Which salt does the name sodium citrate usually mean?

In commerce it almost always means trisodium citrate, the fully neutralised salt, and most often the dihydrate with CAS 6132-04-3 and a molar mass of 294.10 g/mol. The anhydrous form of the same salt is CAS 68-04-0 with a molar mass of 258.07 g/mol. Monosodium and disodium citrate exist and behave differently, because the number of acid protons left on the molecule is what sets the working pH range. If a specification says only sodium citrate, ask which salt and which state of hydration before anything else is discussed, because assay, dosing and solubility all move with the answer.

What is the difference between the dihydrate and the anhydrous form?

They are the same salt with different amounts of water of crystallisation, which means they are different raw materials in a batch sheet. The anhydrous form carries more citrate per unit mass, so substituting one for the other without recalculating changes the dose. Beyond the arithmetic, the two forms differ in how they behave in a dry blend and how they respond to humid storage. The anhydrous form is usually chosen where water has to be kept out of a dry formulation; the dihydrate is the common general-purpose grade.

How does it differ from citric acid in use?

Citric acid supplies acidity, the salt supplies buffering. Used together they form a buffer pair whose ratio, not whose total quantity, sets the pH that the system holds. That is the practical reason both appear on so many formulations at once. Where a formulator wants a defined pH that survives dilution, temperature change and the addition of other ingredients, the pair does the job that neither component does alone. Where the goal is only to acidify, the acid alone is sufficient and cheaper.

What does sequestration mean in a detergent context?

It means binding metal ions in solution so that they cannot do something unwanted. Calcium and magnesium in hard water reduce surfactant efficiency and leave deposits; iron and copper traces catalyse oxidation and can drive discoloration. A sequestrant holds those ions in a soluble complex and keeps them out of the reaction. Citrate is one option among several and it is not the strongest, but it is widely accepted, readily available and straightforward to document, which is why it appears in formulations where a stronger builder would be harder to justify.

Which grade should be ordered?

Grade follows the end use and its documentation requirements, not the price list. A technical grade suits industrial cleaning, water treatment and general chemical use. A food grade is defined by a food-chemicals monograph and brings the corresponding heavy metal and impurity limits, allergen and origin statements, and traceability. A pharmacopoeial grade brings a defined monograph, tighter testing and a documentation set to match. State the intended use and the market of sale at enquiry stage so that the grade quoted is the grade you can actually use.

Sodium citrate: what it does and how to specify it

Technical article · Eapearl Chemical · 9 July 2026

One salt does three unrelated jobs in a formulation, which is why it appears on so many batch sheets and why so few specifications for it are written properly.

One citrate, several salts

Citric acid has three acid groups, so it forms three sodium salts. The one that commerce means when it says sodium citrate is the fully neutralised trisodium salt, and most commonly the dihydrate: CAS 6132-04-3, molar mass 294.10 g/mol. The anhydrous form of the same salt is CAS [CAS?] with a molar mass of 258.07 g/mol. Monosodium and disodium citrate are real products with their own uses, and they behave differently because the number of acid protons left on the molecule is what fixes the pH range in which the material buffers well.

This is the first place a specification goes wrong. A batch sheet that says only sodium citrate leaves the salt, the hydration state and therefore the citrate content undefined. A purchasing department filling that line has a genuine choice to make and no information with which to make it. Write the salt, the hydration state and the CAS number, and the ambiguity disappears before it reaches a supplier.

What the citrate ion actually does

The molecule earns its place through three distinct mechanisms, and most formulations use more than one of them at the same time without saying so.

Buffering. Paired with citric acid, the salt forms a buffer whose ratio sets the pH that the system holds. The buffer resists the pH shift that would otherwise follow dilution, temperature change or the addition of another ingredient.
Sequestration. The citrate ion binds polyvalent metal ions in solution. Calcium and magnesium from hard water are the usual targets in cleaning; traces of iron and copper matter in oxidation-sensitive systems, where they act as catalysts for exactly the degradation the formulator is trying to avoid.
Crystallisation and texture control. By holding ions in complex and by altering the ionic environment, citrate influences how and whether other salts crystallise out. In several food and detergent systems this is the real reason it is present, even where the formulation notes call it a buffer.

A fourth, softer property drives much of its commercial success: the salt is well established, readily documented and generally uncontroversial across markets, which makes it an easier item to defend in a regulatory file than several functionally stronger alternatives.

Where it earns its place

Food and beverage. As an acidity regulator and as an emulsifying salt in processed dairy, and in beverages where a stable pH is what keeps flavour and colour consistent through shelf life. Food use brings a monograph grade and a documentation set with it.
Detergents and household cleaning. As a builder and co-builder alongside or in place of phosphate chemistry such as sodium tripolyphosphate, contributing water softening and alkalinity control at once.
Personal care. As the pH control half of a system where the acid alone would be too aggressive, and as a chelant in products with oxidation-sensitive actives.
Industrial water treatment and metal cleaning. Where the job is to keep hardness ions in solution rather than on a surface.
Pharmaceutical and medical device manufacture. Under a pharmacopoeial monograph, where the grade, the testing and the documentation are set by that monograph and not by the buyer.
Chemical processing. As a pH control agent in reactions and in downstream washes, frequently alongside sodium bicarbonate where a mild alkalinity source is also needed.

Choosing between the dihydrate and the anhydrous grade

The two forms are the same salt and are not interchangeable in a batch sheet. The anhydrous grade carries more citrate per unit of mass, so swapping one for the other without recalculating quietly changes the dose. That arithmetic is easy to fix; the behaviours around it are what actually decide the choice.

Dry blends. Water of crystallisation is water that can be released under the right conditions. Where a dry formulation contains a moisture-sensitive component, the anhydrous grade is usually the safer choice.
Caking and flow. Both forms are sensitive to humid storage, and particle size and packaging matter more than the hydration state in practice. Ask for the particle size distribution, not just a mesh description.
Dissolution. Where the salt is added to water, particle size drives dissolution rate more than the hydration state does. A fine grade dissolves faster and dusts more.
Cost per unit of function. Compare on active citrate delivered, not on delivered mass, or the comparison is meaningless.

Reading the certificate of analysis

A certificate for a citrate salt is short, which makes it easy to skim and easy to misread. The lines worth attention are these.

Assay, stated on a defined basis: as-is or dried. Two certificates quoting different bases are not comparable, and this is the most common cause of a dispute that turns out to be arithmetic.
Loss on drying or water content, which is the line that confirms the hydration state actually delivered.
pH of a defined solution, which is the quickest confirmation that the salt is the one ordered rather than a partially neutralised one.
Chloride and sulfate, the process residues that indicate how the neutralisation and the washing were run.
Heavy metals and specified elemental impurities, whose limits come from the applicable monograph for food or pharmacopoeial grades.
Appearance, particle size and bulk behaviour, which are what your handling equipment actually experiences.
Microbiological limits, where the grade and the end use require them.

Storage, handling and materials

Treat it as a hygroscopic crystalline solid. Keep bags and drums sealed, keep them off the floor and away from steam lines and washdown areas, and rotate stock properly, because caking is almost always a storage outcome rather than a quality defect in the delivered lot. Where the material is dissolved, remember that solutions of the salt are mildly alkaline and that citrate is an active sequestrant: it will slowly attack some metal surfaces and will strip protective deposits from others. Stainless steel and suitable plastics are the usual choices for solution handling; check any coating or elastomer against the actual solution concentration and temperature rather than against the dry salt.

Where orders go wrong

Ordering sodium citrate without stating the salt and the hydration state, then discovering the assay basis differs from the last supplier.
Substituting anhydrous for dihydrate without recalculating the dose, and attributing the resulting pH drift to something else.
Comparing quotations on delivered mass rather than on active citrate delivered.
Buying a technical grade for a use that will later need food or pharmacopoeial documentation, then trying to retrofit the paperwork.
Ignoring particle size until the material fails to flow or fails to dissolve at the rate the process assumed.
Treating caking as a supplier defect without first checking how and where the stock was stored.

Our related catalogue items include sodium citrate itself, together with anhydrous citric acid for the acid half of the buffer pair. Grade selection, specifications and lot certificates are quoted against a named application through our contact page.