Analysis of chromium in waters using voltammetry

Chromium can make its way into different water sources from a variety of outlets, and historically chromium has been measured using spectroscopic or colorimetric techniques.

Chromium enters the different water supplies via discharges from industries, leaching from hazardous waste sites or naturally from the erosion of natural deposits.

Two distinct forms of chromium occur in these water sources; the non-toxic trivalent chromium (III) an essential trace nutrient, and hexavalent chromium (VI), a toxic species that can cause great harm to individuals and the surrounding environment.

The toxicity of chromium (VI) serves as the basis of setting the (total) chromium standard for drinking water currently 50mg/l in the United Kingdom.

With voltammetry one also has the option to speciate between the two chromium species, but usually chromium (VI) is the reported analysis due to its environmental and health impact.

Over recent years, voltammetry has undergone a tremendous surge in popularity and today represents a refined, clean, simple technique that offers outstanding limits of detection and is now the fastest growing analytical technique for trace analysis being eminently suited to chromium analysis.

The nature of the water sample being quantified does not restrict the voltammetric analysis, it makes little difference (other than sample preparation) as to whether the samples are potable, effluent or saline in nature. Voltammetry - a brief overview.

Heyrovsky first introduced polarography in 1922.

The term voltammetry is applied to designate the current-voltage measurement obtained at a given electrode.

Polarography is a special case of voltammetry referring to the current-voltage measurement acquired using a dropping mercury electrode with a constant flow of mercury drops. Stripping voltammetry uses the same instrumentation as traditional polarography but a stationary electrode such as the Hanging Mercury Dropping Electrode (HMDE) is used.

The voltammetric measurement is performed on a stationary mercury drop allowing one to achieve considerable increases in sensitivity.

The jump in sensitivity is possible by electrochemical preconcentration of the metals in question at the surface of the stationary electrode before the current-voltage curve is recorded.

The recorded current is the redissolution (reoxidation) current of the preconcentrated metal traces.

With the Metrohm Multi Mode Electrode (MME) the mercury is hermetically sealed in the reservoir and suffices for around 200,000 drops ensuring low laboratory running costs.

Applications of chromium.

Chromium is a heavy metal used in a variety of processes such as electroplating, leather tanning, wood treatment, the manufacture of pigments, and water cooling towers for use in corrosion control.

Chromium is a very hard transition metal and can be amalgamated with titanium to manufacture replacement hip joints.

Chromium is used extensively by the plastics industry as a catalyst during the manufacture of high density polyethylene (HDPE) resins that can be moulded, possess good taste and odour properties before being used as beverage bottles and industrial chemical containers.

Chromic acid is a mixture of sulphuric acid and sodium dichromate and is used extensively in the electroplating industry as an additive and is a powerful oxidising agent due to the presence of chromium (VI). Chromic acid is used both in chrome plating and wood preservatives.

Chromium plating occurs on a wide range of metals and plastics to produce a durable, tarnish resistant and high lustre finish.

The uses include domestic appliances, plumbing fixtures, automobile accessories, and hospital hygiene equipment where wipe-clean surfaces are essential.

For heavy industrial applications, a hard chrome plate is applied in much thicker layers to increase wear and corrosion resistance while creating a lower friction coefficient.

Some of the common uses of chrome plating include metal working machinery, cutting tools, engine cylinders, and hydraulic ramp coatings.

As an active ingredient in wood preservatives, chromic acid acts as a fixative to bind vital biocides to the wood to aid protection against attack from insects and fungi.

In certain climates, untreated woods in contact with the ground can last only six months - yet treated wood can extend the lifetime to over 30 years.

Estimates have evaluated that chromium based timber preservatives prevent the destruction of some 250 million trees every year.

Some common applications of the use of wood preservatives include utility poles, decking, porches, bridges, playground equipment, and fences.

As with all industrial processes, it is inevitable that a waste product is formed.

Chromium compounds are toxic in large quantities and their effect and impact on the surrounding environment requires monitoring.

In the chromium plating and manufacturing industries it is usually the chromium (VI) compound that is released with smaller quantities of the reduced chromium (III) and solid chromium.

Chromium and its environmental impact. Chromium occurs naturally in the environment and is present in air, water, rocks, and soil.

Natural sources of water contain very low concentrations of chromium and the two most common types are chromium (VI) and chromium (III), the form present depends upon the pH.

The waste products from the different applications of chromium can find their way into natural water sources - either illegally or intentionally - and elevate the levels of chromium present.

Chromium (III) is an essential mineral that is important in carbohydrate metabolism and is an active component of glucose tolerance factor having beneficial effects on the blood sugar control mechanisms.

It helps to maintain normal cholesterol levels and improves high-density lipoprotein levels as well as being important in maintaining muscle and reducing obesity through the promotion of a serotonin neurotransmitter that controls the human appetite and curtails cravings for sugar. Chromium is difficult to store with around 3% of dietary chromium being retained in the body primarily in the spleen, kidneys, and testes with smaller amounts in the heart, pancreas, lungs, and brain.

Chromium is lost from the body through urination and, as one increases in age, the amount of chromium that the body is able to store decreases.

The use of chromium supplementation is necessary when conditions of impaired glucose tolerance such as diabetes are encountered.

Recent scientific evidence suggests that chromium (VI) present in the human body may be reduced to chromium (III) in the acidic environment of saliva and gastric juices.

Chromium (III) is much less toxic than chromium (VI) and is seldom found in potable waters.

Chromium (VI) is often found in waters and has been associated with birth defects and cancer. It has been shown to be toxic in aerosol form, causing damage to the skin and upper respiratory system possibly leading to lung cancer.

Plants and animals do not tend to accumulate chromium so high levels in the environment can cause toxicity to them.

The toxicity may be expressed as skin lesions or rashes as well as potential liver and kidney damage.

Chromium (VI) can enter waterways from industrial cooling towers where chromic acid is used as one of the water treatment chemicals to inhibit metal corrosion.

In the United States, the Environmental Protection Agency has recently banned the use of chromium (VI) from building roof cooling towers that were leaking coolant into the air; it was estimated that these had contributed to some 20 deaths from cancer.

In California, concerns over chromium (VI) have increased in recent years after the release of the movie 'Erin Brockovich', based upon the real life account of a woman who exposed contamination in a Californian desert town.

This was coupled with a health scare in the city of Davis, southern California, that forced state health officials to draft emergency regulations when it was discovered that public drinking wells contained levels of chromium (VI) much higher than those previously thought.

The fact that trivalent chromium is an essential mineral for the wellbeing of everyday activity, while hexavalent chromium remains a suspected carcinogen, ensures that one has to be conscious of the type of chromium being quantified as well as being aware that both forms may actually be present. Thus, organisations should never be complacent in terms of effluent chromium waste and a strict adherence to quality control should be ensured and maintained at all times to ensure compliance with local regulations in force.

Method for analysis of chromium in (effluent) waters.

Samples of effluent or wastewaters often contain large amounts of organic material that may require additional sample treatment before analysis can be undertaken.

The Metrohm 705 UV digester is eminently suited to this task and can eliminate moderate to high (after dilution) amounts of dissolved organic matter that can disturb the voltammetric analysis of chromium.

1ml of effluent sample and 9ml of deionised water were added to the quartz cuvettes in the UV digester.

The pH of the solution was reduced to 1.9 with concentrated hydrochloric acid before the addition of 100ml of hydrogen peroxide to aid the photolytic generation of OH radicals.

The samples were then digested at a temperature of 90C for a period of 90 minutes.

7.5ml of deionised water, 2.5ml of UV digested sample, and 2.5ml of electrolyte buffer were added to the reaction vessel in the Metrohm 757 VA Computrace.

The pH of the solution was buffered to 6.2 +/-0.1 using sodium hydroxide. The role of the electrolyte and additional solutions in voltammetry is crucial.

Many determinations are pH-dependent and the electrolyte can increase the conductivity and selectivity of the solution.

The solution was then degassed with nitrogen for a period of five minutes to remove the electrochemically-active oxygen, before the chromium was determined with two standard additions using the HMDE.

The HMDE is an electrode mode of the MME.

Four mercury drops of a defined size are formed in succession at the MME and the last drop remains suspended on the end of the capillary.

The entire voltage sweep is then performed on the single stationary drop.

The digestion of organic matter is achieved through photolytic generation of OH radicals that in turn react with the organic compounds and decompose them. Hydrogen peroxide serves as an initiator of the radical reaction and a mercury lamp provides radiant energy that is converted to heat accelerating the digestion process.

The advantage of UV photolysis over other sample digestion techniques is that only a little hydrogen peroxide has to be added ensuring that blank values can be kept low.

This is crucially important for analysis of effluent (and other) waters as the low content of chromium coupled with the extreme sensitivity of the technique means that the samples are prone to contamination if extreme care is not taken.

Chromium speciation studies.

It is possible to do chromium speciation investigations upon chromium (III) and chromium (VI) with voltammetry.

Chromium (III) and chromium (VI) ions form a complex with diethylenetriamine pentaacetic acid (DTPA). Depending on the sample preparation procedure and the waiting time employed after the addition of the complexing agent then the different chromium species can in theory be distinguished.

In order to speciate, one has to perform two voltammetric analyses upon the sample of interest.

First the total chromium quantified as chromium (VI) is measured by oxidation of chromium (III) to chromium (VI).

Chromium (III) can be oxidised to chromium (VI) using oxidation by UV irradiation.

The acidified sample is first digested by UV digestion at a temperature of 90C for a period of 60 minutes, then the pH is raised to allow oxidation by further irradiation at 90C for 30 minutes.

Secondly chromium (VI) is measured, but after the addition of DTPA a waiting time of 20 minutes is employed.

During this period, the chromium (III) - DTPA complex becomes electrochemically active.

The value reported for chromium (III) is then the difference between the two values obtained for total chromium and chromium (VI).

As a final comment, it should be noted that chromium (VI) is extremely unstable in solutions with low pH and the success of chromium speciation depends upon the amount of chromium present in any sample.

Chromium analysis using voltammetry with the HMDE, is an ultra sensitive technique capable of reaching detection limits well below 100ng/l and as a result, the linear range is relatively limited.

Conclusion.

Chromium ions are introduced into different waterways from industry as waste from various processes and their release must be monitored and carefully controlled.

The different types of water do not pose problems for chromium analysis with voltammetry, due to the reliable, durable nature of the method and quantification can be performed whether the water samples are potable, effluent or saline in their origins. Voltammetry is an increasingly popular technique that in many instances offers unrivalled detection limits even when compared to vastly more expensive analytical techniques.

On samples other than effluents, then often voltammetry requires little or no sample preparation and the result determined by standard addition obtained in less than ten minutes.

The advantage of using standard addition as a means of calibration and quantification is that matrix effects present in the sample are taken into account. Voltammetry requires no specialist laboratory infrastructure like expensive fume extraction.

All that is required is a sturdy bench top on which to mount the instrument and a regulated flow of an inert gas.

The running and maintenance costs of voltammetry are minimal, ensuring a cost-effective analytical solution to surpass the demands required by those organisations interested in quantifying chromium from a variety of water sources.