teoria agua purificada

7/29/2019 Teoria Agua Purificada

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The water molecule has another interesting characteristic:its ability to dissociate itself. This isexpressed in the equation shown:

TheoryWATER-PURIFICATION

Water is a very simple molecule (two hydrogenatoms combined with one oxygen atom), but thatsimple combination allows the emergence ofexceptional properties that have made water themost important solvent on earth and an essential

support to the development of life.The oxygen atom of the water molecule is moreelectronegative than the hydrogen atom (i.e., itattracts more the electrons). As a result, the side ofthe molecule with the oxygen atom has a partialnegative charge. A molecule with such a chargedifference is called a dipole.

The dipole moment of thewater molecule is shown inthe drawing:

The Water Molecule

Figure 1:Water moleculewith dipole

This dipole moment causes water molecules to beattracted to each other (the slightly positivehydrogen side being attracted to the slightlynegative oxygen side) and to other polarmolecules. This attractionis named hydrogenbonding, and explainsseveral of the water properties.

Figure 2:Hydrogen Bonding -Schematicrepresentation

The water molecule dipole and the resultinghydrogen bonding explains several of the waterproperties such as water high melting and boilingpoint temperatures that are due to the energyrequired to break the hydrogen bonding, the factthat ice has a lower density than liquid water, thehigh surface tension, heat of vaporization andviscosity of water and, to some extent, the ability ofwater to dissolve many substances.

Water is an excellent solvent for salts because theseare made of negatively and positively charged ionsthat will be surrounded by the dipolar watermolecules

Figure 3: Salt dissolved by water molecules

Figure 4:Demonstrationof ultrapurewater easy& fastcontamination

Water is also a good solvent for neutral organicmolecules and establishes hydrogen bonding withmany molecules involved in the life processes suchas glucose, proteins, nucleic acids....

Because water is an excellent solvent, ultrapure

water is easily contaminated.In a classical experience, ultrapure water with aresistivity of 18.2 Mcm @ 25C was left in a beakerand the evolution of its resistivity measured overtime. The results are shown in FIG4 and demonstratethe rapid contamination of water by carbon dioxidefrom the air, leading to the production of HCO3- in

What is true for CO2 is also true for other chemicalsubstances that may be present in the air of a laboratorysuch as acid fumes (from nitric acid or chlorhydric acid)and volatile solvents (such as toluene, acetone ortetrahydrofuran).

Ultrapure water is also easily contaminated by materials

extracted from containers such as sodium and silica fromglass, plasticizers and ions from polymeric materials (forinstance, phthalate esters from PVC pipes, fluoride from PTFEpipes) and metallic ions from metallic containers.

This is the reason why, in order to minimize the risks ofcontamination and the experimental variability that can becaused by these contaminants, ultrapure water should beproduced just before usage and used at once forglassware final rinsing or the preparation of solutions.

Water Physico-Chemical PropertiesFor many physico-chemical parameters, water hasoutstanding values; many of these values can be explainedby the hydrogen bonds that link water molecules together.Some key parameters of high purity water are listed below:

PropertyUltrapure

WaterValue

Unit Comment

Boiling Point 100 C at 1kg/cm

Conductivity 0.055 S/cm at 25C

Critical Point 374 C

Density (ice) 920 g/L

Density (liquid) 997.07 g/L at 25C



Dielectric Constant 78.4 at 25C

Ebullioscopic constant 0.51 K kg/mol

Entropy of vaporization109 J K-1 mol-1

Heat of fusion 5.98 kJ mol-1

Heat of vaporization 40.7 kJ mol-1

Melting Point 0 C

Molarity 55.346 moles/L

Molecular mass 18.0153 g/mole

Molecular Weight 18.0153 Dalton

pH 7 pH units at 25C

Resistivity 18.2 Mcm at 25C

Surface Tension 72.75 mJ/m

Surface Tension 72.75 mJ/m at 20C

Thermal Conductivity 0.58 J-K-1m-1s-1

Triple Point 0.01 C at 611.73Pa

Viscosity 0.89 cP at 25C

Water Dissociation & pH

the water and the resulting decrease of resistivity from 18.2Mcm @ 25C to 4 Mcm @ 25C in about one hour.


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WATER-PURIFICATIONTheory

With Kw as the equilibrium constant of the waterdissociation equilibrated reaction. Due to the law ofmasses action: [ H+] [ OH-] = Kw [ H2O]

As the concentration of water is a constant for agiven temperature is a constant, the equation canbe simplified as follows: [ H+] [ OH-] = K

With K as theionization constant ofwater, expressed in(moles/L). The valueof K changes withtemperature.

Temperature (C) K (mol/L)

18 0.77 10-14

25 10 -14

50 5.5 10-14

This means that at the reference temperature of25C, at equilibrium, [H+] = [OH-] = 10-7 moles/L.

The value of pH (from the French potentialhydrogene) is defined as the logarithm base 10 ofthe proton concentration: pH = - log [ H+]

And therefore, the value of pH for pure water iscalculated as follows: pH = - log [ 10 -7] = 7.0

Questions are often raised about the value of pHfor ultrapure water and attempts to measure thepH of ultrapure water in laboratories sometimesdeliver puzzling results. This is simply due to the factthat regular laboratory pH meters have not beendesigned to measure the pH of ultrapure water thatcontains very little ions. Therefore the results obtainedare a mix between the effects of quickcontamination of ultrapure water by carbon dioxidefrom the ambient air and artefacts generated by themeasuring device.

The fact that the pH of ultrapure water with resistivityequal to 18.2 Mcm @ 25C can only be equal to7.0 (actually, it is equal to 6.998) has been

demonstrated in an article published in AmericanLaboratory News June/July 2006 : High-Purity Waterand pH by Estelle Riche, Aude Carrie, Nicolas Andinand Stephane Mabic.

In this article, it is demonstrated that the pH ofultrapure water with resistivity equal to 18.18 Mcm@ 25C is equal to 6.998 (lets say 7.0 as a quickapproximation). Any attempt to change that pHvalue, for instance the introduction of a strong acidsuch as HCl or a strong base such as NaOH, whiledecreasing or increasing the pH, will also result in adecrease of resistivity because additional ions areintroduced in the water.

A resistivity value of resistivity equal to 18.18 Mcm@ 25C is therefore a synonym of a pH value equal

to 6.998 at 25C and ultrapure water cannot haveanother pH value.Water on Earth

The volume of water on earth is evaluated in thetable below:

Water Reserve Volume in km3 Volume (%)

Oceans and seas 1 320 000 000 97.2

Ice (glaciers, ice caps) 25 000 000 1.8

Ground Water 13 000 000 0.9Surface Water

(Lakes, swamps, rivers) 250 000 0.02

Atmospheric Water(clouds, rain) 13 000 0.001

Overview of ContaminantsInorganic Ions:Inorganic ions commonlypresent in tap water arecations, such as sodium,calcium, magnesium or iron,and anions, such asbicarbonate, chloride andsulfate. Many other ions can

be present depending on the water source. Inorganic ions,even at trace levels, may affect both organic andbiochemical reactions by acting as catalysts.Organics:Dissolved organic moleculespresent in tap water are mainlyof biological origin. Moleculesincluding humic acids, tannins,and lignin are the by-productsof the decay of plants.However, man-madecontaminants may be introduced by the pipes carrying thewater. For example, PVC pipes may leak their phthalateesters plasticizers into the water. Dissolved organics can

affect biological experiments such as cell culture & disturbanalytical techniques. Even moderate organiccontamination present in water used to prepare LiquidChromatography eluents can cause baseline instability anddecrease sensitivity and resolution, therefore decreasingchromatography column lifetime.

Particulates and Colloids:Natural water usually containssoft particulates (vegetal debris)and hard particulates (sand,rock) as well as colloids thatcan interfere with instrumentoperation.

Bacteria and their By-Products:Bacteria contaminate naturalwater, especially surface water.The chlorination process willensure removal of harmfulbacteria, but tap water stillcontains live micro-organisms.

Bacteria can cause different issues in laboratoryexperiments either directly or through their by-products,such as pyrogens, nucleases or alkaline phosphatase.

Earth is sometimes named the Blue Planet due tothe large amount of water present on its surface.Oceans and seas cover more than 70% of the earthsurface.

Gases:Natural water containsdissolved gases such as

nitrogen, oxygen and carbondioxide. The concentration ofoxygen can affect specificbiochemical reactions andnitrogen can form bubbles thatare detrimental to processes such as particulate countingor spectrophotometric measurements.


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Overview of MonitoringTwo classes of contaminants, inorganic salts anddissolved organics, are known to affect mostlaboratory experiments and it is therefore importantthat they are monitored online in laboratory water

systems.Measuring Resistivity to Detect IonsElectrical conductivity is a measure of a materialsability to conduct an electric current. Water itselfhas a weak electrical conductivity. Electric currentis transported in water by dissolved ions, makingconductivity measurement a quick and reliable wayto monitor the total amount of ionic contaminants inwater. As conductivity also is related to ion mobility(which increases with temperature), conductivityvalues usually are reported compensated at 25C, in &mrico;S/cm @ 25 C. Resistivity, expressedin Mcm @ 25 C, is the inverse of conductivity.Traditionally, conductivity is used for values above 1

&mrico;S/cm @ 25 C, and resistivity for conductivityvalues below 1 &mrico;S/cm @ 25 C. Water qualityof high purity therefore will be expressed as resistivityin Mcm @ 25 C.

Measuring TOC to Detect OrganicsTOC means Total Oxidizable Carbon and issometimes referred as Total Organic Carbon.Water may contain hundreds of different organicsubstances at different levels of oxidation, and atdifferent concentrations. TOC expresses the organiccontamination of water with a single figure.

Monitoring

Lab Water Grades

Norms define different laboratory water grades for

both technical and economical reasons. Thepurpose of these norms is to ensure that the rightwater quality is used for a specific application, whilelimiting laboratory operating costs - Type 1 water ismore expensive to produce than Type 2 or Type 3water.

Type 3

Type 3 water is the lowest laboratory watergrade, recommended for glassware rinsing,heating baths and filling autoclaves, or to

feed Type 1 lab water systems.

Type 2

Type 2 water is the grade used in generallaboratory applications such as buffers, pHsolutions and microbiological culture media

preparation; as feed to Type 1 water systems,clinical analyzers, cell culture incubators and

weatherometers; and for preparation of

reagents for chemical analysis or synthesis.

Type 1

Type 1 water is the grade required for criticallaboratory applications such as HPLC mobile

phase preparation, blanks and sampledilution in GC, HPLC, AA, ICP-MS and other

advanced analytical techniques; preparationof buffers and culture media for mammaliancell culture and IVF; production of reagents

for molecular biology applications(DNA sequencing, PCR); and preparation of

solutions for electrophoresis and blotting.

Using Type 1 water for Type 2 water applications isa common laboratory practice in order to decreasethe risk of artifact generation during experimentalprocedures.

Laboratory Water SpecificationsDifferent published norms define the quality requiredfor specific laboratory water applications: ASTMand ISO 3696 for laboratory applications; CLSIguidelines for clinical laboratories. Some laboratorieswill also use norms defined in the European or the USPharmacopoeia.

The table below outlines the different waterspecifications based on the different water types:

Contaminant Parameter & unit Type 3 Type 2 Type 1

IonsResistivity

(Mcm @ 25C)>0.05 >1.0 >18.0

Organics TOC (ppb)


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Microporous filters canbe classified in threecategories: depth,surface and screen.Depth filters are

matted fibers ormaterials compressedto form a matrix thatretains particles by random adsorption orentrapment. Surface filters are made from multiplelayers of media. When fluid passes through the filter,particles larger than the spaces within the filter matrixare retained, accumulating primarily on the surfaceof the filter. Screen filters (also called membranefilters) are inherently uniform structures which, likea sieve, retain all particles larger than the preciselycontrolled pore size on their surface.The distinction between filters is importantbecause the three serve very different

functions. Depth filters are usually used asprefilters because they are an economical wayto remove 98 % of suspended solids andprotect elements downstream from foulingor clogging. They owe their high capacity tothe fact that contaminants are trapped andretained within the whole filter depth. Surfacefilters remove 99.99 % of suspended solids andmay be used either as prefilters or clarifyingfilters. Screen (microporous membrane)filters are 100 % efficient at retainingcontaminants larger than their pore size. Thesefilters are placed at the furthest possible point ina system to remove the last remaining traces of

resin fragments, carbon fines, colloidal particles

WATER-PURIFICATIONTheory

Microporous Filters:

Depth Filter

Screen Filter

& microorganisms. Forexample, 0.22 mMillipore membrane filters,which retain all bacteria,are routinely used tosterilize intravenoussolutions, serums andantibiotics.

Elix Continuous Deionization:

This technology is acombination of

electrodialysis and ionexchange, resulting in aprocess which effectivelydeionizes water, while theion-exchange resins arecontinuously regeneratedby the electric current in theunit.This electrochemicalClick to englarge

regeneration replaces the chemical regeneration ofconventional ion-exchange systems.

The Elix module consists of a number of cellssandwiched between two electrodes. Each cellconsists of a polypropylene frame onto which arebonded a cation-permeable membrane on oneside, and an anion-permeable membrane on theother.

The space in the center of the cell, between theion-selective membranes, is filled with a thin bed ofion-exchange resins. The cells are separated fromone another by a screen separator.

The feed water entering the module is split into three parts.A small percentage flows over the electrodes, 65-75 % ofthe feed passes through the resin beds in the cell, and theremainder passes along the screen separator between thecells.

The ion-exchange resins capture dissolved ions in the feed

water at the top of the cell. Electric current applied acrossthe module pulls those ions through the ion-selectivemembrane towards the electrodes. Cations are pulledthrough the cation-permeable membrane towards thecathode, and anions through the anion-selectivemembrane towards the anode. These ions, however, areunable to travel all the way to their respective electrodessince they come to the adjacent ion-selective membranewhich is of the opposite charge. This prevents furthermigrations of ions, which are then forced to concentratein the space between the cells. This space is known as theconcentrate channel, and the ions concentrated in thisarea are flushed out of the system to the drain.

Ion Exchange:

The ion-exchange process percolates water throughspherical, porous bead resin materials (ion-exchangeresins). Ions in the water are exchanged for other ionsfixed to the beads. The two most common ion-exchangemethods are softening and deionization. Softening is usedprimarily as a pretreatment method to reduce waterhardness prior to reverse osmosis (RO) processing.The softeners contain beads that exchange two sodiumions for every calciumor magnesiumion removedfrom softenedwater.

Figure 1:Deionization

Deionization (DI) beads exchange either hydrogen ions forcations, or hydroxyl ions for anions. The cation-exchangeresins, which are made of polystyrene chains cross-linkedby divinylbenzene with covalently bound sulfonic acidgroups, will exchange a hydrogen ion for any cations theyencounter (e.g., Na+, Ca++, Al+++). Similarly, the anion-exchange resins, which are made of polystyrene polymerchains with covalently bound quaternary ammoniumgroups, will exchange a hydroxyl for any anions (e.g.,Cl-, NO3-, SO4--). The hydrogen ion from the cationexchanger unites with the hydroxyl ion of the anionexchanger to form pure water.

These resins may be packaged in separate bed exchang-ers with separate units for the cation and anion exchangebeds. Or, they may be packaged in mixed bed exchangerscontaining a mixture of both types of resins. This last con-figuration enables more efficient ion removal and provideshigher water resistivity values.

The resin may be regenerated by strong acid and basesonce it has exchanged all its hydrogen and / or hydroxylions for charged contaminants in the water. This regen-eration reverses the purification process, replacing thecontaminants bound to the DI resins with hydrogen andhydroxyl ions. However, this is a harsh chemical process that

may damage the polymerchains constituting the beads,leading to contamination ofthe resin by organics andparticulates, and creating anissue in the production of highpurity water.


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Ultrafiltration:

Ultrafiltration

Ultraviolet (UV) Radiation:Ultraviolet radiation has been widely used asa germicidal treatment for water. Mercury lowpressure UV lamps generate light atdifferent wavelengths, including 185 and 254nm. UV lamps with a regular quartz sleeveallow passage of 254 nm light. These lampsare an effective means of sanitizing water.The adsorption of UV light by the DNA in themicrobial cells results in the inactivation of themicroorganism.

UV lamps with a very pure quartz sleeve allowpassage of both 185 and 254 nm UV light. Thiscombination of wavelengths is necessary for thephotooxidation of organic compounds, whichultimately allows conversion of dissolved organicsubstances into carbon dioxide. With these speciallamps, Total Oxidizable Carbon (TOC) levels in highpurity water can be reduced to 5 ppb.

Activated Carbon:Activated carbon is made of organic materialporous particulates containing a maze of smallpores, which account for the substances highlydeveloped surface. One gram of activated carbonhas a surface of up to 1 000 m2. Organic moleculesdissolved in water may enter the pores and bind totheir walls due to van der Waals forces. The

adsorption process is controlled by the diameter ofthe pores in the carbon filter and by the diffusionrate of organic molecules through the pores. Therate of adsorption is a function of molecular weightand the molecular size of the organics.

Natural activated carbonproduced by treating vegetalproducts such as coconut shellsat high temperature. The resultof this process is a fine powdermade of irregularly shaped

grains. Natural activated carboncontains a high concentration ofionic contaminants & is thereforeused only as a pretreatmentstep to remove excess chlorinefrom tap water by a reductionreaction and, to some extent, toreduce organic contamination.Synthetic activated carbon ismade by the controlled pyrolysisof polystyrene spherical beads.This cleaner material is used forthe removal of trace organics oflow molecular weight.

Reverse Osmosis: Reverse osmosis (RO) is the mosteconomical method ofremoving 95 % to 99 % of allcontaminants. The pore structureof RO membranes is muchtighter than that of UFmembranes. RO membranes arecapable of rejectingpractically all particles,bacteria and organics > 200Dalton molecular weight(including pyrogens) at a rate

close to 99 %. Natural osmosis occurs when solutions withtwo different concentrations are separated by a semi-

permeable membrane. Osmotic pressure drives waterthrough the membrane; the water dilutes the moreconcentrated solution; and the end result is an equilibrium.

In water purification systems, hydraulic pressure isapplied to the concentrated solution to counteractthe osmotic pressure. Pure water is driven from theconcentrated solution at a flow rate proportional toapplied pressure and collected downstream of themembrane.

Because RO membranes are very restrictive, theyyield slow flow rates per surface unit. Storage tanksare required to produce an adequate volume in areasonable amount of time.

RO also involves an ionic exclusion process. Onlysolvent (i.e., water molecules) is allowed to passthrough the semi-permeable RO membrane, whilevirtually all ions and dissolved molecules are retained(including salts and organic molecules such assugars). The semi-permeable membrane rejects salts(ions) by a charge phenomenon action: the greaterthe charge, the greater the rejection. Therefore, themembrane rejects nearly all (> 99 %) strongly ionizedpolyvalent ions but only 95 % of the weakly ionizedmonovalent ions like sodium. Salt rejection increasessignificantly with applied pressure up to 5 bar.

Different feed water may require different types of ROmembranes. Membranes are manufactured fromcellulose acetate or thin-film composites of

polyamide on a polysulfone substrate.If the system is properly designed for the feed waterconditions & the intended use of the product water,RO is the most economical and efficient method forpurifying tap water. RO is also the optimumpretreatment for reagent-grade water polishingsystems.

A microporous membrane filter removes particlesaccording to pore size. By contrast, an ultrafiltration(UF) membrane functions as a molecular sieve. Itseparates dissolved molecules on the basis of theirsize - often reported as the molecular weight (both

parameters are related, but not always directly) - bypassing a solution through an infinitesimally fine filter.

The ultrafilter is a tough, thin, selectively permeablemembrane that retains most macromolecules abovea certain size (Nominal Molecular Weight Limit, orNMWL) including colloids, microorganisms andpyrogens. Smaller molecules, such as solvents andionized contaminants, are allowed to pass into thefiltrate. Thus, UF provides a retained fraction(retentate) that is rich in large molecules and afiltrate that contains few, if any, of these molecules.

Ultrafilters are availablein several selective ranges.In all cases, the membranes

will retain most, butnot necessarily all,molecules abovetheir rated size.In water purification,ultrafilters are routinelyused to providepyrogen-free andnuclease-free water for critical cell culture ormolecular biology experimentation. The key pointhere is the validation process, which ensures that theultrafilter, when challenged by pyrogens, RNases orDNases at levels far above those likely to occurduring regular operation, will be able to reliablydeliver water within specification.

Activated carbon used in water purification isavailable in two forms:

MRC.VER.01-6.12

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