Polyaluminium chloride is an inorganic coagulant used in drinking-water treatment, municipal and industrial wastewater treatment, and paper manufacture. It is not a single compound. PAC is a partially neutralised, partially polymerised aluminium chloride — a distribution of hydroxo-aluminium species in equilibrium, produced by controlled addition of base to an aluminium chloride solution and stabilised at a defined degree of neutralisation. Because the product is a mixture rather than a discrete chemical, it is specified by composition parameters — aluminium content expressed as Al2O3, and basicity — rather than by a purity assay. Understanding those two numbers is the whole of PAC specification.

Substance identity

Field	Value
Common name	Polyaluminium chloride (PAC); polyaluminum chloride
Also called	Basic aluminium chloride, aluminium chlorohydroxide, polyaluminium chlorohydrate (higher-basicity types)
General formula	Aln(OH)mCl(3n−m), commonly written [Al2(OH)nCl6−n]m
CAS number	1327-41-9
PubChem CID	none applicable — see below
Physical forms	Aqueous solution (typically yellow to colourless, sometimes with a slight haze) or spray-dried powder (pale yellow to white)

Substance identity, stated honestly. PAC is a polymeric mixture, not a discrete compound. There is no single molecular formula and no single molar mass, and consequently PubChem returns no single CID for it — the database indexes defined molecular species, and PAC is a composition-dependent distribution of them. Any source that quotes a specific molecular weight for “polyaluminium chloride” as though it were a defined compound has over-specified the material.

CAS 1327-41-9 is broad. It designates aluminium chloride, basic — that is, the general class of partially neutralised aluminium chlorides — and therefore covers a wide span of basicity and aluminium content under one registry number. Two products can share CAS 1327-41-9 and behave very differently in a treatment plant. Related and higher-basicity materials, such as aluminium chlorohydrate, may be listed under their own registry entries. The consequence for procurement is direct: the CAS number does not specify this product. The basicity and Al2O3 figures do.

What is actually in the solution. Controlled neutralisation of aluminium chloride produces hydrolysed aluminium species ranging from simple monomers through dimers and oligomers to large polycations. The most studied and most significant of these is the tridecameric Keggin polycation, commonly written Al13O4(OH)24(H2O)12 with a 7+ charge and referred to as “Al13”. Its high positive charge per aluminium atom is the principal reason pre-hydrolysed coagulants outperform simple aluminium salts. The proportion of aluminium present as Al13 and other polymeric species depends on how the product was made and on its basicity — which is why basicity is a performance parameter and not a bookkeeping figure.

Specification

Typical commercial ranges. These are market-typical figures for orientation, not guaranteed limits. Grade requirements for potable-water service are set by the applicable drinking-water treatment chemical standard in the destination market, and the certificate of analysis for the specific grade governs.

Liquid grades

Parameter	Typical range	Note
Aluminium as Al2O3	approx. 10.0–12.0 % w/w	the standard way aluminium content is expressed
Basicity	approx. 45–75 % for common water-treatment grades	higher-basicity types exist
Density at 20 °C	approx. 1.19–1.25 g/cm3	needed to convert volumetric dosing to mass
pH, 1 % solution	approx. 3.5–5.0
Water-insoluble matter	typically ≤ 0.3–1.0 %
Appearance	clear to slightly hazy liquid	pronounced haze or sediment can indicate ageing

Spray-dried powder grades

Parameter	Typical range	Note
Aluminium as Al2O3	approx. 28–31 % w/w	roughly 2.5–3× the liquid on a mass basis
Basicity	approx. 40–90 % depending on type	must be stated; the span is wide
pH, 1 % solution	approx. 3.5–5.0
Water-insoluble matter	typically ≤ 0.5–1.5 %
Appearance	pale yellow to white powder

Impurity limits for potable-water grades — iron, arsenic, lead, cadmium, mercury, chromium and manganese are limited, because the coagulant is dosed directly into water intended for human consumption and any contaminant it carries is added to that water. The limits are set by the applicable standard, not by convention. Grades intended for potable water are normally qualified against the applicable drinking-water treatment chemical standard in the destination market. Confirm which standard, which edition and which certification body applies to the specific grade, and require it on the certificate of analysis. Do not infer potable-water suitability from a technical-grade specification.

Properties

Basicity — the single most important parameter. Basicity expresses how far the aluminium chloride has been neutralised toward aluminium hydroxide:

Basicity (%) = [OH] / (3 × [Al]) × 100, on a molar basis

At 0 % basicity the material is aluminium chloride with no hydroxide substitution. At 100 % it would be aluminium hydroxide, which is insoluble. Commercial PAC sits between, and where it sits determines its behaviour:

Lower basicity — more chloride, more residual acidity, more alkalinity consumed from the treated water, generally stronger charge neutralisation per unit aluminium in some waters, and a lower price per unit Al2O3.
Higher basicity — less alkalinity consumed, less pH depression of the treated water, a larger fraction of aluminium present as pre-formed polymeric species, better performance in cold and low-alkalinity waters, generally better floc formation — and a greater tendency toward instability on prolonged storage, since the material sits closer to the point where aluminium hydroxide precipitates.

Al2O3 content is a concentration, not a quality. It states how much aluminium the product contains, which is what allows dose comparison on a like-for-like basis. Two products at the same Al2O3 and different basicity are different coagulants. Comparing PAC products on Al2O3 alone — or worse, on price per tonne of product — is the most common specification error with this material.

Why pre-hydrolysed coagulants differ from alum. Aluminium sulphate (alum) must hydrolyse in the treated water itself, which consumes alkalinity, depresses pH, and works well only in a fairly narrow pH window. PAC arrives pre-hydrolysed. In practice this generally yields:

a wider effective pH window, broadly in the region of pH 5–9, against a narrower window for alum;
lower alkalinity consumption and less pH depression, often removing the need for supplementary alkali;
lower dose on an aluminium basis for equivalent turbidity removal in many waters;
better performance in cold water, where alum hydrolysis slows markedly — often the decisive argument in temperate and cold climates;
faster floc formation and stronger, denser floc, improving settling and filter runs;
less sludge produced per unit of water treated, and lower residual aluminium in the treated water when properly dosed.

None of these is universal. Coagulant selection is water-specific and must be settled by jar testing on the actual water, across the actual seasonal range of temperature, turbidity, alkalinity and organic content. A supplier comparison table is not a substitute for a jar test.

Ageing. PAC solutions are metastable. The species distribution continues to shift slowly during storage, more quickly at high basicity, high temperature or high concentration, and eventually aluminium hydroxide precipitates. Performance declines before visible precipitate appears. Suppliers state a shelf life for each grade — confirm it, and manage stock to it.

Applications

Drinking-water treatment. Primary coagulant for turbidity removal, natural organic matter removal (and therefore disinfection by-product precursor control), and colour removal. Used in conventional clarification, dissolved air flotation and direct filtration. Requires a grade certified for potable-water contact.
Municipal wastewater treatment. Phosphorus removal by precipitation of aluminium phosphate; enhanced primary treatment; suspended-solids removal; sludge conditioning; and control of filamentous bulking in activated sludge.
Industrial wastewater. Removal of suspended solids, colloids, colour, oil and emulsified oil, and metals (through co-precipitation and sweep flocculation) from process effluent across textiles, food processing, metal finishing, paper and petrochemicals.
Pulp and paper. Retention and drainage aid; pitch and stickies control; sizing.
Other: swimming-pool water clarification; cooling-water clarification; certain cosmetic and personal-care applications for specific high-basicity aluminium chlorohydrate types (which are distinct products with their own specifications and are not interchangeable with water-treatment PAC).

Dosing practice. PAC is normally dosed as a dilute solution into rapid mix, with sufficient mixing energy at the injection point to disperse it before floc formation begins. Optimum dose and optimum pH are water-specific and season-specific. Overdosing wastes reagent, increases sludge, can restabilise the colloids, and raises residual aluminium in the finished water — which is itself a regulated parameter in drinking water.

Handling and storage

Corrosivity — the primary hazard. PAC solutions are acidic. Concentrated liquid product causes skin and eye damage and is corrosive to many materials. Eye protection, face protection, chemical-resistant gloves and appropriate clothing are required for handling; eyewash and safety shower must be available at the dosing point. The supplier’s current safety data sheet governs classification, exposure controls and first aid.

Materials of construction. Suitable: HDPE, polypropylene, PVC, GRP/FRP, PTFE, rubber-lined steel, and specific high-alloy grades where the supplier confirms compatibility. Not suitable: carbon steel, aluminium, galvanised steel, copper and copper alloys, and unprotected concrete — the acidic chloride solution attacks all of these. Aluminium is a particularly common and particularly bad mistake in this service. Confirm gasket and seal elastomer compatibility as well as the pipe and tank material.

Storage of liquid grades.

Store in vented, dedicated, clearly labelled HDPE or GRP tanks within chemical-resistant bunding.
Protect from cold. Liquid PAC can crystallise, gel or partially freeze in cold storage; heated or insulated storage, or trace heating on lines, may be needed in cold climates. The onset temperature is grade-specific — obtain it from the supplier rather than assuming it.
Protect from heat and direct sunlight, which accelerate ageing and precipitation.
Observe the stated shelf life and rotate stock. Do not top up an old tank with fresh product as a matter of routine; mixing old and new material makes ageing untraceable.
Provide adequate venting; do not seal storage tanks.

Storage of powder grades. Spray-dried PAC is hygroscopic. It absorbs moisture, cakes, and once caked does not redissolve readily. Keep sealed, dry, off the floor, under cover; close bags and FIBCs between withdrawals. When making up solution, add powder to water under agitation.

Incompatibilities.

Alkalis (caustic soda, lime, soda ash) — immediate precipitation of aluminium hydroxide as a gel, which will block a dosing line permanently. Never mix PAC with an alkali in a common line, a common make-up tank or a shared spill bund. Where both are dosed, use separate injection points with adequate separation.
Sodium silicate and some coagulant aids — gelling on direct contact in concentrated form; dose separately.
Sodium hypochlorite and other oxidants — do not mix in concentrated form; keep storage, dosing and spill containment separate.
Reactive metals — aluminium, zinc and galvanised surfaces are attacked by the acidic solution with hydrogen evolution.
Concrete — attacked; bunds and floors require an acid-resistant coating.

Notes

This is a mixture, and every honest specification for it says so. There is no single molecular formula, no single molar mass, and no single PubChem CID. Basicity and Al2O3 content define the product; the CAS number does not.
CAS 1327-41-9 spans a wide range of materials. Do not treat a shared CAS number as evidence that two products are equivalent, and do not accept a specification that names only the CAS number.
Never compare PAC products on price per tonne of product. Compare on cost per unit of Al2O3 delivered at a stated basicity, and validate by jar test on the actual water, across the seasonal range. A liquid at 10 % Al2O3 and a powder at 30 % Al2O3 are not comparable by mass, and two 10 % liquids at different basicities are not comparable by aluminium either.
Potable-water service requires a certified grade. Technical-grade material may carry heavy-metal levels that are irrelevant in an industrial effluent duty and unacceptable in drinking water. Name the applicable standard on the specification and require the certification on the CoA.
Residual aluminium in treated drinking water is a regulated parameter in most jurisdictions. Coagulant dose optimisation is a compliance matter, not only a cost matter.
PAC is not a drop-in substitute for alum without re-optimisation. Dose, optimum pH, mixing energy and sludge handling all change. Plan a trial period, not a switchover.

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Frequently asked questions

We have quotes for PAC at very different prices per tonne. How do we compare them fairly?

Compare on active content, not on tonnage. The working ingredient is alumina, reported as % Al2O3. Spray-dried solid grades commonly sit around 28-30% Al2O3 and liquid grades around 10-12% (typical commercial ranges — always confirm against the actual certificate of analysis). A tonne of liquid therefore carries roughly a third of the alumina of a tonne of powder, before freight is even counted. Convert every quote to delivered cost per kilogram of Al2O3, then adjust for basicity and water-insoluble matter, which decide how much of that alumina is actually usable. Close the comparison with a jar test on your own raw water, dosing each candidate at equal Al2O3 rather than equal product weight.

What does “basicity” on the spec sheet actually mean, and is a higher number better?

Basicity, or degree of neutralisation, is the ratio of hydroxide to aluminium in the polymer expressed as a percentage. It describes how far the aluminium chloride has already been hydrolysed into polynuclear species in the reactor, before the product ever reaches your water. Commercial grades typically fall somewhere between roughly 40% and 90%. Higher basicity means less hydrolysis has to happen in your tank, so less alkalinity is consumed and pH drops less on dosing — an advantage in soft, low-alkalinity water. It is not universally better: lower-basicity grades often perform well in high-turbidity or cold water. Treat basicity as a selection parameter matched to your water chemistry, not as a quality score.

Should we buy spray-dried powder or liquid PAC?

The chemistry is the same; the decision is logistics and dosing. Powder carries roughly three times the alumina per tonne, ships as a dry good and stores longer, but it must be dissolved and allowed to stand before use, needs make-up equipment and dust control, and incomplete dissolution shows up directly as poor performance. Liquid arrives ready to meter, removes the make-up step and gives steadier dosing, but you pay freight on mostly water, need corrosion-resistant bulk storage and accept a shorter practical shelf life. As a rule of thumb, long haul, intermittent use or limited tankage favours powder; continuous high-volume dosing near the supply point favours liquid.

Our supplier offers a white powder and a yellow powder at different prices. What is the difference?

Colour mainly tracks iron content and the raw-material route. Yellow to light-brown powders generally come from bauxite or calcium aluminate routes and carry more iron and more water-insoluble matter; they are normally sold as industrial grade for wastewater, papermaking or oilfield service. White grades are made from purer aluminium sources, contain little iron and have lower insolubles. For potable water the decisive question is not colour but whether the product is certified to the drinking-water standard in force in your market, supported by a certificate of analysis covering arsenic, lead, cadmium, mercury and chromium. Industrial grade is not a cheaper substitute in that application, whatever its appearance.

How should PAC be stored, and what goes wrong in storage?

Solid PAC is strongly hygroscopic. Keep bags sealed, off the floor, in a dry ventilated store out of direct sun; absorbed moisture cakes the powder and makes dissolution slow and incomplete. Liquid PAC is an acidic solution: store it in HDPE, FRP or rubber-lined tanks, never in bare mild steel or aluminium, and check that gaskets and pump wetted parts are compatible. In cold conditions liquid grades can thicken or throw crystals — warm and re-homogenise the whole container rather than decanting the clear layer. Slight haze or fine sediment in stored liquid is normal; a growing gel or a hard settled layer means the product is ageing. Rotate stock and re-check assay after long storage.

What must never be mixed with PAC?

Never combine PAC with alkaline chemicals in the same tank or dosing line. Caustic soda, lime, soda ash or ammonia neutralise it immediately and precipitate aluminium hydroxide, blocking lines and destroying the coagulant. Do not blend it with concentrated anionic polyacrylamide either: coagulant and flocculant belong at separate injection points, PAC first at the rapid-mix stage and polymer afterwards in slow mix. Keep it away from strong oxidisers such as hypochlorite, and from sulphuric acid or sulphate concentrates, which can throw insoluble solids. Use dedicated dosing equipment and flush lines when switching products — residue of a previous alkaline chemical is a common reason a new batch appears not to work.

When is PAC the wrong choice for us?

It is a coagulant for suspended and colloidal matter, not a treatment for dissolved species. It will not remove nitrate, chloride, hardness or most soluble salts, and raising the dose does not change that. In very soft, low-alkalinity water it still depresses pH and may need alkali support. In agriculture it is not a fertiliser and supplies no plant nutrient; aluminium is not a nutrient element, and in soil below roughly pH 5.5 soluble aluminium is phytotoxic and inhibits root growth, so water treated with it and applied to acid soils warrants a residual-aluminium check. Where coagulation pH cannot be held near the aluminium-hydroxide solubility minimum, residual dissolved aluminium rises and an iron-based coagulant is often the better fit.

Polyaluminium Chloride (PAC) — Basicity, Al2O3 Content, Liquid and Powder Grades