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Offline dan2000

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what is purity?
« on: March 19, 2015, 10:39:05 PM »
This is quite a dumb question to ask...but my googling skills failed me.

When companies like Sigma Ald. sell their stuff, whether its solvents or chemicals, all has purity percentage on them (99%, 99.5% etc)

What are the other 1%,0.5%etc contained in it?

Say, for instance, acetone 99%. What is other 1%? water, methanol, or other stuff arising from its synthesis?

Is it possible that the impurity may affect the reaction? (like say, if u are using organic solvent and cant have water contamination but 1% impurity turns out to be watere etc)

What tests do they use to confirm the purity, and is it possible to know what the impurities are beforehand?

Also, (this is kinda difference from others) stuff like Pt/C 40% : whats the other 60%? If the answer is carbon im just dumb.

Offline Corribus

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Re: what is purity?
« Reply #1 on: March 19, 2015, 11:50:32 PM »
In general, it depends, but most of the time you can find or request specification and analysis sheets that specify the biggest impurities. For companies like Aldrich and Fisher, these are usually available online. If in doubt, you can also call them. They are generally pretty helpful about this kind of thing.

Impurities can obviously make a difference, which is why chemicals are sold in different grades. It all depends on what you're doing with it. For particularly sensitive applications, most solvents are available in anhydrous grade, but for the most sensitive uses, you are better off distilling it yourself over a drying agent and storing under inert atmosphere.
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Offline Borek

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Re: what is purity?
« Reply #2 on: March 20, 2015, 04:10:25 AM »
What are the other 1%,0.5%etc contained in it?

As Corribus wrote - it can be anything.

The higher the reagent grade the more specific the additional information will be.
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Offline Arkcon

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Re: what is purity?
« Reply #3 on: March 20, 2015, 06:54:35 AM »
Often a sigma Aldrich bottle gives the possible impurities, usually giving max levels.  As was said, it either doesn't matter, or the end user uses something better.  This is a very general question, you'll have to be more specific.  For solvents sold in glass bottles on the surface of the planet Earth, there is usually some water contamination.  So that's what you may try to remidiate yourself at point of use, as was said.  For solid salts, there is usually some trace of sodium and or chloride, given that the surface of the Earth is 4/5 a solution of those ions.  They may even list the test they used to determine the impurity (GC, MS, Karl-Fisher.)
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Offline Dan

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Re: what is purity?
« Reply #4 on: March 20, 2015, 07:48:33 AM »
Also, (this is kinda difference from others) stuff like Pt/C 40% : whats the other 60%? If the answer is carbon im just dumb.

Yes, this is a bit different, it means is is 40/60 Pt:C (by mass, i.e. 40% wt Pt on carbon). So yes, the answer is carbon (but it's not dumb to check).
« Last Edit: March 20, 2015, 10:46:46 AM by Dan »
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Offline jeffmoonchop

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Re: what is purity?
« Reply #5 on: May 18, 2015, 09:01:33 AM »
Usually in analysis you want to use the highest grade possible because techniques these days are so sensitive the impurity can definitely affect the data. You might spend ages wondering what a little peak is that you keep getting only to realise its that impurity. In drug formulation analysis its often important to report what impurities are in your product and impurities coming from your solvent or initial standard API can complicate matters.

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