Chemists, how is a purity of >100% achieved?

[NeoPTLD]

I can't see how it is logically possible, but some chemicals are chemicals have a range of asasy where upper range exceeds 100%.

Like this

Anyone know the reasoning behind >100% assay?

 

[TerryMathews]

Not sure on the technical explnation, but probably in the same manner that you can get gasoline with an octane rating higher than 100.

 

[Peter]

Octane aren't a percentage, so why not have a value >100 there?

 

[TerryMathews]

It is, or more properly it was. It measured the percentage of octane in the gasoline. Now it's just a measurement.

 

[Peter]

Yes and no - the rating is comparing the combustion of the given fuel with isooctane. There are fuels that burn "better" than pure isooctane, hence the comparison factor goes beyond 1.0 or 100 percent as soon as you stop making your fuel just from natural gasoline.

Mathematically, you can of course achieve 150 percent in a comparison, but there is no such thing as 100.0001 percent purity of something. It's either entirely homogenous which is 100 percent, or something below that.

I guess the ratings given on those bottles are also in comparison to some reference quality, not the absolute purity of the contained chemical.

 

[bendixG15]

Only if the definition of 100% purity is flawed.

 

[NickE]

Probably a reflection of variation in the measurement method, so an analysis of 100% of the substance could give a result of 98-102%.

 

[gsellis]

Originally posted by: Peter
Yes and no - the rating is comparing the combustion of the given fuel with isooctane. There are fuels that burn "better" than pure isooctane, hence the comparison factor goes beyond 1.0 or 100 percent as soon as you stop making your fuel just from natural gasoline.

Mathematically, you can of course achieve 150 percent in a comparison, but there is no such thing as 100.0001 percent purity of something. It's either entirely homogenous which is 100 percent, or something below that.

I guess the ratings given on those bottles are also in comparison to some reference quality, not the absolute purity of the contained chemical.

And the 0 is compared to heptane.

Otherwise, anything more than 100% pure is an abuse of statistics.

 

[NickE]

Originally posted by: gsellis
Otherwise, anything more than 100% pure is an abuse of statistics.

Like sportsmen etc. that all give 110% - clever trick! For some reason that annoys me a lot more than it probably should.

 

[Mark R]

It may be because the substance is anhydrous - and there may be great difficulty in driving out all the water - hence a significant range of variability of water content in both the sample under test, and a reference sample.

 

[AbsolutDealage]

Originally posted by: Mark R
It may be because the substance is anhydrous - and there may be great difficulty in driving out all the water - hence a significant range of variability of water content in both the sample under test, and a reference sample.

That would be my assumption as well.

 

[Calin]

Let's assume we are talking about sulfuric acid (H2SO4) with excess of SO3 (dissolved). When you add water (dilute it), you will obtain pure (100%) sulfuric acid.
DISCLAIMER: do not add water in any kind of acid - when needed to dilute acid, add acid to water. Doing otherwise may be bad for your health (flying droplets of acid).

Calin

 

[Eraserface]

"Full strength" is not physical 100%

 

[JediJeb]

The 98%-102% reflects the margin of error in the testing method. If you weigh something to 1 gram and your balance is accurate to +/- 2% then you have to assume the weight is between 0.98 and 1.02 grams because that is as accurate as you can give the number. If your balance is accurate to +/- .001 grams then something that weighs 1 gram could be expressed as being from 0.999 - 1.001 grams in weight. In analytical chemistry you can never say something has a value with 100% accuracy because it just can't be measured without some degree of inaccuracy.