Analysis of carbonate in washing powder by IC

Ion chromatography with Metrohm represents a robust, precise quantitative technique for the analysis of carbonate in a variety of different samples matrices that include washing powders (detergents).

In recent years the cost of ion chromatography instruments have fallen drastically and they now represent a cost effective and rapid alternative to titrimetric methods that are currently widely used for carbonate analysis.

Modern fabric washing powders contain a number of chemical ingredients in various proportions and are complex mixtures to reflect the demands of modern day living in the new millennium.

They have to deliver high standards of cleaning, be safe towards people, fabrics and the environment in which they are utilised.

Carbonate present as sodium carbonate is a key component present in washing powders and the manufacturers of such products require a simple yet reliable way of quantifying its presence in a given product to ensure that quality control tolerances are maintained.

The sodium component could also be determined with ion chromatography simply by using a different eluent and separation column. Composition of washing powder.

No two brands of washing powder have exactly the same composition and the same brand in different countries may well have a different formulation.

A washing powder is a mixture of many different components.

Surfactant.

The surfactant (short for surface active agent) is a key ingredient of any washing powder improving the wetting ability of water as it loosens and removes dirt.

The surfactant emulsifies and helps suspends soil in the water preventing re-deposition.

Surfactant molecules contain a hydrophilic (water soluble) and a hydrophobic (water resistant) part.

The hydrophobic end attaches to oil and grease surrounding these particles to form micelles which dissolve in the water due to the attraction between the hydrophilic ends and the surrounding water.

A particular detergent may contain more than one surface active agent and they are often classified by their ionic character in water.

Anionic detergents are typified by alkyl aryl sulphonates which ionise in water to produce a large anion.

Cationic surface active agents produce a large cation but in reality are used less in detergents, often they are quaternary ammonium compounds or fatty acid amide derivatives.

Non ionic detergents generally consist of condensation products of alcohols or phenols with ethylene oxide. Builder.

The builder in washing powders enhances the effect of the surfactant by deactivating calcium and magnesium ions which would otherwise use up the surfactant molecules.

Builders are water softeners and work by complexation (eg sodium tripolyphosphate or nitriloacetic acid), precipitation (eg sodium carbonate) or ion exchange (eg zeolites); most of them produce alkaline solutions which aids the cleaning as most detergents work effectively around pH10-11.

The most common builders used today are synthetic zeolites which are solid ion exchangers that trap the divalent ions inside the solid particles.

Fillers. Fillers are materials added to washing powders to alter their physical properties like making the powder more free flowing as is the case of sodium sulphate or to lower the freezing point as for alcohols and liquid detergents.

Bleaches.

Bleaches (often sodium perborate) are added to bleach coloured stains that are not easily removed by washing but these work effectively only at temperatures above 60C so activator compounds such as tetra acetyl ethylenediamine are added which lower the temperature at which the bleach operates.

The following minor components in washing powder are important for the functioning of the detergent.

Optical brighteners.

Optical brighteners are organic molecules that are added to the detergent as a result of their fluorescent properties to create a visual whitening effect.

Their function is to off-set the yellowing of white cottons and linens by absorbing ultraviolet light and re-emitting it as visible light in the blue region increasing the total amount of light reflected by the surface to give a 'whiter white'. Enzymes.

Enzymes are added to biological detergents as catalysts to help break down biological stains such as blood and grass marks that may be present on the fabrics, thus making it easier for the other ingredients present in the washing powder to lift the dirt away from the clothing.

Fabric softeners.

Fabric softeners impart softness to the human touch by controlling the amount of static electricity that is present in the washed article; typically these are quaternary ammonium compounds. Softeners also reduce the amount of water retained after the spin dry cycle and so less energy is required to dry the clothes after washing.

Corrosion inhibitors.

A corrosion inhibitor like sodium silicate is added to reduce corrosion of the appliance surfaces and is incorporated into washing powder to protect the metal parts and finishes inside the washing machine.

Antifoaming agent. An antifoaming agent may be added to reduce the amount of foam produced particularly when detergents are drained to waste. Colourants and fragrances.

Colourants and fragrances are added to improve the appearance of the washing powder and give the washed garments a distinctive aroma in character with the product as well as masking any unpleasant odours from the wash water. Manufacture of sodium carbonate.

Sodium carbonate is used by many different industries as a raw material and approximately one million tonnes is produced each year in the United Kingdom. Industrially, sodium carbonate is referred to as soda ash and is produced in two grades; light ash - a fine powder, and heavy ash which has a bigger particle size and is denser making it more efficient to transport.

Sodium carbonate is manufactured using the Solvay process dating from the late nineteenth century also being known as the ammonia-soda process.

Sodium carbonate (Na2CO3) is produced from sodium chloride (NaCl) and calcium carbonate (CaCO3) via the Solvay process:- 2NaCl + CaCO3 —> Na2CO3 + CaCl2.

The Solvay process does not occur directly as written in the reaction above, but rather consists of a series of five individual reactions and several intermediate compounds.

To a solution of brine (sodium chloride in water), ammonia is added followed by carbon dioxide resulting in the precipitation of sodium hydrogen carbonate that decomposes to sodium carbonate when elevated to a temperature of 175C.

The ammonia is recovered by treating the ammonium chloride solution with calcium hydroxide slurry.

The whole process forms hydrates, anhydrous sodium carbonate is known as soda ash and the decahydrate form known as washing soda.

The role of carbonate in washing powder.

The role of sodium carbonate in washing powder is crucial; it may be used as a water softener and it is also used to raise the pH of the detergent as other ingredients that are present in the washing powder (particularly zeolites) tend to work better at elevated pHs.

Sodium carbonate softens the water by precipitating insoluble calcium or magnesium carbonate.

The benefit of softened water is that it provides real cost savings in terms of service, maintenance and replacement of the use of washing machines extending their lifespan.

Hard water tends to fur pipes as a result of the carbonate formed during heating and more washing powder is required for washing as the hard water forms a scum with calcium or magnesium compounds formed with the detergent.

It is estimated that softened water saves about 50% on washing powder consumption as well as reducing the cleaning time required and leaving the laundry looking brighter.

What is ion chromatography? Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction. Chromatography techniques are divided up according to the physical states of the two participating phases.

The term ion exchange chromatography or ion chromatography (IC) is a subdivision of high performance liquid chromatography (HPLC).

A general definition of ion chromatography can be applied as follows: "ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector".

A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces.

For anion chromatography these are quaternary ammonium groups.

In theory ions with the same charge can be exchanged completely reversibly between the two phases.

The process of ion exchange leads to a condition of equilibrium, the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.

Direct ion chromatography.

For analysis of carbonate using ion chromatography it is not possible to work with chemical suppression as the carbonate would be converted to the acid form by the suppressor and no peak observed.

In ion chromatography without chemical suppression (direct ion chromatography) very high demands are placed on the conductivity detector since the background conductivity (which must be compensated for) is relatively large in comparison with the measured signal.

Of prime importance for electronic suppression is the constancy of the background and as conductivity is extremely temperature dependant, then a good temperature constancy of the eluent flowing in the detector measuring cell is required (<0.01C). The sensitivity of conductivity measurements depends on the difference between the equivalent ionic conductivities of the sample and eluent ion.

In direct anion determinations salts of benzoic acid are often employed as these have a low equivalent ionic conductivity.

Thus should an anion with a higher equivalent ionic conductivity appear in the detector cell, the conductivity will increase and a positive peak will be observed and then recorded by the data acquisition software.

Method for the determination of carbonate in washing powder.

25mg of the dried analytical sample was weighed accurately into a 100ml volumetric flask before being up to the mark with deionised water.

The sample was sonicated for a period of ten minutes in an ultrasonic bath to aid homogeneity of the sample as well as removing carbon dioxide.

The diluted sample was injected into the Metrohm 761 Compact IC through a RP sample preparation cartridge and the response for the peaks recorded using an eluent of p-hydroxybenzoic acid/acetonitrile with the Metrohm Super-Sep analytical column.

The calculation was carried out automatically using integration software IC Net 2.2 against a previously prepared calibration plot.

There are no external displays or switches on the instrument, all the hardware is fully controlled via a single RS232 connection between the IC and the PC.

All the instrument parameters can be called upon with a click of the mouse.

The 761 Compact IC consists of a low-pulsation dual-piston pump, pulsation damper, electromagnetic injection valve, two channel peristaltic pump (optional, not required for carbonate analysis), suppressor module (optional, not required for carbonate analysis), conductivity detector, eluent organiser as well as a data recording and processing module.

All the components that come into contact with the eluent and sample are metal-free.

The detector is the heart of every ion chromatograph. The Metrohm detector's temperature varies by less than 0.01C and can be optimally adapted to the ambient conditions.

This outstanding temperature stability reduces interference and allows exact conductivity measurements.

Sample automation with Metrohm. When a large sample throughput is required, it is recommended to automate the ion chromatograph with one of the following Metrohm auto samplers: 766 IC sample processor, 788 IC filtration sample processor, or the 813 compact autosampler.

The prepared samples are placed onto the sample carousel before being processed automatically through the software freeing the user to perform other duties in the laboratory.

Conclusion of carbonate analysis of washing powder using ion chromatography.

Ion chromatography as an analytical technique has seen an enormous surge in popularity, due partly to the simplicity of many of the methods as well as other factors such as market forces driving down the expenditure costs of the equipment and an improved instrument power.

Ion chromatography is a precise technique that requires only a very small amount of sample for the analysis and the quantified results obtained within a matter of minutes.

The samples once in solution can be loaded onto an autosampler and run overnight or during the day freeing the user to perform other duties.

The running costs of ion chromatography with Metrohm instruments are surprisingly low requiring only the acquisition of chemicals required for the eluent as well as a clean, reliable source of deionised water for preparation of the standards and samples. With Metrohm ion chromatography systems it is possible to work non suppressed giving the analyst the option to run applications like carbonate using IC as opposed to current titrimetric methods.

If after the analysis has been completed it is necessary to perform a suppressed anion application, then all one has to do is attach the suppressor module between the separation column and detector to change the chemistry of the Metrohm IC system.

Ion chromatography is a clean technique as all the reagents are enclosed, its robustness and reliability are assured demonstrating precisely the reason why it is rapidly becoming the method of choice for many analysts in a plethora of different industries and is eminently suited to the analysis of carbonate in washing powder.