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Lab-3 Background

Importance of Water

* Solvent
Water is an excellent solvent, which means that many chemicals dissolve easily into it (referred to as dissolved species, Fig. 4.2).
* Dipolar
Water molecules have one end that is more negatively charged, and one end more positively charged (Fig. 4.1), allowing it to bond or complex with most charged species and with itself.
* Benefits
Makes water a useful cleaning agent, and important transporter of chemicals in all living things
* Costs
Water can easily be contaminated with undesirable chemicals

Figure 4.1: Dipolar nature of water molecule and intermolecular hydrogen bond. The two white hydrogens are more positive than the red oxygen (which is not sharing two of its electrons).
Image dipolar-water

Figure 4.2: Dissolution of NaCl in water. After U. Arizona Biology Project .
Image naCl_inWater

pH and Dissociation of Water

It gets more complicated though, because

* Dissociation
each water molecule can ``come apart'' (termed dissociation or ionization of water)
* Acid-Base
dissociation creates acids (the H$^+$ ion) and bases (the OH$^-$ ion)
* Acid-Base Reactions
the exchange (i.e. donation by the acid or acceptance by the base) of the proton (H$^+$) is the basis of many chemical reactions (acid-base reactions), especially in water
* Examples
e.g. eating citric acid (tangy sensation), using muriatic acid on concrete (dissolves stains that water alone can't get out), taking ``Tums'' (a base) to neutralize gastric acid
* pH
the measure of concentration of protons (H$^+$ ion) in water, or essentially the strength of the proton donation reaction.
* pH Definition
pH is the negative logarithm of the concentration of H$^+$. So an acid has low pH, and therefore high concentrations of H$^+$, and can participate more readily in reactions that require donation of a proton.
* neutral pH
at neutral pH there are equal ``concentrations'' of H$^+$ and OH$^-$ in the solution. At room temperature neutral pH is 7. ``Neutral'' really means that there is equal tendency solution for donation or acceptance of protons.
* Consequences
for example metals (which can often be toxic) tend to be immobile in acid environments. If we want to understand the chemical state of a water, we must measure its pH as well as concentrations of dissolved species.

Figure 4.3: Dissociation reaction of H$_2$O.
Image h2o_pH_reaction

Figure 4.4: The pH scale, where high pH indicates high concentrations of protons (H$^+$ ions), and a high potential for proton donation.
Image pH_scale

Chemical Reactions and Temperature

We must also measure temperature of our water because:

Figure 4.5: Variation of pH vs. temperature.
Image pH_vs_T


Since many dissolved species of interest are metals, we must also characterize the oxidation (``rusting'' and transport) potential of the water

Other Parameters

Some other parameters are useful:

* Turbidity
the cloudiness of the water. An indicator of suspended particulates, which can transport bacteria. EPA Drinking Water limit is 1 NTU (nephelometric turbidity units). Easily controlled by settling or filtration.
* Electrical Conductivity
is an indication of the amount of dissolved ions. The more ions, the easier it is for electricity to move through the water (essentially electrons hop from ion to ion, the more ions, the easier that is). Typical values for drinking water are around 300 $\rm\frac{\mu S}{cm}$ (e.g. Fine Waters website). Specific Conductance is the electrical conductivity adjusted to 25 ${\rm ^\circ C }$, to allow direct comparison of waters that have differing temperatures.
* Salinity
also known as TDS or total dissolved solids, given in ppm. Often inferred from specific conductance or computed as the sum of all dissolved species. The Safe Drinking Water Act limit for TDS is 1000 ppm.
* Nitrate
usually a remnant of fertilizer application, maximum allowed in drinking water is 10ppm. Measured using spectrophotometer (colorimeter)
* Hardness
the potential to form carbonate scale, this is the sum of Ca and Mg, usually dominated by Ca
* Alkalinity
essentially the concentration of the anion HCO$_3^-$, which is dominant in many surface water systems. Alkalinity is reported as ppm CaCO$_3$

next up previous contents
Next: Lab-3 Activities Up: Water Quality Measurements (Lab Previous: Water Quality Measurements (Lab
GEOS 3110 Professor's Notes, Summer 2007
Dr. T. Brikowski, U. Texas-Dallas. All rights reserved.