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In Breaking Bad, Walter White can cook $99.1\%$ pure meth. In general what does it mean for a substance to be pure? In this case, what exactly does it mean for meth to be pure? Does higher purity mean less unhealthy? More addictive? Giving more euphoria for the user?

For comparison, is the purity of prescribed methamphetamine or methylphenidate to people with ADHD or narcolepsy close to $100\%$? My guess is that purity is how close Walt's product is with legal meth.

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When a compound is "pure" it means the contents of that compound are exactly what we claim them to be. If something is $99\%$ pure, that means $99\%$ of it is the expected material (in this case, meth), and 1% is other non-meth compounds.

Purity matters in chemistry and biology because the other compounds can be very nasty. In theory, if you had a bag of 100% pure meth, and a bag of $50\%$ pure meth, you could simply double the dose with the second bag and end up with the same amount of methamphetamine in your system. However, you would also have a double dose of $50\%$ "other" stuff.

Sometimes "other" stuff isn't a big deal. It all depends what that "other" stuff is. I don't know much about the methamphetamine trade, but the effect of purity is a big deal in the ethanol world. Moonshiners know to discard the "head" and the "tail" of the distilation, the first and last parts of the distilation, because they contain fusel oils (which taste really bad), and methanol (which can make you go blind in large enough dosages). They want very "pure" ethanol.

To you talk about the purity of prescription drugs, typically what matters more for the drug companies is that they fully understand what "other" things are in the pills or injections, and they are confident they are safe. That's why, when you look at many pill vials they will list their "inactive ingredients." They want to make sure you are very confident that the only things you are putting in your body are the "right" things. Going back to the alcohol industry, "pure" ethanol is actually $95.6\%$ ethanol by volume, and $4.4\%$ water. That is the limit for how "pure" ethanol can get with fractional distillation. To have ethanol in any higher percentages than that you have to go to exotic processes to create anhydrous alcohols. However, that is deemed "pure" because we still know what's in it. We know its $95.6\%$ ethanol and $4.4\%$ water.

Of course, $100\%$ purity isn't always the desire. There are myriad alcoholic spirits out there. All of them are sold at 80 proof ($40\%$ alcohol by volume), so they contain the same amount of ethanol. Its the "other" stuff that makes each spirit unique. You pay a great deal of money for the particular impurities that make a great well aged scotch. You pay very little for the impurities in a cheap tequila that leave your head pounding the next morning.

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  • $\begingroup$ Except that Absolut (and Finlandia?) are also sold at $50~\%$ and absynthe goes all the way up to $90~\%$. And I love how we basically crafted core points of our answers around the same general ideas ;) $\endgroup$ – Jan Oct 16 '16 at 0:55
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    $\begingroup$ @BCLC There's certainly a lot to that side of things that I don't fully understand (and will likely choose not to understand). However, I think the cheap Tequila example may be the closest to something I can relate to. Cheap Tequila (silver/gold) is mixed with 50% "other" alcohols (it's only 50% agave). The other alcohols tend to be cheaper, and have impurities like fusel alcohols. So, while you want the toxicity of ethanol, the hangover is actually mostly caused by the fusel alcohols, even though they make up a small part of the liquid. I can only assume the solvents used in making... $\endgroup$ – Cort Ammon Oct 16 '16 at 1:27
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    $\begingroup$ ... meth are particularly nasty, so having some small amount of them left over (impurities) would markedly change the crap you are putting into yourself. To use the Coke Zero analogy, it's like ordering a Coke Zero, and wanting to minimize how much drain-cleaner is in the liquid you drink. $\endgroup$ – Cort Ammon Oct 16 '16 at 1:28
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    $\begingroup$ With pills, also, pretty often the absolute quantity of the active ingredient is tiny, so they make a pill of cornstarch or something else harmless that's physically large enough to handle, and then add just the right dose of the drug. $\endgroup$ – zwol Oct 16 '16 at 19:09
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A pure compound is one that does not have anything else accompanying it — and thus is something entirely unreachable if your detection system is good enough unless you’re going for supercooled $\ce{^3He}$. However, typically lab grade chemicals are sold in purities of ${99+}~\%$ and extra special care is taken so that the remaining impurities do not affect the chemical’s intended use.

After performing a reaction typically a mix of compounds is obtained and some method of purification (e.g. precipitation and filtration, recrystallisation, chromatography) is required to arrive at a pure substance from the reaction mixture. Chemists then routinely record NMR spectra to check for any impurity peaks — and if they did it right, they will only find a water peak and thus deem their compound ‘pure’.

There are two main benefits of using pure compounds, one of which is alluded to above:

  1. More ‘bang for the buck’, i.e. if you have a compound at $95~\%$ and another at $97~\%$, you need to weigh in less of the purer $97~\%$ compound to have the same amount in your flask

  2. Less unintended side reactions from impurities present.

Transferred onto the crystal meth case, number 1 tells us that drug users would need less of the substance to reach the same effect, while 2 tells us that there is less of a potential for side effects. Note however that a higher purity compound can cause worse side-effects if it has impurities present that are responsible for worse side-effects. So $40~\%$ vodka bought from the store ($60~\%$ ‘impurities of the alcohol’; mostly water) won’t do anything aside from getting you drunk but if you run across $45~\%$ ethanol that contains $5~\%$ methanol as an impurity, it is much more harmful although it is purer.

For comparison, most medicines are, by purity standards, very impure — but it of course depends on the method of administration. Most of the time, however, the active ingredient is given in percent on the package; the drug I just checked had only $1~\%$ active ingredient. So technically, going by the above definition $99~\%$ impurities. Of course, in pharmaceuticals all the ‘impurities’ will either help or just be there to dilute the active ingredients. For example, many surface sprays have to contain a base as most active ingredients are added as hydrochloric acid salts. But going into too much detail here would not fit within this margin.

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    $\begingroup$ "1." really isn't a concern when you're at 99% vs 97%. The main benefit of using purer compounds is to avoid side reactions. Moreover, medications are typically more impure, but the kinds of impurities present are controlled to be a lot less harmful (ie, free of heavy metal catalysts or toxic side products). Moreover, if a medication has 1% active ingredient it means the rest of it are usually binding compounds that prevent product decay, assist drug delivery, prevent toxicity of overdose, as well as fillers. Those substances are controlled for during production and are not impurities. $\endgroup$ – user7798 Oct 16 '16 at 3:54
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    $\begingroup$ @chell You are right about your first point (maybe I should have taken a more extreme example) and the second point should be in my answer somewhere. On the remainder of your comment: of course, drugs are typically manufactured to have the least harmful side-products in them. But from a purely analytical point of view anything that is not ‘the ingredient’ is an impurity whether it serves a purpose or not. Of course, the contents of a pill or a cream are chosen by the pharmaceutical company to have exactly the right dose and smallest size possible, but that’s almost a side point. $\endgroup$ – Jan Oct 17 '16 at 21:23
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Organic reactions (reactions involving fairly complex compounds of carbon) like the ones used in drug synthesis, are notoriously complicated and almost always produce various side-products and impurities. The matter is of real concern because almost all such syntheses involve several steps, where A is treated to form B and B is then treated to form C, and then C is treated to form D... (you get the picture.) Any impurities or byproducts produced in any or all steps must be removed to insure product purity, which rare;y happens in illicit syntheses. So important is this purification that Isolation & Purification is a science in its own right in organic chemistry, and very often more difficult that the chemical reactions themselves.

The FDA maintains extremely stringent controls on drugs manufactured for humans and even animals, and every batch manufactured by pharmaceutical makers, whether in the US or abroad for export to the US, whether brand-name or generic, must be exhaustively tested in multiple ways to ensure it passes requirements for purity, content, stability, and even physical state. Not only that, but the purity and identity of all chemicals used in the synthesis must likewise be tested and proven as to purity and identity.

The chemistry in "Breaking Bad" is quite accurate as far as it goes. A common purification technique for drugs of the methamphetamine family is to treat the crude, unpurified product in such a way that the meth forms crystals, which can then be collected and then washed free of the grossest impurities. The result is "crystal meth," which is still too contaminated to ever be okayed by the FDA even if it were legal, but is much cleaner and therefore more potent than the usual slop.

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