The F Test
3.2 Methodology, Terms and Definitions
3.2.2 Qualifications for a Trace Analysis Laboratory
3.2.2.1 Water Purification
Pure water is an essential element of a chemical analysis laboratory. Since purity is a rather relative concept, each laboratory should try to produce water that has suffi- cient purity for the analyses on agenda. Most laboratories start this process by using tap water. Undesired impurities consist of cations, anions and uncharged molecules including organic compounds and free chlorine, Cl2. Majority of elements that are subject of trace analysis are present as ions, while most metals are in cationic form, some elements are in anionic form, such as MnO4⫺, AsO33⫺and CrO42⫺. One crite- rion for water purity is low conductivity (or high resistivity); many manufacturers claim that the product has a resistivity as high as 18 MΩ. However, it must be remembered that this does not ensure the absence of many uncharged species, such as organic molecules. Another point worth noting is that high resistivity is indicated by sensors built in the water purification systems. As soon as the pure water is out of the purification system, carbon dioxide is rapidly dissolved in pure water, result- ing in the production of bicarbonate and carbonate ions. Therefore, if a resistivity measurement is to be made on a pure water sample outsidethe purification system, values much lower than 18 MΩwill be observed. This result does not mean that the system is not efficiently purifying water, but is an indication of the difficulty for preservation of high resistivity in ambient air. In such a case, the water will still be pure enough to have a sufficiently low blank for many analyses, because the pres- ence of dissolved carbon dioxide lowers resistivity, but does not introduce any con- tamination except CO2, HCO⫺3and CO32⫺species.
Elimination of organic components is necessary especially when chromatographic techniques are used, as some detectors are capable of detecting the organic impuri- ties. Therefore, in order to avoid any unpleasant surprises, most chemists prefer to obtain water at the highest level of purity. The following principles of operation or their several combinations in a certain logical sequence are used in water purification.
Hardness removal. This is usually a preliminary step, rather than an effort to lower conductivity. Usually, columns containing inorganic ion exchangers in Na⫹form are used. Most of Ca2⫹ and Mg2⫹are eliminated but an equivalent amount of Na⫹is introduced. The resulting resistivity is not much altered. However, the possibility of scaling in any further distillation process is minimized. Ion exchangers used in these systems can and should be regenerated by NaCl solution.
Distillation. Water is boiled, its vapour is transferred and is then condensed. Most of the ionic impurities are eliminated. Molecules more volatile than water are not eliminated by this method. Tap water should be introduced into a distiller after elim- ination of cations causing hardness, as explained above. Otherwise, scaling lowers the efficiency of heating elements and slows down the process of distillation; unit cost of distilled water increases as more electrical power will be consumed. Sometimes, oxi- dizing compounds such as KMnO4and Na2CrO7are used in the boilers; therefore,
organic molecules are oxidized and decomposed. This system, although was very common in past, is not used very much since attention is required during operation.
Another approach is subboiling distillation; water is evaporated by infrared radiation below its boiling point. The process is slower as compared to normal distillation by boiling, but the resulting purity is higher. This system is also successfully used in purification of mineral acids.
Ion exchange. Mostly polymeric cation and anion exchangers, which are in H⫹ and OH⫺forms, respectively, are used for removing ionic impurities in water. The relevant equations are given below for removal of cationic, M⫹, and anionic, A⫺, species. Similar equations can be written for ions of multiple charges:
R⫺H⫹⫹M⫹R−M⫹⫹H⫹ (3.1)
R⫹OH⫺⫹A− R⫹A⫺⫹OH⫺ (3.2)
Water that is purified by ion exchange is called deionized water and the process is often called deionization. Ion-exchange resins have a capacity that will be exceeded after a certain degree of loading; at this point, the resin material should be renewed or regenerated if possible. Working principles of ion-exchange resins are illustrated in Figure 3.1. If separate columns are used in series, as shown in Figure 3.1A, the regen- eration of each column material can be easily done by using HCl for cation and NaOH for anion resins. If a mixed bed is used, regeneration is not practical since the respec- tive resin materials must first be separated; columns operating on the principle of
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Figure 3.1 Water purification by deionization using cation and anion exchangers. (A) sequential beds, (B) mixed bed
mixed bed must therefore be renewed. However, in mixed beds, released H⫹and OH⫺ combine to form water; this constitutes another driving force for more efficient removal of cations and anions, resulting in an obvious advantage for using a mixed bed. All the commercial systems use mixed resins:
H⫹⫹OH⫺H2O,K⫽(Kw)⫺1⫽1.00⫻1014 (3.3) Filtration. A column filled with a fine particular material that is not soluble in water is used to filter any solids present in water. Pure quartz is commonly used for this purpose. In addition, most commercial systems have a 0.22-µm filter at the end, just prior to the exit of pure water.
Activated carbon. Activated carbon reduces free Cl2 to Cl⫺ that can be easily removed by ion exchange. In addition, many organic molecules or even transition metals are adsorbed on carbon surface. Such an element is usually placed before the ion exchangers.
Reverse osmosis. The principle of reverse osmosis has been used in purification of seawater to obtain water that is free of NaCl for common use. The same principle is also used in laboratory water purification systems. Pressure is applied on water, and molecules are forced to pass through a size-selective membrane; while water mole- cules are small enough to pass, larger ions or compounds are eliminated. In most commercial systems, chemical species larger than 100 Da are eliminated.
Electrodeionization. This is a rather novel approach to obtain water pure enough to be fed to high-cost–high-performance water purification systems. The mechanism is illustrated in Figure 3.2. In this system, membranes selectively permeable to cations or anions are used. An electrical field is applied to attract the cations to cath- ode and anions to anode. The pathway full of both cation- and anion-exchange resins
Figure 3.2 Water purification by electrodeionization AM, anion permeable membrane; CM, cation permeable membrane (Adapted from Ref. 3 with permission form Millipore Corporation, France)
are in contact with these membranes. The location of these membranes is designed in such a way that purified water and separated ionic contents are directed into dif- ferent channels, called as ‘product’ and ‘waste’, respectively. The purified water can be stored until further use. The presence of ion-exchange resins facilitates ionic migration. It should be noted that since the ions are not permanently retained on the ion-exchange materials, these columns do not need to be regenerated.
Ultraviolet radiation. High-energy UV radiation damages bacteria, by either killing them or preventing their reproduction. Water is subjected to ultraviolet radi- ation as it passes through a channel that is radiated by an UV source lamp. This part is usually located towards the end of a purification train of several steps.
Typical water purification systems and general considerations. Water purification apparatuses are commercially available in the form of either separate modules or combined instruments, capable of functioning according to one or several of the principles discussed above. The following points should be considered in order to build and/or buy a water purification system.
Most elements in a water purification system require continuous attention and service. Ion-exchange columns should be checked for efficiency and replaced or regenerated when necessary. Distillation systems should be serviced and periodi- cally cleaned from residues and scaling products that lower efficiency. While inter- nal sensors such as a resistivity-measuring device sometimes indicate the purity, external checks should also be made. Any increase in water blanks should be care- fully followed, and the performance of the water purification system should be kept at the desired level.
In most cases, tap water is the initial material to obtain pure water. Tap water con- tains impurities at high levels. The first step should be an inexpensive way of purifi- cation so that the product, water of intermediate purity, can be fed into more expensive, high-purity systems. This approach ensures a higher lifetime for more expensive components such as renewable cartridges.
Most important and abundant species causing water hardness and thus scaling are Ca2⫹and Mg2⫹. If any kind of distillation is to be used, these cations should be elim- inated prior to being fed into a distiller; this can be done by water softening or elec- trodeionization. Theoretically, an electrodeionization system does not require regeneration of ion-exchange material and therefore is expected to have a longer lifetime.
Proper storage of purified water is as important as its production. Regarding mate- rial for storage containers, PTFE and some other fluorinated polymers are best but they are expensive. Many other polymeric materials such as high-density polyethyl- ene are also suitable for production of storage containers. Period of storage should be kept to minimum. Glass should not be used for the storage of high-purity water;
otherwise, high blanks for some elements such as boron should be expected.
Sequential systems for water purification can be used either as a continuous train or with some intermediate stages of storage, especially if the speeds of water pro- duction are not compatible for components.
Some systems for water purification are schematically given in Figure 3.3. The general strategy is first to prepare water of intermediate purity that can be fed into a high-purity water production system or used for analyses that do not demand very
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high purity. At this stage proper storage has to be used. The final stage is used for production of high-purity water; the lifetime of components and their performance is highly improved if the preceding steps are properly designed and used.