Glossary of Terms for Ion Exchange Resins

We use many terms in our industry and we know that it can sometimes be confusing. In this section we have collected the most common terms for your reference.

ACIDITY: An expression of the concentration of hydrogen ions present in a solution.

ADSORPTION: The attachment of charged particles to the chemically active groups on the surface and in the pores of an ion ex changer.

ALKALINITY: An expression of the total basic anions (hydroxyl groups) present in a solution. It also represents, particularly in water analysis, the bicarbonate, carbonate, and occasionally, the borate, silicate, and phosphate salts which will react with water to produce the hydroxyl groups.

ANION: A negatively charged ion.

ASH: The residual mineral content of resin after incineration at 800° C.

ASTM – The American Society of Testing and Materials

ATTRITION: The rubbing of one particle against another in a resin bed; frictional wear that will affect the size of resin particles.

BACKWASH: The upward flow of water through a resin bed (i.e., in at the bottom of the exchange unit, out at the top) to clean and reclassify the bed after exhaustion.

BASE: The hydroxyl form of a cation or a compound that can neutralize an acid.

BATCH OPERATION: The utilization of ion exchange resins to treat a solution in a container where in the removal of ions is accomplished by agitation of the solution and subsequent decanting of the treated liquid.

BED: The ion exchange resin contained in a column.

BED DEPTH: The height of the resinous material in the column after the exchanger has been properly conditioned for effective operation.

BED EXPANSION: The effect produced during backwashing: The resin particles become separated and rise in the column. The expansion of the bed due to the increase in the space between resin particles may be controlled by regulating backwash flow.

BICARBONATE ALKALINITY: The presence in a solution of hydroxyl (OH ) ions resulting from the hydrolysis of carbonates or bicarbonates. When these salts react with water a strong base and a weak acid are produced, and the solution is alkaline.

BREAKTHROUGH: The first appearance in the solution flowing from an ion exchange unit of unadsorbed ions similar to those which are depleting the activity of the resin bed. Breakthrough is an indication that regener- ation of the resin is necessary.

BRINE: A salt solution, generally sodium chloride in a saturated solution.

BS&D: A procedure for resin volume measurement where in an ion exchange resin bed is f first backwashed, then allowed to settle and then drained of water. The resultant bed- height is measured for volume calculations.

BUFFER – A solution that is capable of opposing small changes in chemical composition; particularly in pH.

CALCIUM – A metallic element often found in water, usually as dissolved calcium carbonate (CaCO3). Soluble in water, it causes hardness and subsequent scaling.

CAPACITY: The ability of an ion exchange material to absorb ions. Usually expressed in kilograins per cubic foot or milliequivalents per milliliter.

CAPACITY, OPERATING: the portion of the total capacity utilized in practical ion ex change operation.

CAPACITY, SALT-SPLITTING: The portion of total capacity to split neutral salt.

CAPACITY, TOTAL: The ultimate exchange capacity of the resin.

CARBONACEOUS EXCHANGERS: Ion exchange materials of limited capacity prepared by the sulfonation of coal, lignite, peat, etc.

CATION: A positively charged ion.

CHANNELING: Cleavage and furrowing of the bed due to faulty operational procedures, in which the solution being treated follows the path of least resistance, runs through these furrows, and fails to contact active groups in other parts of the bed.

CHEMICAL STABILITY: Resistance to chemical change which ion exchange resins must possess despite contact with aggressive solutions.

COLUMN OPERATION: Conventional utilization of ion exchange resins in columns through which pass, either upflow or downflow, the solution to be treated.

CONDENSATE POLISHERS: Ion exchange resins being used to remove or exchange ions as well as to filter condensate for reuse in the steam cycle.

CONDUCTIVITY – The reciprocal of resistivity, it is a measure of the ability to conduct an electric current. Since ionized impurities increase the conductivity of water, it is also an accurate measure of ionic purity. To measure it, current is passed between two electrodes a fixed distance apart. Conductivity is normally expressed as microsiemens/cm, identical to micromhos/cm.

CROSSLINKAGE: The degree of binding of a monomer or set of monomers to form an insoluble tri-dimensional resin matrix.

CYCLE: A complete course of ion exchange operation. For instance, a complete cycle of cation exchange would involve: exhaustion of regenerated bed, backwash, regeneration and rinse to remove excess regenerant.

DEASHING: The removal from solution of inorganic salts by means of adsorption by ion exchange resins of both the cations and the anions that comprise the salts. See deionization.

DEIONIZATION: Deionization, a more general term than deashing, embraces the removal of all chargedconstituents or ionizable salts (both inorganic and organic) from solution. See deashing.

DEMINERALIZATION – Sometimes used interchangeably with deionization, it refers to the removal of minerals and mineral salts using ion exchange.

DENSITY: The weight of a given volume of exchange material, backwashed and in place in the column.

DIFFUSION: Usually referred to ion exchange resins as the diffusion of ions through the ion exchange resin beads.

DISSOLVED SOLIDS – Also referred to as Total Dissolved Solids (TDS), it is the amount of nonvolatile matter dissolved in a water sample, and is usually expressed in parts-per-million by weight.

DOWNFLOW: Conventional direction of solutions to be processed in ion exchange column operation, i.e., in at the top, out at the bottom of the column .

DRY SOLIDS: The matter, usually expressed in weight percent, remaining after liquid removal.

EFFICIENCY: The effectiveness of the operational performance of an ion exchanger. Efficiency in the adsorption of ions is expressed as the quantity of regenerant required to effect the removal of a specified unit weight of adsorbed material, e.g., pounds of acid per kilograin of salt removed.

EFFLUENT: The solution which emerges from an ion exchange column.

ELECTROLYTE: A chemical compound which dissociates or ionizes in water to produce a solution which will conduct an electric current; an acid, base or salt.

ELUTION: The stripping of adsorbed ions from an ion exchange material by the use of solutions containing other ions in relatively high concentrations.

EQUILIBRIUM REACTIONS: The interaction of ionizable compounds in which the products obtained tend to revert to the substances from which they were formed until a balance is reached in which both reactants and products are present in definite ratios.

EQUIVALENT WEIGHT: The molecular weight of any element or radical expressed as grams, pounds, etc., divided by the valence.

EXCHANGE SITES: The reactive groups on an ion exchange resin.

EXCHANGE VELOCITY: The rate with which one ion is displaced from an exchanger in favor of another.

EXHAUSTION: The state in which the resin is no longer capable of useful ion exchange; the depletion of the exchanger’s supply of available ions. The exhaustion point is determined arbitrarily in terms of: (a) a value in parts per million of ions in the effluent solution; (b) the reduction in quality of the effluent water determined by a conductivity bridge which measures the electrical resistance of the water.

FMA: The free mineral acidity, or sum of the mineral acids.

FINES: Extremely small particles of ion exchange materials.

FLOW RATE: The volume of solution passing through a given quantity of resin within a given time. Usually expressed in terms of gallons per minute per cubic foot of resin, as milliliters per minute per milliliter of resin, or as gallons per square foot of resin per minute.

FREEBOARD: The space provided above the resin bed in an ion exchange column to allow for expansion of the bed during backwashing.

GRAIN: A unit of weight; 0.0648 grams.

GRAINS PER GALLON: An expression of concen- tration of material in solution, generally in terms of calcium carbonate. One grain (as calcium carbonate) per gallon is equivalent to 17.1 parts per million.

GRAM-MILLIEQUIVALENTS: The equivalent weight in grams, divided by 1000.

GEL: Ion exchange resins that are made up of firm gel structure in a solid bead form allowing for the diffusion of ions through the gel.

HARDNESS: The scale-forming and lather- inhibiting qualities which water, high in calcium and magnesium ions, possesses. Temporary hardness, caused by the presence of magnesium or calcium bicarbonate, is so called because it may be removed by boiling the water to convert the bicarbonates to the insoluble carbonates. Calcium sulfate, magnesium sulfate, and the chlorides of these two metals cause permanent hardness.

HARDNESS AS CALCIUM CARBONATE: The expression ascribed to the value obtained when the hardness forming salts are calculated in terms of equivalent quantities of calcium ] carbonate; a convenient method of reducing all salts to a common basis for comparison.

HEADLOSS: The reduction in liquid pressure associated with the passage of a solution through a bed of exchange material; a measure of the resistance of a resin bed to the flow of the liquid passing through it.

HYDRAULIC CLASSIFICATION: The rearrangement of resin particles in an ion exchange unit. As the backwash water flows up through the resin bed, the particles are placed in a mobile condition wherein the larger particles settle and the smaller particles rise to the top of the bed.

HYDROGEN CYCLE: A complete course of cation exchange operation in which the adsorbent is employed in the hydrogen or free acid form.

HYDROXYL: The term used to describe the anionic radical (OH-) which is responsible for the alkalinity of a solution.

HYDROXYMETHYL FURFURAL, HMF: 5 (Hydroxymethyl) -2-furaldehyde, a precursor of the coloring matter from the decomposition of glucose and also thereby assisting in the color development in sugars. HMF is produced during contact with strong acid cation resins in H+ form at elevated temperatures.

INFLUENT: The solution which enters an ion exchange unit.

ION: Any particle of less than colloidal size possessing either a positive or a negative electric charge.

IONISATION: The dissociation of molecules into charged particles.

IONISATION CONSTANT: An expression in absolute units of the extent of dissociation into ions of a chemical compound in solution

KILOGRAIN: A unit of weight; one thousand grains.

LEAKAGE: The phenomenon in which some of the influent ions are not adsorbed or exchanged and appear in the effluent when a solution is passed through an under- regenerated exchange resin bed.

MACROPOROUS: Resins that have a rigid polymer porous network in which there exists a true pore structure even after drying. The pores are larger than atomic distances and are not part of the gel structure.

MONOMER: A single reactive molecule capable of combining with another different monomer to form a polymer. Where two different monomers combine the resulting polymer is called a polymer.

NEGATIVE CHARGE: The electrical potential which an atom acquires when it gains one or more electrons; a characteristic of an anion. pH: An expression of the acidity of a solution; the negative logarithm of the hydrogen ion concentration (pH I very acidic; pH 14, very basic; pH 7, neutral) .

pH: The measurement of the acidity of a solution where 1 is very acidic, 7 is neutral and 14 is very basic.

PHYSICAL STABILITY: The quality which an ion exchange resin must possess to resist changes that might be caused by attrition, high temperatures, and other physical conditions.

POROSITY: An expression of the degree of permeability in ion exchange resins to liquids and large organic molecules. Gel resins, even when referred to as highly porous, have a negligible porosity in comparison to the macropores inherent in the macroporous resins .

POSITIVE CHARGE: The electrical potential acquired by an atom which has lost one or more electrons; a characteristic of a cation.

QUATERNARY AMMONIUM: A specific basic group [-N(CH3)3+] on which depends the exchange activity of certain anion exchange resins. RAW WATER: Untreated water from wells or from surface sources.

REGENERANT: The solution used to restore the activity of an ion exchanger. Acids are employed to restore a cation exchanger to its hydrogen form; brine solutions may be used to convert the cation exchanger to the sodium form. The anion exchanger may be rejuvenated by treatment with an alkaline solution.

REGENERATION: Restoration of the activity of an ion exchanger by replacing the ions adsorbed from the treated solution by ions that were adsorbed initially on the resin.

Resistivity – A measure of specific resistance to the flow of electricity. In water, it is an accurate measure of ionic purity. See “Megohm-cm”.

RINSE: The operation which follows regeneration; a flushing out of excess regenerant solution.

SALT SPLITTING: The conversion of salts to their corresponding acids or bases by passage through ion exchange resins containing strongly acidic or strongly basic functional groups.

SELECTIVITY: The difference in attraction of one ion over another by an ion exchange resin.

SILICEOUS GEL ZEOLITE: A synthetic, inorganic exchanger produced by the aqueous reaction of alkali with aluminum salts.

SPHERICITY: Relating to the spherical nature and whole bead content of a resin.

SULFONIC: A specific acidic group (S03H) on which depends the exchange activity of certain cation exchange resins

SWELLING: The expansion of an ion exchange bed which occurs when the reactive groups on the resin are converted into certain forms.

THROUGHPUT: The amount of solution treated prior to the exhaustion of the ion exchange resin.

THROUGHPUT VOLUME: The amount of solution passed through an exchange bed before exhaustion of the resin

TOTAL IONIZED SOLIDS – Concentration of dissolved ions in solution, expressed in concentration units of NaCI (sodium chloride). It determines the operating life of ion exchange resins, and is calculated from measurements of Specific Resistance.

TOTAL ORGANIC CARBON – A measure of the level of organic impurities in water, which determines the operating life of activated carbon beds.

UPFLOW: The operation of an ion exchange unit in which solutions are passed in at the bottom and out at the top of the container.

VALANCE: A measurement of the number of atoms or ions of hydrogen it takes to com- bine with or be replaced by an element or radical. In short, the number of positive or negative charges of an ion.

VOID VOLUME: The space between particles of ion exchange resins in a settled bed, also called interstitial volume.


amp – Unit of electrical current.

cm – Unit of length. Abbreviation for centimeter. 1 centimeter = 1/100 meter.

°C – Unit of temperature. Abbreviation for degree Centigrade. °C = 5/9 (°F –35).

°F – Unit of temperature. Abbreviation for degree Fahrenheit. °F = 9/5 (°C +32).

gpm – Unit of water flow. Abbreviation for U.S. gallons per minute.

grain – Unit of ion-exchange capacity. Also, unit of weight.

Hz – Unit of frequency. Abbreviation for Hertz. 1 Hertz = 1 cycle per second.

megohm-cm – Unit of specific resistance of water. 1 megohm-cm = 1,000,000 ohm-cm.

mho/cm – Unit of specific conductance of water.

μmho/cm – Unit of specific conductance of water. Abbreviation for micromho/cm. 1 micromho/cm = 1/1,000,000 ohm-cm.

μs/cm – Unit of specific conductance of water. Abbreviation of microsiemens/cm. 1 microsiemens/cm = 1/1,000,000 mho/cm.

mg – Unit of weight. Abbreviation for milligram. 1 milligram – 1/1,000 gram. mL – Unit of volume. Abbreviation for milliliter. 1 milliliter = 1/1,000 liter. NPT – Abbreviation for National Pipe Thread.

ohm-cm – Unit of specific resistance of water.

ppb – Unit of concentration. Abbreviation for parts per billion. 1 part per billion = 1 micro-gram per liter.

ppm – Unit of concentration. Abbreviation for parts per million. 1 part per million = 1 milligram per liter.

psi – Unit of pressure. Abbreviation for pounds per square inch.

S/cm – Unit of specific conductance of water. Abbreviation for seimens per centimeter. 1

siemens/cm = 1mho/cm.

TDS – Abbreviation for total dissolved solids.