning into a beautiful, shiny mirror.

Speaking of which, according to @MaChemGuy on Twitter, this is the perfect, foolproof, silver mirror method:
° Place 5 cm³ 0.1 mol dm⁻³ AgNO₃(aq) in a test tube.
° Add concentrated NH₃ dropwise untill the precipitate dissolves. (About 3 drops.)
° Add a spatula of glucose and dissolve.
° Plunge test-tube into freshly boiled water.

Silver nitrate stains the skin – wear gloves!

One word of warning: be careful with the silver nitrate and wear gloves. Else, like me, you might end up with brown stains on your hands that are still there three days later…


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Are you ok? You look a little flushed.

PrintYesterday was World Toilet Day (yes, really). This is actually an admirable campaign by WaterAid to raise awareness of the fact that one in three people around the world don’t have access to a safe and private toilet. This, of course, leads to unsanitary conditions which results in the spread of infection and disease. You’ve probably never given it a second thought, but loos literally save lives.

portaloo

Has the TARDIS’ replicator function gone funny?

So, with the topic of toilets in mind, I started thinking about chemical loos. If you live in the UK, the name Portaloo ® will probably spring to mind. This has practically become a generic word for a portable toilet, but it is (like Hoover, Sellotape and others) actually a brand name. I’m told that in America they call them porta-pottys or honey-buckets, which I rather like. In any case, all the chemicals and plastic make them seem like modern inventions, surely?

Actually, not at all. The idea of a self-contained, moveable toilet that you can pick up and take from place to place may be newer, but people have been using chemical toilets for hundreds of years. For example after, ahem, ‘business’ had been completed in an an old-fashioned wooden outhouse – basically a tall box built over a hole in the ground – the user would sprinkle a little lye or lime down the hole to help with the smell.

SodiumHydroxide

Don’t get sodium hydroxide on the toilet seat.

Both of these are strongly basic chemicals. Lye is either sodium hydroxide or potassium hydroxide, and lime is calcium oxide. Both mix with water to form extremely corrosive, alkaline solutions and, incidentally, give out a lot of heat in the process. Both are very damaging to skin. These were the days before health and safety; whatever you did, you had to try not to spill it on the seat.

Urea, a key chemical in urine, reacts with strong alkalis in a process known as alkaline hydrolysis. This produces ammonia, which is pretty stinky (if rather tough on the lungs), so if nothing else that helped to cover up other smells. Ammonia also kills some types of bacteria (which is one reason it’s popular in cleaning products). Flies generally don’t like high concentrations of it either, so that’s another plus.

Alkalis also have another effect in that decomposition of human waste is pH dependent; it works better in acidic conditions. Adding lye or lime raises the pH and slows down this decomposition. On top of this (literally) both lime and lye are hygroscopic: they absorb water. This keeps moisture down and allows a solid ‘crust’ to form on the surface of the waste, making it difficult for any volatile, smelly chemicals to escape. Lovely.

Bleach and ammonia could result in a rocket up your...

Bleach and ammonia could result in a rocket up your…

One word of caution: it’s very, very important you don’t try to clean such an outhouse with any kind of bleach. Bleach, which contains sodium hypochlorite, reacts with ammonia to form hydrogen chloride, chlorine gas and chloramine. None of which are good for your health. Even more dramatically (if this is more dramatic than death – you decide) if there’s lots of ammonia you might get liquid hydrazine, which is used in rocket fuels because it’s explosive. Who knew that toilet chemistry could also be rocket science?

But you don’t find buckets of lye in modern chemical toilets (although, apparently, there are still some people out there using it). So what’s in there? At one time, formaldehyde, otherwise known as methanal, was common. You probably recognise it as embalming fluid; the stuff that Damien Hirst floated that shark in. It’s an extremely effective preservative. Firstly, it kills most bacteria and fungi and destroys viruses, and secondly it causes primary amino groups in proteins to cross-link with other nearby nitrogen atoms, denaturing the proteins and preventing them from breaking down.

shark

Don’t worry, this won’t appear in your chemical toilet.

Interestingly, whilst definitely toxic in high concentrations, formaldehyde is a naturally-occuring chemical. It’s found in the bloodstream of animals, including humans, because it’s involved in normal metabolism. It also appears in fruits and vegetables, notably pears, grapes and shiitake mushrooms. The dose, as they say, makes the poison. I mention this because there are certain campaigners out there who insist it must be completely eliminated from everything, something which is entirely unecessary not to mention probably impossible (just for the hell of it, I’m also going to point out here that an average pear contains considerably more formaldehyde than a dose of vaccine).

All that said, because formaldehyde is extremely toxic in high concentrations, and because it can interfere with the breakdown processes in sewage plants (because it destroys bacteria), formaldehyde isn’t used in toilets so much anymore. In fact, many of the mixtures on sale are explicitly labelled “formaldehyde-free”. Modern formulations are enzyme-based and break down waste by biological activity. They are usually still dyed blue (if you work your way though the colour spectrum, it’s probably the least offensive colour), but usually using food-grade dye. As a result, what’s left afterwards is classed as sewage rather than chemical waste, making it easier to deal with.

Toilet twinning So, this has been brief tour around the fascinating world of toilet chemistry. You’d never have guessed there was so much to it, would you? Now, have you considered twinning your toilet?

After Waco: why are fertilisers so dangerous?

Yesterday there was news of a huge fertiliser explosion in the town of West, near Waco, Texas and as I write the search for survivors is ongoing.  It’s a dreadful tragedy:  the blast all but destroyed a school and a nursing home a few hundred metres away, and dozens of homes were also levelled.  More than 160 people have been injured and so far twelve have been found dead.

ammonium nitrateAt the moment the full details are still unknown.  Fertilisers have long been associated with explosives, and terrorists have been known to use fertiliser bombs (something I shall not be discussing in more detail for fear the men in dark suits might come knocking), although it seems that there’s no indication of malicious intent in this case.  Obviously factories make fertiliser all over the world, and they don’t all blow up on a regular basis, so clearly something went very wrong at 8pm local time on the 17th of April.

So why is fertiliser such potentially dangerous stuff?  Can we make it safer?

First of all, we should probably clarify what we mean by ‘fertiliser‘ (or fertilizer, for our American cousins).  Actually the clue is in the name; it’s something which makes the soil more fertile.  In essence, anything that’s added to the soil to supply one or more of the nutrients that plants need.  In particular, most fertilisers supply nitrogen.  If you were paying attention at school, you’ll remember that most of the solid stuff in plants actually comes from the air in the form of carbon dioxide (see that wooden table over there? A plant made most of that out of air. Air. How cool is that?)

However, just like us, plants also need to make protein for growth, and to do that they need nitrogen.  Unlike us, they can’t (with a few notable exceptions) get that protein from eating animals or other plants, on account of not having teeth, the ability to move and so on.  Except for triffids and that plant in Little Shop of Horrors obviously.  But good old air is about 80% nitrogen, so surely if they can get the carbon from carbon dioxide from air they can get nitrogen too?

Well, there are a few plants that can do that, but most can’t.  The problem is that the nitrogen in air, N2, has one of the strongest bonds between its atoms.  It’s very difficult to break, which means it doesn’t get involved in chemical reactions very easily.  And since growing is basically one big complicated mix of chemical reactions, plants can’t easily use the nitrogen in the air.  Before we started chucking fertiliser on the soil plants managed of course, because useable forms of nitrogen do get into the soil from natural processes.  But if you want to grow large quantities of crops year after year, you need to provide a bit of a helping hand, and that’s what fertiliser does, whether it comes from a factory or, ahem, the back of a cow.

nitrogenBut, and here’s the thing, it’s that strong, triple, bond in N2 that makes fertilisers potentially explosive.  Because if it takes a lot of energy to break those bonds, then exactly the same amount of energy is released when they’re formed.  There is no way around this: energy cannot be created or destroyed, or made to disappear.  (Not in real life, anyway – Harry Potter and co follow different rules.  But they’re not real.  Sorry.)

Why do things explode?  Essentially an explosion occurs when a chemical reaction produces lots of hot gases, very quickly.  If these gases have nowhere to go, because they’re in an enclosed space, they put immense pressure on their immediate surroundings as they rapidly expand.  Ultimately those surrounding are apt to give way, with a bang. (High explosives, like dynamite and TNT, are a little different – but fertilisers aren’t high explosives, so we’ll save that topic for another day.)

ParticleTheoryCompounds that contain nitrogen have the potential to produce nitrogen gas.  Gases take up a lot more space than solids because their particles are further apart and, as I’ve already mentioned, when that hugely strong nitrogen triple bond forms lots of energy is released.  So there you are, hot (that’s the energy bit) gas.  Lots of it.  Surround it with walls – say in a container in a factory – and you have the potential for an explosion.

The fertiliser in this case appears to have been ammonium nitrate.  This is made by reacting ammonia (if you remember, Fritz Haber figured out how to produce that) with nitric acid.  Ammonium nitrate’s chemical formula is NH4NO3 – so plenty of nitrogen there.  In fact when ammonium nitrate decomposes it forms water vapour, nitrogen gas and oxygen gas (via some nitrous oxide, aka laughing gas, along the way).  Lots of gases.  Lots of heat.

The factory also contained lots of anhydrous ammonia.  Not especially surprising this, since you need ammonia to make ammonium nitrate – this was a fertiliser factory.  Anhydrous just means ‘no water’, in other words pure ammonia, NH3.  The boiling point of pure ammonia is -33 oC, so you have a bit of a problem right there if your cooling systems fail; it will quickly turn into vapour at room temperature.  This vapour is pretty nasty.  You know that smell when you use hair dye or perming solution (if you’re still in the 80s)?  That.  Times a hundred.  It’s toxic and corrosive (it poisons you while damaging your lungs), and environmentally damaging.  Oh yes, and flammable.  Not as flammable as say, petrol, but flammable enough.

Reports are that there was a fire at the plant before the explosion, so it looks as though the ammonia might have caught fire.  Ammonium nitrate isn’t easy to ignite, but if the fire is contained and it’s exposed to sustained heat it’ll start reacting.  It decomposes at about 210 oC and once it’s started it’s very difficult to stop, because the reaction gives out a lot of heat which causes the surrounding material to react, and so on in a catastrophic spiral – something chemists call a runaway reaction – ultimately leading to detonation.

So fertilisers are potentially dangerous because they contain nitrogen in a more reactive form, which plants can use.  There’s nothing you can do to make fertilisers explosion-proof.  You can’t say, put additives in to make them less explosive.  It’s in their nature.  Take away their explosiveness and you take away their ability to act as fertilisers.

Factories, though, should be following detailed safety procedures and have numerous protective backup systems to prevent disasters like this.  We don’t yet know what went wrong here, but let’s hope some serious lessons are learned.