Chenodeoxycholic Acid Specification: A Quick-Reference
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Chenodeoxycholic acid (CDCA) has its own European Pharmacopoeia monograph, numbered 1189, separate from the UDCA monograph (1275). It is bought and sold as a monographed substance in its own right, not only as an impurity or a starting material.
This is a reference sheet: every parameter the monograph sets, its limit, and the four figures that differ from UDCA — the ones most often carried across between the two products by mistake. The most important single fact is that the two monographs are not interchangeable, and a specification table copied from one to the other is a documented risk.
Values are from the Ph. Eur. monograph for chenodeoxycholic acid (1189). Where a figure is deliberately unstated, it is flagged rather than filled in.
The monograph parameters in one table
| Parameter | Ph. Eur. requirement |
|---|---|
| Chemical name | 3α,7α-dihydroxy-5β-cholan-24-oic acid |
| CAS number | 474-25-9 |
| Appearance | White or almost white powder |
| Content (dried substance) | 99.0% to 101.0% |
| Specific optical rotation | +11.0° to +13.0° (dried substance) |
| Related substances — UDCA (impurity A) | Not more than 1% |
| Related substances — cholic acid (impurity B) | Not more than 0.5% |
| Related substances — lithocholic acid (impurity C) | Not more than 0.1% |
| Any other impurity | Not more than 0.25% |
| Loss on drying | Not more than 1.5% |
| Sulphated ash | Not more than 0.1% |
| Polymorphism | Stated in the monograph |
That is the whole panel. It is short, and every line matters, because a CDCA certificate that omits one of them is incomplete against the monograph rather than simply abbreviated.
Content, and why the basis matters
Content is specified as 99.0% to 101.0%, calculated on the dried substance. The range and the basis are two separate requirements, and both show up as errors on real certificates.
The basis is the one that causes more trouble. Assay on the dried substance means the water content is measured separately and excluded before the result is calculated. A certificate that reports content without stating the basis has given a number whose reference point is unclear — and a result calculated on the undried substance will sit systematically lower for a hygroscopic powder, which CDCA is.
The tight range is the other point worth noting. A 99.0% to 101.0% window is a content range for a substance, not a purity ceiling that can be met by any material “above 98%”. A specification that quotes an assay as “≥98.0%” is applying a different definition of the acceptance range, and the two are not interchangeable.
Specific optical rotation — the fastest identity check
CDCA is specified at a specific optical rotation of +11.0° to +13.0°, measured on the dried substance in methanol.
This is the parameter that most quickly distinguishes CDCA from UDCA, whose range is +58.0° to +62.0°. The two are one hydroxyl configuration apart at C-7, but that single difference pushes their rotation ranges far apart — enough that a single measurement separates them.
That makes rotation more than a formality. It is a first-line identity test, and for a buyer comparing a CDCA specification against material received, it is one of the most efficient checks available. A CDCA certificate showing a rotation near +60° would be describing a different substance.
Related substances: what the 2022 revision changed
For years, the related-substances test in monograph 1189 was a thin-layer chromatography method. A revised version was published as a draft in 2019 and took effect in January 2022, replacing the TLC test with two liquid-chromatography methods using a differential refractometer.
The practical consequence is a documentation question, and a specific one. If a certificate still cites a TLC related-substances method for CDCA, and does not show a liquid-chromatography result, the certificate is being produced against an obsolete method. That is a checkable, binary question — far more useful than asking a supplier whether they are “compliant”.
The lettered impurities in the CDCA monograph
The CDCA monograph letters its impurities too, and the letters are assigned relative to the CDCA monograph itself — not carried over from UDCA. Position A is where this shows most clearly.
| Letter | Identity in the CDCA monograph | Limit |
|---|---|---|
| A | Ursodeoxycholic acid | Not more than 1% |
| B | Cholic acid | Not more than 0.5% |
| C | Lithocholic acid | Not more than 0.1% |
| Any other | Unspecified | Not more than 0.25% |
The reversal at position A is the point to hold on to: in the UDCA monograph, CDCA is impurity A; in the CDCA monograph, UDCA is impurity A. A specification that says “impurity A” without naming the substance is ambiguous, and the meaning depends entirely on which product’s monograph it comes from. Naming the substance as well as the letter removes the ambiguity in one word.
Loss on drying and sulphated ash
Loss on drying is specified at not more than 1.5%, and sulphated ash at not more than 0.1%.
The loss on drying figure is the clearest evidence that a specification table should not be copied between the two molecules. UDCA’s monograph allows not more than 1.0%; CDCA’s allows 1.5%. Both are correct for their own substance, and a CDCA material released against a 1.0% ceiling would be held to a limit the monograph does not impose.
Sulphated ash, by contrast, happens to match at 0.1% in both monographs — which is exactly the kind of coincidence that makes copied tables look plausible.
Polymorphism, and what the monograph says about it
CDCA is polymorphic, and the monograph states this. That matters more than it first appears.
A different polymorph is not a different substance — it is the same molecule in a different crystal arrangement. But the crystal arrangement affects how the powder behaves: its melting behaviour, its dissolution rate, and its handling under a press or in a formulation step can all shift between forms.
For a buyer, the practical implication is that a supplier should be able to say which crystalline form is produced and how crystallisation, drying and milling are controlled. A specification that reports a compliant assay while saying nothing about form has not answered the question.
The copy-paste trap: CDCA and UDCA side by side
Because the two substances are closely related, their specification tables are easy to merge — and wrong to merge. Setting the comparable parameters side by side shows which lines are safe and which are not.
| Parameter | UDCA (1275) | CDCA (1189) | Interchangeable? |
|---|---|---|---|
| Content (dried substance) | 99.0% to 101.0% | 99.0% to 101.0% | Same |
| Specific optical rotation | +58.0° to +62.0° | +11.0° to +13.0° | No — far apart |
| Loss on drying | Not more than 1.0% | Not more than 1.5% | No |
| Sulphated ash | Not more than 0.1% | Not more than 0.1% | Same |
| Impurity A | CDCA — not more than 1.0% | UDCA — not more than 1% | No — reciprocal |
| Impurity C | Lithocholic acid — 0.1% | Lithocholic acid — 0.1% | Same |
| Polymorphism | — | Stated in the monograph | No |
The two columns agree on content and on two impurity lines, and disagree on rotation, loss on drying, the identity of impurity A, and the polymorphism statement. A table assembled by averaging the two columns will be wrong on more lines than it is right on.
Reporting, identification and qualification thresholds
Separate from the limits above, the general Ph. Eur. monograph for substances for pharmaceutical use sets thresholds that determine when an impurity must be reported, identified and qualified. For CDCA these are based on the maximum daily dose:
- Reporting threshold: above 0.05%
- Identification threshold: above 0.10%
- Qualification threshold: above 0.15%
These answer a different question from the related-substances limits. A peak can sit below a product limit and still cross a reporting threshold, which is why mixing the two frameworks up is a common source of disagreement between a supplier and an incoming-goods laboratory.
Questions buyers ask
What is the CDCA content specification?
99.0% to 101.0%, calculated on the dried substance, under Ph. Eur. monograph 1189.
What is the specific optical rotation of chenodeoxycholic acid?
+11.0° to +13.0°, on the dried substance. This is well below the +58.0° to +62.0° of UDCA, making rotation a fast identity check.
What is the loss on drying limit for CDCA?
Not more than 1.5%. This differs from the UDCA monograph, which allows not more than 1.0%.
What are the related substance limits?
UDCA as impurity A at not more than 1%, cholic acid at 0.5%, lithocholic acid at 0.1%, and any other impurity at 0.25%.
Did the CDCA related-substances method change?
Yes. A draft published in 2019 took effect in January 2022, replacing thin-layer chromatography with two liquid-chromatography methods.
Can a CDCA specification table be reused for UDCA?
No. Rotation, loss on drying, the identity of impurity A and the polymorphism statement all differ between the two monographs.
What to ask a supplier
The specification is fixed by the monograph, so the useful questions are about whether the certificate in front of you actually reflects it. Three checks do most of the work.
Ask for the related-substances method to be dated to the current monograph — the change took effect in January 2022, so a certificate citing only a TLC method is working from the older edition. Ask for the impurity to be named alongside its letter, so “impurity A” cannot be misread. And ask for the assay basis to be stated, since content on the dried substance and content as-is are not the same number.
For the wider specification context, the UDCA panel is set out in the full UDCA specification, and the reason CDCA and UDCA each appear as the other’s impurity A is explained in CDCA as UDCA impurity A. The general discipline of checking a supplier against its own documents is set out in how to verify a bile-acid supplier. Our chenodeoxycholic acid grade is described with these parameters, and you can request a quotation against your own acceptance criteria.
The values in this sheet are taken from the current European Pharmacopoeia monograph for chenodeoxycholic acid. The report, identification and qualification thresholds come from the general monograph on substances for pharmaceutical use, whose framework follows the ICH quality guidelines; reference standards are listed by the United States Pharmacopeia.
This article describes pharmacopoeial requirements and analytical interpretation only. It is not a statement of therapeutic use, and it does not set out commercial guarantees. Always work from the pharmacopoeia edition in force at the time of your release.
Frequently asked questions
Why do CDCA specifications differ between suppliers showing the same purity?
Assay and purity measure different things, and neither names the impurities. Two certificates can both read 99% while one carries a named impurity table with individual limits and the other reports a single total. The difference between those two documents is not cosmetic; it is the difference between a controlled process and an unmeasured one.
Which impurities matter most in CDCA?
The epimeric and oxidation-related ones: ursodeoxycholic acid and the keto bile acids. Their limits show how well isomerisation and oxidation are controlled in the route. A specification without them is a specification that has not looked.
Is a USP-style COA layout proof of pharmacopoeial grade?
No. Layout proves nothing. Some certificates borrow a pharmacopoeial format while the limits behind the lines are looser. The check is line by line: compare each limit against the corresponding monograph and note which lines are missing entirely, because missing lines are usually the ones that matter.
What decides identity for CDCA?
Typically an infrared spectrum compared against a reference plus a chromatographic retention match. One test without the other leaves room for closely related bile acids to pass. If a COA shows identity by a single method, ask what the second confirmation is.
What does the melting range actually tell a buyer?
It is a rough identity and crystal-form check, not a purity number. A narrow range consistent with the specification supports the identity claim; a drifting range across batches is an early sign that crystal form or residual solvent is moving. Read it as a trend, not as a verdict.
Two CDCA certificates disagree on the same batch. What is checked first?
The method. Titration and HPLC produce systematically different assay figures, and the difference is explainable once the method is named. A dispute that starts with two numbers usually ends with two method lines, and the supplier who states the method up front never has that argument.