Laboratory Grade Water and Dishwashing

The first essential to perform any chemical analysis of water, geological or biological materials is ultra-clean water free of contamination from the analyte of interest.     The degree of water purity necessary will vary with the analyte and the sensitivity required, but in general must be roughly 10 times lower in concentration than the lowest concentration to be measured or the lowest standard.    There are several different methods for removing contaminants from water, which can be split into three categories:  distillation, ion exchange and reverse osmosis.   Often it is advantageous to use a combination of these to get the best attributes of each.

Distillation

               Laboratory stills are typically either of stainless steel or glass construction.   The former generally have copper elements, while the latter have glass coated elements.    Glass stills produce cleaner water, particularly for trace metal analysis, but produce much less water and are more expensive.    Stills work by boiling water in a large chamber, producing steam which enters a condenser.   In high capacity stills, the initial cool feed water goes into the condenser first, where it causes condensation of the steam and is itself pre-warmed.    It then enters a split valve which maintains the water level in the still, while the excess goes to waste.    Inside the still, two metal rods are attached to a voltage source and extend down into the water.   Ions in water conduct electricity (conductuctivity) so that if the water gets low there will be a broken circuit between the rods and the still will automatically shut off.   As steam is produced, the remaining water in the still becomes progressively more concentrated.    In most hard waters, the predominant ions are Ca²⁺ and HCO₃^- (bicarbonate).   As this mixture is heated, CO₂ degasses (most gasses are less soluble in hot water) giving the following reaction:

                     Ca²⁺     +     2HCO₃^-              Ca²⁺     +      CO₂(g)        +    OH^-       +      HCO₃^-

Note that as this reaction occurs, OH- is produced which will raise the pH, which will in turn affect the speciation of the weak acid bicarbonate, converting it to carbonate:

                     Ca²⁺     +      OH^-      +     HCO₃^-             Ca²⁺     +      CO₃^2-     +     H₂O

As the concentration of Ca²⁺ and CO₃^2- increase, the solubility of calcium carbonate (calcite) is exceeded and it precipitates out, covering the elements and the entire bottom of the still. 

                     Ca²⁺     +      CO₃^2-             CaCO₃(s)

Consequently, most stills have to be cleaned with acid (1N HCl) to dissolve the CaCO₃ in the reverse of these reactions about once a month.   The same processes occur in many hot water heaters using hard water, keeping lots of plumbers employed doing acid flushing.   A valve at the top of the water level on the side of the still is called the deconcentrator valve.   It is usually set to drip slowly, which bleeds off some of the concentrated water and reduces the rate of precipitation.    If this valve is set too fast, however, it will cool the water and reduce output.

               All stills exhibit the a property called carryover, in which rapid boiling produces small, undistilled droplets which are carried over with the steam into the condenser, also carrying their load of dissolved solids.   For this reason, most stills only average about 95 - 98% efficiency at dissolved ion removal.   Stills can, however, remove nonionized compounds such as silica, which ion exchanges columns cannot do.    Some trace metal labs use multiple distillation in glass stills (double-distilled water) producing very high quality water.

Ion Exchange

               Most labs use commercial ion exchange units to produce so-called “Type I” water for ion analysis.   The most common of these, in the LCRI lab, is manufactured by Millipore, producing “Milli-Q” deionized (DI) water.    Cation Exchange resins consist of small (0.5 mm) resin beads with surface charged sulfonate sites (where R is the resin particle, and   refers to adsorption, often an electrostatic attraction between two entities of opposite charge):

                                             2R-O-SO₃^- 2H⁺      +       Ca²⁺         2R-O-SO₃^- Ca²⁺   +     2H⁺

               The principle of electroneutrality dictates that any charge (dissolved ion or surface) must have an equal and opposite quantity of charges in its vicinity or an electrical potential will develop.   The latter situation will generally result in a flow of electrons or ions, eventually restoring electroneutrality.    The principle of mass action means that any reaction can be driven to the right if large quantities of one of the reactants are put in.    Since the resin comes in pure H⁺ form, it can remove all of the Ca²⁺ and other cations from solution until a significant amount of the H⁺ has been replaced.  

               The other part of an ion exchange system is the anion exchange resin, which often has quaternary (N with four bonds) ammonium functional groups on a resin bead surface:

                                                            R-NH(CH₃)⁺ OH^-     + Cl^-    R-NH(CH₃)⁺  Cl^-     +    OH^-

The H⁺ and OH^- produced by cation and anion exchange reactions will combine to form H₂O.   They are always produced in equal amounts, due to the principle of electroneutrality.    Milli-Q and similar systems often have four bowls.   In a typical configuration, the first bowl is a pretreatment cartridge, containing activated charcoal and mixed bed (anion and cation exchange resins mixed together), to remove most of the contaminants.   This first column is replaced most often and serves to protect the later columns.   The next two are pure, mixed bed ion exchange columns, and the last is an organic removal cartidge.   These columns contain activated charcoal (charcoal powder treated with acid), which strongly adsorb uncharged organic compounds via London forces.    A set of four replacement cartridges typically runs about $160.00 and lasts 3-6 months.   Their life and performance is greatly extended by using distilled water as the feedstock, in which most of the ions have already been removed.   This also has the added advantage of removing uncharged substances such as silica, which exchange columns do not remove.   In the student lab, we use relatively cheap, non-mixed bed resins, which we can recharge with acid or base (see reactions above), and use over and over again.

Conductivity

The ionic purity of water exiting a still or exchange column can be easily monitored using a Conductance meter.     Conductivity is the ability of a substance (in this case water) to transmit electricity and is the inverse of resistance.    Resistance is measured in units of Ohms, so fittingly conductivity is measured in units of “mhos”.   A conductance probe consists of two square platinum foils 1cm by 1cm exactly 1 cm apart (a volume of 1 ml).   A voltage (electrical potential) is appied to these plates, causing ions in solution to move toward them, which in turn allows electrons to flow onto or off of the corresponding plate, producing an electrical current which is measured with a current meter.    Through Ohm’s law:

                                             V     =      IR

where, V is the voltage applied to the plates (in volts), I is the current flow (in amps) and R is the resistance in ohms.   Resistance (and consequently conductance) can be calculated because two of the three terms are known.

               Theoretically pure water has a resistance corresponding to approximately 18,000,000 ohms or 18 megaohms.   This small conductivity is due to the natural dissociation of water into H⁺ and OH^-.   The extent of this equilibrium reaction, shown below, can be calculated from the K_w (Water dissociation constant):

                              H₂O          H⁺      +     OH^-                              K_w    =      [H⁺] [OH^-]     =     10^-14

Since in absolutely pure water, all H⁺ and OH^- must come from water dissociation, their concentrations have be equal (electroneutrality).   Therefore [H⁺] (the concentration of H⁺ in moles/L, M) and [OH-] must both be 10^-7 M.     The “p” function always refers to the negative log of any concentration, therefore the pH of pure water will be 7.0, as will the pOH.    Milli-Q and equivalent systems have a conductance meter on them which monitors conductivity of the water as it exits.   When all the columns are new, conductance will typically vary between 10 and 18 megaohms.   These values should be monitored daily to assess column performance.

Water, upon contact with air has CO₂ gas dissolve in it, which combines with water to form carbonic acid.    This in turn dissociates to form H⁺ and HCO₃^- which lowers the pH to approximately 5.6:

               CO₂     +      H₂O          H₂CO₃              H⁺     +     HCO₃^-

Normally, water to be used for most analyses is adequate if the measurement with a conductivity probe is less than 1umho.   Use the conductivity meter in the lab to determine the conductance of tapwater,

distilled water, “cheap” deionized water, and Milli-Q deionized water

Glassware Washing

     Water quality analysis requires meticulously clean glassware in every step of the process to avoid contamination.   Cleaning procedures vary, however, depending on the analyte under consideration.   For most inorganic analytes measured at the sub-ppm level (e.g. Ca²⁺, Mg²⁺, Na⁺, K⁺,SO₄^2-, Cl^-), it is adequate to rinse (or soak) the glassware in 1 N (Normal) HCl, which is roughly a 10% HCl solution.   All glassware (unless silanized) has surface active groups in which Si-O-Si bonds have been broken, producing surface active sites (Si-O^-) which can adsorb and retain contaminant cations, even when rinsed copiously with deionized water.   Acid washing exchanges all of the adsorbed cations with H⁺, effectively removing these conaminants. 

Acid-washing procedure:

1.      Use one of the red squirt bottles containing 10% HCl to rinse all of the inside surfaces of the container.   Use a funnel to pour the acid back into the reagent bottle next to the sink so the acid can be reused.   The squirt bottle can also be used to rinse the inside of pipets.    For large glassware, the acid can be poured in directly from the reagent bottle.

2.      Rinse the container 4-6 times with DI water.   It is only necessary to place a small amount of water in the container each time and swirl it to contact the sides.    It is the number of rinses, more than the volume of water, that is important.

3.      Cap the container with parafilm so that dust will not enter, or store upside down on a clean paper towel.   Immediately before using it is wise to do one more rinse unless it is essential to have dry glassware.   Pipets can be stored on their side without capping or they can be stored wet in a large graduated cylinder.

               For trace-metal analysis (sub ppb), it is common practice to use 20% HNO₃ rather than HCl.   The stronger acid will remove strongly bound or highly insoluble trace metals more effectively, and nitric acid is a stronger oxidant than HCl and can attack and oxidize insoluble organic acids on the glass surface which will retain trace metals very effectively.   Since surface active sites on glassware can actually remove significant amounts of trace metals from solution, it is common practice to use teflon dishware for trace metal analysis since it will not adsorb ions from solution.

               For organic analyses, entirely different techniques are used to remove organic deposits or scums.   Nonpolar organic compounds dissolve most readily in organic solvents (remember “like dissolves like” from organic) so dishwashing usually involves the use of organic solvents.

Dishwashing for Organic Analysis 

1.      Using a concentrated soap solution and a scrub brush, meticulously clean the surface with very  hot soap and tap-water.  For trace analysis, the glassware should be heated in this soap solution.

2.      Rinse the glassware repeatedly with very hot tap-water.

3.      Using a squirt bottle in the hood, rinse the sides with acetone (a universal solvent which is miscible in water as well as most other less polar organic solvents).

4.      Rinse repeatedly with milli-Q water.

Questions

1.      Explain why it is necessary to acid-wash glass dishware.

2.      Using chemical reactions explain why stills get coated with calcite.

3.      What is the pH of “pure” water produced by ion exchange both before and after equilibration with atmospheric CO₂?

4.      What is the typical efficiency of a still at removing dissolved ions?

5.      How “pure” does water need to be for a particular analysis?

6.      Explain, in chemical terms, how ion exchange resins work.