A surprising number of ordering mistakes involve the right compound in the wrong form. The buyer asked for copper sulfate and received the pentahydrate when the method assumed anhydrous material. The lab ordered an API as the free base, but the pharmacopoeial standard is the hydrochloride. Both are the same molecule in a loose sense, but they are different substances on a balance and in a specification.
This article explains how these forms differ, how they appear in identifiers, and how to make sure your order says exactly what you mean.
One molecule, several substances
The same core structure can be supplied in several forms:
- Free base or free acid, the neutral molecule.
- Salts, where an acidic or basic group is paired with a counter ion such as chloride, sulfate, sodium, potassium, maleate or mesylate.
- Hydrates and solvates, where a fixed amount of water or solvent is part of the crystal.
- Stereoisomers, such as a single enantiomer versus the racemate.
Each of these generally carries its own CAS Registry Number. That is useful, because it lets you specify the form precisely, but it also means a quick web search for the parent name can lead you to the wrong number.
Real examples
- Metformin is 657-24-9, while metformin hydrochloride is 1115-70-4.
- Copper(II) sulfate anhydrous is 7758-98-7, and the pentahydrate is 7758-99-8.
- Sodium carbonate anhydrous is 497-19-8, the monohydrate is 5968-11-6 and the decahydrate is 6132-02-1.
Why the form matters in the lab
Mass and molarity
The molecular weight changes with the form, and so does the amount of active molecule per gram. Copper(II) sulfate anhydrous has a molar mass of about 159.6 g/mol, while the pentahydrate is about 249.7 g/mol. If a method asks for 1.596 g of anhydrous salt to make a 0.1 M solution in 100 mL, you need about 2.497 g of the pentahydrate to deliver the same amount of copper.
Conversion factors
To convert a mass of salt to the equivalent mass of free base, multiply by the ratio of molecular weights. For metformin hydrochloride (about 165.6 g/mol) to metformin (about 129.2 g/mol), the factor is about 0.780. So 100 mg of the hydrochloride contains about 78.0 mg of metformin.
Pharmacopoeial monographs and product labels often express strength as the free base even when the salt is used. Make sure your calculations use the same basis as the specification you report against.
Physical behaviour
Different forms dissolve differently, absorb moisture differently and can have different stability. A hydrochloride salt may be freely soluble in water where the free base is barely soluble. Some anhydrous salts are strongly hygroscopic and slowly become a hydrate if left open.
Stereochemistry and naming
Single enantiomers have distinct CAS numbers from their racemates. The same applies to E and Z isomers. If your method or synthesis depends on stereochemistry, quote the stereodescriptor in the name as well as the correct CAS number, for example (S)- or (R)-, and ask for optical rotation or chiral purity data.
Melting point and solubility are also quick ways to tell forms apart on receipt. A hydrate often loses water before melting, and a salt usually dissolves far more readily in water than its free base. Comparing these simple observations with the certificate catches many form mix-ups before they reach a reaction or a calibration.
How to order the correct form
- Quote the CAS number of the exact form, not the parent compound.
- Write the form in words as well, for example "sodium carbonate, anhydrous" or "sertraline hydrochloride". A CAS typo is then easy to spot.
- State the molecular formula including water of crystallisation, such as CuSO4·5H2O.
- Say whether substitution is acceptable. Sometimes a different hydrate is perfectly fine if you correct the mass, and saying so can shorten lead time.
- Check the certificate on arrival, especially the water content, which confirms the hydrate state.
Special cases worth knowing
Solutions sold by concentration
Acids, ammonia and some reagents are sold as aqueous solutions. Hydrochloric acid at about 37 percent, ammonia solution at about 25 percent and hydrogen peroxide at 30 percent all share CAS numbers with the pure substance, so the concentration must be stated separately on your order.
Undefined hydrates
Some products are sold as "hydrate" or "xH2O" because the water content varies between lots. Here the molecular weight on the label is nominal, and you should use the measured water content from the certificate to calculate the true amount of anhydrous substance.
Salts with variable stoichiometry
Certain salts, such as some trifluoroacetate or acetate salts of peptides and amines, carry a variable number of counter ions. Content is then usually stated as free base, and you should check how the supplier determined it.
Solvates
Crystalline APIs sometimes trap ethanol, acetone or other solvents in a defined ratio. A solvate behaves like a hydrate for calculation purposes, and the residual solvent result on the certificate helps confirm which form you have.
Watch out for outdated and duplicate identifiers
Occasionally a CAS number is deleted and replaced, or a supplier lists an older number. Some products are registered under a number for an undefined hydrate. When identifiers do not match your expectation, compare the structure, formula and molecular weight rather than relying on the number alone.
Tools that help
Zemixal's product pages show the CAS number, formula and molecular weight for each form, and you can search by name, CAS, formula or ZX catalogue number. The CAS number guide explains how the identifiers are built and validated. To check a whole list of forms at once, paste up to 200 numbers into the bulk CAS lookup. The molar mass and dilution calculators on the lab tools page are handy for salt and hydrate conversions.