Reef Chemistry Question of the Day #253 Purity, what does it mean?

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #253

Purity means different things to different people. We use products that have purity grades (such as pharmaceutical grade (USP), food grade (FCC), reagent grade), some that give numbers (e.g., 99%) and many that say nothing.

For example, let's take one supplier of one chemical, potassium nitrate, and see how many different "grades" are available. When I pulled up the link, there were 17!

https://www.sigmaaldrich.com/catalog/substance/potassiumnitrate10110775779111?lang=en&region=US

I'd like to start a discussion of what these mean and how we might interpret them, and let's start with a seemingly reasonable question.

Which of the following has the highest assurance of purity, meaning the least numerical potential for chemicals that are not on the label.

A. Potassium nitrate, minimum purity 99%
B. Potassium nitrate, minimum purity 99.99%
C. Potassium nitrate, 100%
D. Potassium nitrate, 100.0%
E. Potassium nitrate, 99.5 - 100.5%

I have, in fact seen all of these.

Good luck!



































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[Gareth elliott]

A seems the most reasonable. This might because its the purity of the potassium nitrate i have lol.

Also how is something 100.5% pure? Lol

 

[beaslbob]

Obviously the 99.99% should have the highest potassium nitrate. Everything else could be less than 99.99%

100% could be 96-100%
100.0 could be 99.6-100%

 

[Randy Holmes-Farley]

Also how is something 100.5% pure? Lol

https://www.sigmaaldrich.com/catalog/product/sigald/12648?lang=en&region=US

assay 99.0-100.5%

 

[Cory]

I think b but i dont understand the rest.

 

[Acorral]

I would go with B, not pretty sure about it but it states pretty clearly that at least 99.99% of the product is potassium nitrate...

I suspect the difference in all of them is the method or set of standards to define purity... And how to measure it...

At the end of the day, all those numbers are found by some kind of testing, and all tests have their limitations, so guess statistics and error tolerances are involved with different criterias to set some kind of knlwn standard to define purity...

 

[Dburr1014]

B, the others don't say they have a minimum, just that it's potassium and that label can have other meanings.
Kinda like a fruit roll up package saying 100% real fruit, it may contain a fruit you didn't think would be in the product.

 

[Randy Holmes-Farley]

Without getting into the answer to the specific question yet, let's talk about what a product means when it says 100%.

Here's a product, for example:

https://www.amazon.com/Superior-Lab...rd_wg=MfvKu&psc=1&refRID=9F49V8PNB05TTXEDXC0F

It has a big 100% Pure right on the label.

Sounds great!!!! But what does that mean?

To them it means:

• ABSOLUTE PURITY - ZERO SYNTHETIC ADDITIVES, FILLERS, HEAVY METALS, ADULTERANTS - You will never see unneccessary additives such as stearates, dioxides, glycerides or fillers other brands use to make manufacturing cheaper, faster and easier. In addition we test all ingredients and products for heavy metals pollution and harmful bacteria common in plant and ingredient material.

Ah, it means no "unnecessary" additives. But apparently, there are some "necessary" additives:

It says elsewhere:

Ingredients
200mg Elemental Magnesium (from 1,250mg Magnesium Citrate), Plant Cellulose (vegetable capsule), natural rice flour.

Ah, so it is not 100% magnesium citrate. It is magnesium citrate plus cellulose and rice flour.

They have apparently made the decision that you care about excluding stearates (salts of the fatty acid stearic acid), but that rice flour is OK.

Curiously, rice flour is known to contain stearic acid (as would nearly any animal or vegetable product):

http://www.foodcomp.dk/v7/fcdb_details.asp?FoodId=0222

So does it contain stearates or not? Sure it does. They just didn't intentionally add any.

That is the crux of this particular 100% claim:

There is no analysis that shows that there is not any calcium present, or any of many other possible impurities, it simply means they added only magnesium citrate (plus some other things which you are supposed to assume are unimportant).

While it never makes the specific claim anywhere I could find, this product almost certainly meets USP and/or FCC purity guidelines since it is an intended food/drug and is made in an FDA approved facility.

What does that imply?

A lot more than the label does.

The USP requirements for magnesium citrate are listed here:

http://www.pharmacopeia.cn/v29240/usp29nf24s0_m46730.html

some that might be of specific interest to a reefer are:

Heavy metals: limit is 50 ug/g
Iron: limit is 200 ug/g
calcium: limit is 1%
loss on drying: limit is 2%
arsenic: limit is 3 ug/g

So there are limits here based on the presumed grade used, and this grade certainly doesn't imply 100% exactly. It could have 1% calcium, could have 2% water, definitely also has rice flour and cellulose in unknown quantities.

So in the end, the 100% purity claim means close to nothing about actual purity. Given the rice flour and cellulose additives, it might only be 75% magnesium citrate. We have no evidence otherwise. It is the implied USP grading that gives people assurance of what is actually in the product, at least in terms of the specific tests that USP requires.

 

[rkpetersen]

B. It's all about the decimals, baby...

 

[Cory]

100% pure seems like false advertising then, no?

 

[MnFish1]

The answer is B - using the rule of 9's.

 

[Randy Holmes-Farley]

100% pure seems like false advertising then, no?

There is enough wiggle room in what it means that they likely wouldn't get in trouble, but it certainly seems misleading to me to say 100% pure magnesium citrate and then list more ingredients.

 

[Cory]

Perhaps the proper word is misleading?

 

[Randy Holmes-Farley]

Continuing the purity discussion, let's look at what a claim like

Potassium nitrate, 99.99%

actually means.

It actually can mean somewhat different things when written in slightly different ways. Perhaps surprisingly, it may not mean the product is 99.99% pure potassium nitrate.

Let's look at an example from my original link:

https://www.sigmaaldrich.com/catalog/substance/potassiumnitrate10110775779111?lang=en&region=US

Potassium nitrate 99.995

https://www.sigmaaldrich.com/catalog/product/mm/105065?lang=en&region=US

This is actually a very extensively characterized, highly purified material. It lists about 30 specific impurities that have set limits.

1. One claim there says "Purity (metallic): ≥ 99.995 %"

That's presumably the basis for the 99.995 in the label name. What that means is that they tested for potassium and found the cationic portion of the product (potassium and all other cations they tested) to be more than 99.995% potassium (presumably this is by weight, but they do not specify).

It is not entirely clear whether this means that

1. They tested potassium very accurately and found that, compared to the theoretical amount, the potassium is within 0.005% of the theoretical level for pure potassium nitrate (which could be misleading if the nitrate part is not equally precise since, if the molecular weight of the impurity in place of nitrate (say, chloride) was lower than that of nitrate, the potassium might actually be present at higher than the theoretical level)

or

2. They tested for potassium and most other cations that might be there, and found that the others added up to less than 0.005% of the total cations.

I strongly expect it is the latter.

This designation says nothing about the nitrate part. It might be all potassium chloride and get the same rating "Purity (metallic): ≥ 99.995 %". It could also be 50% water and get that claim .

2. The next claim down says

Assay (acidimetric): ≥ 99.0 %

This is an assay for the nitrate part of the material. It converts the nitrate to ammonia then titrates the ammonia with acid to count it. Note that the nitrate assay is much less precise than the potassium part of the assay. It suggests that the material might have almost 1% something else in it. Could be water, for example. That wouldn't interfere with the potassium purity either, if it was done by the second method I suggested above. In fact, I see no other assay for moisture or loss on drying. So it seems possible that the material might theoretically have up to 1% water in it.

3. In the list of possible impurities is rubidium. It's a fairly uncommon material. In the ocean it is naturally present at about 0.1 ppm (100 ug/L). But in the list of possible impurities in our potassium nitrate sample, it has the highest possible allowance (up to 30 ppm). Why would that be?

The reason is the rubidium looks very similar, chemically, to potassium, and so many of the things you would do to purify potassium nitrate (say, recrystallization) will not be as effective at excluding rubidium as nearly every other ion. So as this material is more and more highly purified, the relative amount of rubidium compared to, say calcium (at <0.1 ppm), likely rises significantly.

In summary, while this material is very highly pure, one might want to look at the actual specifications to see what 99.995% actually means. It does not appear to mean that the actual product must have at least 99.995% of the theoretical amount of potassium nitrate in it.

We'll continue the discussion with other sorts of purity labals soon...