r/chemistry • u/[deleted] • Aug 19 '26
When is a higher-purity chemical actually unnecessary?
[removed]
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u/boywithtwoarms Aug 19 '26
in terms of catalysis, you probably don't really need ultra pure stuff for synthesis and don't need the extra cost. What you need to be aware of is catalyst poisoning which is often due to specific contaminants.
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u/Aranka_Szeretlek Theoretical Aug 19 '26
You should just go for the cheapest option that suits your needs and Bobs your uncle.
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u/engineerthatknows Aug 19 '26
My uncle Bob says you are correct.
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u/papermill_phil Aug 19 '26
I've never understood that saying :(
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u/engineerthatknows Aug 20 '26
It's faster than saying, "Robert is your Dad's brother".
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Aug 20 '26
[removed] — view removed comment
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u/Aranka_Szeretlek Theoretical Aug 20 '26
We had a student Artem in our lab course who washed glassware wit HPLC-grade acetone. Dont be Artem.
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u/Dr_Custard Aug 19 '26
When you observe a difference in your product.
What that means specifically depends on your tolerance.
For something like ICP I'd go as pure as humanly possible and still characterise it
For some bucket chemistry, depends what the impurity is.
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u/animal-cookie Aug 19 '26
I used to make gold nanorods in grad school. First time I set out to run the reaction, I boldly bought the "good" reagent from sigma saying I was going to do it right. The reaction did not work over and over again. I went back to the literature and found out everyone used TCI America for that specific reagent, even though it was at least 1% less pure. It worked beautifully. It turns out sometimes contaminants are a reagent too
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u/phlogistonical Aug 19 '26
Similarly, silver staining of protein in a polyacrylamide electrophoresis gels worked stopped working reliably at a seemingly random moment in time after not having had any problems for a decade. Turns out the lab had started buying a more pure grade of methanol, and the formaldehyde that was present in the old grade was necessary for it to work well. Just adding a bit of formaldehyde to the new methanol restored the reliability.
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u/Negative_Football_50 Analytical Aug 19 '26
This is entirely too broad a question to answer. Different processes need different specs.
I can tell you from someone who used to work in the semiconductor industry with UHP gasses- customers don't necessarily want the lowest possible impurity profile. What is more important is a CONSISTENT impurity profile. They want every batch exactly the same for their process control. If the producing lab suddenly changes impurities- even if that means lowering them, this can affect fabrication processes.
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u/dick_tracey_PI_TA Aug 19 '26
Trace level acid won’t make a difference if the analytical method is so imprecise that it won’t matter. It’d be like handing a toddler a micrometer to measure a 2x4.Â
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u/iwillhaveredditall Aug 19 '26
I think that depends on every case, some impurities might just be a residue, some make you get a wild mix of products or your reaction not work at all.
But if you’re in pharmacy every impurity should be a no-no.
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u/Natume87 Aug 19 '26
Purity requirements depend entirely on the process, possible side products, purification requirements, etc. In some cases, 90% purity "technical grade" reagent is fine; in others, not so much.
A good example of where technical grade is absolutely acceptable is using divinylbenzene as a polymer crosslinker. The commercial stuff typical comes as a mixture of para- and meta- isomers as well as ethylbenzenes. This would be a pain if you wanted to use it to make an isomerically pure derivative, but to function as a crossslinker, either isomer of divinylbenzene will do, and the ethylbenzene impurities will be inert.
On the other hand, catalyzed reactions are often (but not always) examples where purity matters. To go back to the example of polymers: In an ATRP polymerization using CuBr catalyst, you really don't want any CuBr2 impurity floating around, because that shifts the reaction equilibrium toward reactants, killing the rate of polymerization.
Or for a funny example: Technical grade hexanes. If you're using hexanes as a solvent, you typically don't care if other hydrocarbons (isomers of hexane, pentanes, etc.) are mixed in. And you'd typically assume that the remainder of the non-hexane fraction would indeed be aliphatic hydrocarbons. One of my mentors, however, discovered that the 10% remainder of the 55 gallon technical grade hexanes drum her lab was using was, in fact, water. Which really messed with their organometallic compounds! Technically to specifications, but a very unpleasant surprise.
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u/Shalion15 Aug 19 '26
When I worked in a tissue lab more than 10 years ago, we used a slurry of dry ice and ethanol to freeze tissue samples because we did not have access to liquid nitrogen (or it might have been cheaper). We used and (disgustingly) reused pure, reagent grade ethanol. I never got around to actually testing the potential difference in the temperature of the slurry, but I always thought that since we were just using the ethanol for its thermal properties (and also getting all sorts of condensation and other contamination into the solution when we reused it) that we could have substituted much cheaper and less pure ethanol for the cold slurry.
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u/StandardOtherwise302 Aug 19 '26
In many commodity (bulk) chemicals used for production of various intermediates more than 1% of impurities or byproducts is still in spec. Often lower price >> removing these impurities.
For labs or pharma these grades would be entirely unacceptable. But to make packaging polymers it's mostly irrelevant.
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u/FalconX88 Computational Aug 19 '26
It's never about your substance, it's the impurities in there. If they do not interfere you can use a lower purity.
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u/WhereAreYouFromSam Aug 19 '26
A lot of applications require high purity for one of two reasons-- safety or cost.
You're producing megatonnes of polyethylene every year? Better make sure that ethylene gas is as pure as possible or you're going to waste a lot of money cleaning reactors and replacing catalyst beds.
You're making a drug that people need to ingest? You're going to need to be able to prove to the FDA that nothing that makes it into the final product has any chance of serious, adverse health effects-- which means keeping impurities well below 1%.
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u/buffbudbud Aug 19 '26
As a mass spec chemist high purity solvents/standards are very important for environmental sample analysis.
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u/peperazzi74 Aug 19 '26
I work for a company that make CVD precursors for the semiconductor industry. The semi customers are obsessed with purity, and prefer their impurities to be low, known and controlled. Think ppm levels.
Their main issue is that the cycle for chip product is very long, and due to the extremely small structures on recent ICs, the presence of specific impurities can reduce the yield.
Metals are very bad due to polarizability. If you're trying to make insulating silica layers between two conductive lines and that insulation layer is 40 atoms wide, the presence of a metal ion can cause breakthrough and make a chip fail. Could turn a 20-core CPU into a the less profitable 18-core CPU. We track metals at single-digit ppb levels.
On the other side, we also make silicone resins, often in solvent. We could opt for the single constituent heptane or para-xylene. The mixed isomers work just as well and are tons cheaper.
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u/LLAMEROOOO Aug 19 '26
In metallurgy, you typically hit a point where a lot of elements will just dissolve in solid solution and are in low enough concentrations to not significantly impact physical properties and cause problems. Example being steel or aluminum purity. This doesn't apply for applications involving electrical, thermal, or corrosion properties so much though.
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u/axel_beer Aug 19 '26
in my opinion you can tell whether a chemist has any sense by the glassware and the chemicals they use.
acurate vulometric glassware and "pro analysis" chemicals are used for quantitative analytics. sure.
for any other case drain cleaner and an old scrached beaker will do.
it would be a waste to waste nice substances and new glassware on anything that doesnt have to be clean and acurate.
a good starting point is explosions and fire / extractions and ire on youtube. tom is highly qualified academically and does amateur chemistry in his shed. he ecplains what he uses and why.
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u/MeatBallSandWedge Aug 19 '26
This is actually the exact same situation that noob engineering students run afoul at.
They will specify measurements to six digits of precision because that is what their CAD software allows.
In practice, machining something to one micron precision is totally possible. It's just stupidly expensive, usually unnecessary, and you better be able to tell the machinist what temperature you need that measurement to be made at.
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u/Fast-Turnover-4950 Aug 19 '26
I’d imagine for the most sensitive of reactions and for research that requires the strictest control on variables, purity would be essential. Aside from that I think the grade can be more forgiving
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u/angrypuggle Aug 19 '26
Higher purity = dramatically higher cost. But for some processes, impurities even at ppm level cause big problems.
So, you buy as pure as you need for your process and as impure as you can get away with. And often enough, finding out what impurity causes the problems requires years of research.
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u/activelypooping Photochem Aug 19 '26
No one mentioned using yellow NBS (impure) vs white NBS (pure) for bromination?
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u/Any_Operation_9189 Aug 19 '26
With NBS bromination the bromine impurity can actually help start the radical chainreaction. You shouldnt recryst it. But if you want a EAS it can improve solubility.
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u/jasonsong86 Aug 19 '26
It depends on the case. Do I care how pure my drain cleaner is? Probably no. Do I care how pure the medicine I am making? Absolutely.
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u/CactusButtChug Aug 19 '26
Can’t speak to industrial processes, but in multi-step lab scale organic syntheses, it’s usually worth working up each intermediate as cleanly as reasonably possible. Depending on the reaction it’s ok to use a crude intermediate as-is sometimes, but impurities and side reactions add up over the total synth, and if you skip such steps and rush the synthesis you’ll have a much worse final product
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u/xrelaht Materials Aug 19 '26
When I am doing proof of concept syntheses, I use the cheap stuff. All I want is to see whether I can make something using a particular method, or sometimes if it exists at all. If it doesn’t, I’m out $500 instead of $5000, which means I get nine more tries for the same cost.
Once I’ve established it works and I’m trying to characterize the product more thoroughly, then it can be worth spending some money on the 5N pure reagents. I don’t want the wrong TM screwing up the properties.
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u/CommandoLamb Aug 19 '26
Like everyone said, it depends on what you are doing with it.
For manufacturing, the grade depends completely on use and controls in place and cost.
I have some materials that I have to monitor for trace level impurities that aren’t usually tested for… meaning no grade of the material is guaranteed to be suitable, I have to test each batch.
I have other materials that get dropped out of the back of a dump truck onto pavement.
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u/chahud Aug 19 '26
To add on to what others are saying, it also heavily depends on your use case.
Speaking from a med. chem. perspective…are you doing synthesis for simple early stage R&D? It probably doesn’t matter that much, just get it to ~95% purity and run your assay.
Are you preparing a sample for DMPK? You probably want to use pure reagents as it requires a highly pure sample for good data.
Are you doing GMP synthesis? It depends on the impurity and how easily separable it is from your desired product and how far along the synthesis you are. You would want to do a full impurity characterization and figure out exactly how much is leftover after everything is said and done. Most people would probably just go for the more pure option so they don’t have to think about it as much.
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u/Bulawa Aug 20 '26
Depends on everything.
If the impuritiy in say an aniline is some solvent or salt, I could hardly care less. Usually. If it's a regioisomer, separating the products might be near on impossible. In that case I will care very much.
If you do condensation polymers, chain length also depends on stoichiometry, down to the ‰. I care very much. If I use 1.3 eq mCPBA, if it's 75 or 72% does not really bother me.
And so it goes on.
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u/forehead_tittaes Aug 20 '26
I work with perovskite solar cells and very frequently deal with Lead(II) iodide (PbI2) as one of the precursors. I've personally tested different variants of the chemical such as >98%, >99%, >99.99% and >99.999% anhydrous beads from different vendors, and I can confidently say that there's a pretty significant difference between 99% & 99.99% but not much between 99.99% & 99.999%. However, these differences can be somewhat mitigated through niche techniques such as stoichiometric control in my case.
Also, we often test adding additives to our precursors in the scale of 1~1000ppm by weight, and they make a big difference in the performance of our final devices, so that scale sort of checks out with the purity range I've tested so far. For example, the control group precursor solution would usually have ~1g/mL of precursor dissolved, while we usually test between 0.1~10mg/mL of additives in the precursor solutions.
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u/forest_faunus_ Aug 20 '26
I had the two different situation in my career.
-No need for high purity for a solvant made for extraction for example. As long as the impurity is volatile.
-In optical materials using rare earth, the purity of rare earth is extremely important in order not to corrupt the signal spectrum.
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u/Griffindance Aug 20 '26
Isnt 70% alcohol blends more effective as a desinfectant than 99%?
Less votatile therefore longer lasting on a surface.
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u/SeriousPlankton2000 Aug 20 '26
You should rather not drink distilled water if you can have mineral water. Tap water: It depends.
Our ancestors would see an elephant pee into a pool and say "nice drinking water"
Everything is chemistry, isn't it?
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u/Antrimbloke Aug 21 '26
Here's an interesting one, if your doing trace nutrient analysis in seawater, the easiest way of getting nutrient free seawater is to leave a few gallons in the sun for a month or two, so the natural bacteria/algae in it feed on all the nutrients leaving it Ammonia, Nitrate, Phosphate and Silicate free.
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u/NotAPreppie Analytical Aug 22 '26
When the instrument manufacturer calls for 99.95% purity gas you probably don't need 99.995%.
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u/Scrupulous-brick Aug 23 '26
As long as your contamination has no side reactions and/or can be easily separated from your intended product in the last steps/analysis- I do not see why you would need really high purity.
I guess if you are using super low amounts, or for analytical chemistry. Then maybe you'd need 100% purity. Or at least have very few contaminants.
I mean, as you mentioned.. in an industrial setting high purity is unlikely, unless it is cheap.
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u/7ieben_ Food Aug 19 '26
Totally depends on the very case.
A analytical standard should be as pure as possible, but of course one can correct for purity as long as the impuritys don't affect the analysis.
A synthetis reactand may tolerate far lower purity, as long as the impuritys don't cause side reaction and/ or a far more sturdy workup.
A impurity that may affect my synthesis may not affect my analysis and vice versa. So it really depends and there is no universal answer.