8/09/2007

Some Things Are Just Better Without Context

Um

I, uh... what? Last time I checked, something that's 102% pure... isn't pure. Rookie mistake, guys. (Yes, I'm sure there's a perfectly reasonable explanation for this, but c'mon. Citric acid doesn't have 102% of itself anywhere. Ever.)

Also, we hit (half of a tenth of) A MILLION visitors this week! Thanks for reading our blog, guys.

28 comments:

allan said...

could be they're reporting the empirically determined range including whatever experimental error gave 102%

milo said...

Whoa... only 5000 views? Oh wait.... Congratulations!

Paul said...

Dean Toste considers 102% purity too low.

Ryan said...

if it said 100.5% ± 1.5% I still wouldn't feel any safer...

Greg the Chemist said...

Good catch! I actually chuckled out loud about this post.

mevans said...

Half of a tenth of a million is 50,000, right? Or am I just confused?

Excimer said...

mevans-

both. :-P

Uncle Al said...

Get the lot analysis. Don't feel cheated if Aldrich charges extra for anything over 100%. Those New Jersey warning stickers cost money.

In the days of good Doctor Bader a 100 gm bottle invariably contained slightly more than 100 grams. My experience today is that Aldrich is actively regressing toward the mean stated contents averaged over 40+ years.

Hap said...

Maybe they just added some extra, in case there is some antimatter around, so that way, there will be the appropriate mass of citrate left after the explosion (assuming you can find it, I guess).

An alternative explanation would be that the experimental error isn't the only error they're displaying, or their stock market writers were subbing for the usual label writers that week.

Russ said...

Maybe it contains 2% anhydride?

Excimer said...

I think it contains at least 2% of PURE HAPPINESS.

Ψ*Ψ said...

In that case, will you share?

Felix said...

I'll go with the anhydride. only that it had to contain more than two percent.

let's say it's 50% anhydride and 50% monohydride

then you could either say it's 96% pure anhydride or 104% pure monohydride. when you multiply it with the corresponding molar mass and this factor you'd get the correct moles of citric acid

Felix said...
This comment has been removed by the author.
Felix said...

I mean divide the mass by the molar mass and multiply with the factor
n = m / 210g/mol * 1.04 = m / 192g/mol * .96 = m / 201g/mol
either way you like it

Excimer said...

Felix, please read the title of the post, over and over and over, until your head explodes.

milo said...

Felix, do you mean mono-hydrate or hydride (H-)

Ashutosh said...

Maybe the error bars on the 100% are +-2%. They just chose the higher one :D

taitauwai said...

Hey Excimer, why you go buy that for?

Excimer said...

T: I didn't buy it, I just found it in my lab. I don't use it for anything.

milkshake said...

1M aqueous citric acid is good for making Alka Selzer and also for working up acid-senzitive reaction mixtures. Boc and OTBS are safe but non-cyclic acetals and OTES are not. The organic phase needs a aqueous (preferably bicarbonate) wash because citric acid partially extracts into org solvents like EtOAc so it could contaminate your crude product if you don't wash it away.

Anonymous said...

Most chemicals are sold by assay. . .which means that if you get 99-102% citric acid monohydrate you are getting 99-102% of what you paid for (i.e. 495-510 grams of citric acid monohydrate + ???? but I'd guess more water in this case).

I'm shocked that more chemists don't actually know this, and this blog isn't the first time I've had this discussion with people about this issue.

Excimer said...

Okay then. If you buy 99.5% toluene from Aldrich, you get 99.5% toluene and 0.5% other stuff- that much makes sense. 1 liter of 99.5% toluene has 5 mL other stuff. Great.

It's when assays that say there is more than 100% of something that I start getting confused. 1 kg of 102% citric acid monohydrate actually contains 20 extra grams of citric acid monohydrate? What is it that allows an assay to say you have more of something than is possible? And why should we trust any assay that gives that as a valid result? Please elaborate- this makes my head spin.

Jeremy said...

Here's something even more confusing: The Aldrich website lists two values for the purity of this compound. It first says greater than or equal to 99.5%. It then lists the 99.0-102.0% shown in the original post.

If additional water leads to a mass-based purity of greater than 100%, where does that 99.0% come from? A non-hydrated form? How can the purity be 99.0% while simultaneously at a level of at least 99.5%?

This seems like a ridiculous way to label something. There must be a better method of clearly conveying the contents of a bottle without using a mathematical impossibility.

milkshake said...

Be quiet, kids. They could have calculated the yields by integrating it in the complex plane. They are Aldrich and they can get away with it.

In my first job, I put several times something like 108% yield in my chemistry presentation (on a crude intermediate) before being advised by my boss to report just a 'quantitative' yield or at least put the number exceeding 100% in parenthesis or quotation marks. He said that here in a capitalistic country we don't have to exceed the 100% yield quota like we used to, in those five-year plan reports.

kiwi said...

citric acid is an awesome DIBAL quench too

Milo said...

What you all might find someday is that it is very profitable to supply various purities of your product. If you have a "standard" purity of say 98%, most people will buy that for average, everyday use. However, if you have a super peacial purity of 99.5+%, you can charge a whole lot more. What you need to ask yourself is: What do I need and what is the batch to batch average purity Aldrich has. In the case of the latter, you will routinely get 99+% with one purification. Getting 99.995% typically is harder (and overkill for most applications).

As an additional point, be careful when interpretting the %. Is it weight/weight? volume/volume or weight/volume (this is seen in certain industries). Pilot plant guys report yields in w/w, it is easier for them. So they routinely get 150-210% yields.

Ashutosh said...

One Mexican technician woman in Carl Djerassi's lab at Syntex used to get the exact same yield every time- 83.6%. Djerassi wondered how on earth this could be so. Then he found out that sometimes when she got a yield above this number, she used to hide some of the product in her drawer to make it 83.6%. Then the next time when she got a yield below this number, she used to add the difference from the previous product to again make it 83.6%. Talk about consistency.