01Buyer guide

Quercetin 95%: 95% of what, exactly — the dihydrate or the molecule?

Ten percentage points of quercetin hide inside a question nobody asks the supplier: which basis is the assay on?

At a glance

The short answer

Quercetin dihydrate contains 89.35% quercetin by mass — the remaining 10.65% is water of crystallisation — so "95% quercetin" means 95.0% if the basis is the dihydrate and 84.9% if the basis is the molecule. On a one-tonne order that is a 101 kg difference in the compound you are buying, and nothing on a typical certificate of analysis tells you which basis was used. The trap is sharpened by the fact that both the European and United States pharmacopoeial reference standards for quercetin are the dihydrate, so an assay run against them naturally lands on the dihydrate basis unless someone deliberately corrects it.

Anhydrous quercetin: C₁₅H₁₀O₇, M_r 302.24, CAS 117-39-5
Quercetin dihydrate: C₁₅H₁₀O₇·2H₂O, M_r 338.27, CAS 6151-25-3
The mass fraction: 302.24 ÷ 338.27 = 89.35% quercetin; 10.65% is water
What 95% becomes: 0.95 × 0.8935 = 84.9% anhydrous quercetin. On 1,000 kg: 849 kg rather than 950 kg
Why it happens by default: Ph. Eur. Quercetin dihydrate CRS and the USP Quercetin RS are both the dihydrate (CAS 6151-25-3). Quantifying against a dihydrate standard reports a dihydrate-basis result
The other half of the question: Commercial quercetin is made by acid hydrolysis of rutin from Styphnolobium japonicum (Sophora) buds. The related substances to look for are unhydrolysed rutin and partially hydrolysed isoquercitrin — not water

The arithmetic, done once so you never have to trust it again

Quercetin is C₁₅H₁₀O₇ with a relative molecular mass of 302.24. The commercially stable crystalline form is the dihydrate, C₁₅H₁₀O₇·2H₂O, which adds two water molecules at 18.015 each and arrives at 338.27. Divide: 302.24 ÷ 338.27 = 0.8935. So a gram of perfectly pure quercetin dihydrate contains 893.5 mg of quercetin and 106.5 mg of water, permanently, by construction — not because it is wet, but because the water sits in the crystal lattice.

Now apply that to a certificate. "Quercetin ≥ 95.0% by HPLC" against a quercetin dihydrate reference standard, with the result expressed as the dihydrate, means 95.0 g of dihydrate per 100 g of powder, which is 84.9 g of quercetin. The same certificate wording, with the result corrected to the anhydrous molecule, means 95.0 g of quercetin. The difference is 10.1 percentage points and the certificate looks identical either way.

Scale it to a purchase. One tonne at 95% dihydrate basis delivers 849 kg of quercetin. One tonne at 95% anhydrous basis delivers 950 kg. If two suppliers quote the same price for "95% quercetin" and one is on each basis, the second is 11.9% cheaper per kilogram of actual quercetin, and no negotiation happened at all.

Why the default runs against the buyer

This is not usually deception. It is what happens when nobody specifies. The pharmacopoeial reference standards for quercetin — Ph. Eur. Quercetin dihydrate CRS, and the USP Quercetin RS, both carrying CAS 6151-25-3 — are the dihydrate. A laboratory that weighs out that standard, builds a calibration curve and reports against it is reporting on the dihydrate basis by default. Getting an anhydrous-basis number requires someone to notice and divide.

Reference-standard certificates make this manageable if you read them: they typically state the absolute content for both the anhydrous molecule and the dihydrate, precisely because the two differ. The information is available. It is simply not carried forward onto most commercial COAs, which report a bare percentage and a method name.

The honest position for a supplier is to state the basis on the certificate line rather than in a footnote, and to publish a water figure separately so the buyer can check the arithmetic themselves. If assay is 95% and Karl Fischer water is around 10.5%, the two numbers are consistent with a dihydrate-basis result and you have your answer without asking anyone.

What actually varies between suppliers — and it is not the water

Commercial quercetin is not extracted as quercetin. It is made by acid hydrolysis of rutin, itself extracted from the flower buds of Styphnolobium japonicum (formerly Sophora japonica). Rutin is quercetin-3-O-rutinoside; hydrolysis cleaves the disaccharide and releases the aglycone. The process-related substances that follow from this are therefore glycosidic: unreacted rutin, and isoquercitrin (quercetin-3-O-glucoside) from partial hydrolysis where only the rhamnose came off.

These are the peaks worth looking at, and they behave well on an ordinary reversed-phase HPLC — rutin and isoquercitrin are considerably more polar than quercetin and elute earlier, cleanly resolved. Which means the chromatogram tells you the completeness of the hydrolysis, and the completeness of the hydrolysis is the actual quality difference between one supplier's 95% and another's. Ask for the chromatogram, not just the number.

The second real variable is the botanical and geographic source of the rutin, because that is what determines the pesticide and heavy-metal load you inherit. This is a supplier-qualification question rather than an assay question, and it belongs in the audit rather than the specification — but it is the one that a purely chemical specification will never catch.

The specification line that closes the gap

Write the basis into the assay line: "Quercetin ≥ 95.0% (HPLC, calculated on the anhydrous basis, expressed as quercetin C₁₅H₁₀O₇, M_r 302.24)". Add a separate water determination: "Water (Karl Fischer): report result" — you want the number reported rather than merely limited, because it is how you verify the assay basis independently.

Name the reference standard and require the correction. "Quantified against Ph. Eur. Quercetin dihydrate CRS, result corrected for the water content of the standard" removes the single most common source of a 10-point discrepancy between two honest laboratories.

Then add the related substances: "Rutin ≤ x%, isoquercitrin ≤ x%, by HPLC", with the chromatogram supplied. And if you are buying to a target mass of quercetin rather than a target mass of powder, say so in the purchase order — "1,000 kg calculated as anhydrous quercetin" is a different order from "1,000 kg of 95% quercetin", and the difference is worth roughly 101 kg.

For a case where the hidden factor is a counter-ion rather than water, the same reasoning is worked through in the glucosamine salt-factor guide.

Frequently asked

Is quercetin dihydrate the same as quercetin?

Chemically it is quercetin plus two molecules of water held in the crystal lattice. By mass, quercetin dihydrate (Mr 338.27, CAS 6151-25-3) is 89.35% quercetin (Mr 302.24, CAS 117-39-5) and 10.65% water. That water is not removed by ordinary drying to constant weight at ambient conditions, because it is structural rather than surface moisture.

Does 95% quercetin mean 95% of the molecule?

Only if the certificate says the result is on the anhydrous basis. If the assay was run and reported against a quercetin dihydrate reference standard without correction — which is the default, since both the Ph. Eur. and USP reference standards are the dihydrate — then 95% on the dihydrate basis is 84.9% expressed as quercetin. On 1,000 kg that is a 101 kg difference.

What impurities should a quercetin certificate show?

Commercial quercetin is produced by acid hydrolysis of rutin from Styphnolobium japonicum (Sophora) buds, so the process-related substances are glycosidic: unreacted rutin (quercetin-3-O-rutinoside) and isoquercitrin (quercetin-3-O-glucoside) from partial hydrolysis. Both are more polar than quercetin and resolve cleanly on reversed-phase HPLC, so the chromatogram shows how complete the hydrolysis was.

How do I check the assay basis without asking the supplier?

Compare the assay figure against the water figure. If assay is about 95% and Karl Fischer water is about 10.5%, the two are consistent with a dihydrate-basis result. If assay is 95% and water is below 1%, the material is essentially anhydrous quercetin and the figure means what you assumed. This is why a specification should require the water content to be reported, not merely capped.

Request a quote

This page describes raw-material sourcing and specification. It is not a health claim and not medical advice. Last updated 2026-08-07.