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The Chemistry of Natural Water

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INTRODUCTION

The purpose of this experiment is to explore the hardness of the water on campus. Hard water has been a

problem for hundreds of years. One of the earliest references to the hardness or softness of water is in

Hippocrates discourse on water quality in Fifth century B.C. Hard water causes many problems in both in

the household and in the industrial world. One of the largest problems with hard water is that it tends to

leave a residue when it evaporates. Aside from being aesthetically unpleasing to look at, the build up of

hard water residue can result in the clogging of valves, drains and piping. This build up is merely the

accumulation of the minerals dissolved in natural water and is commonly called scale.

Other than clogging plumbing, the build up of scale poses a large problem in the industrial world. Many

things that are heated are often cooled by water running through

piping. The build up of scale in these pipes

can greatly reduce the amount of heat the cooling unit can draw away from the source it is trying to heat.

This poses a potentially dangerous situation. The build up of excess heat can do a lot of damage; boilers

can explode, containers can melt etc. On the flip side of the coin, a build up of scale on an object being

heated, a kettle for example, can greatly reduce the heat efficiency of the kettle. Because of this, it takes

much more energy to heat the kettle to the necessary temperature. In the industrial world, this could

amount to large sums of money being thrown into wasted heat.

In addition to clogging plumbing and reducing heating efficiency, the build up of hard water also

adversely affects the efficiency of many soaps and cleansers. The reason for this is because hard water

contains many divalent or sometimes even polyvalent ions. These ions react with the soap and although

they do not form precipitates, they prevent the soap from doing it's job. When the polyvalent ions react

with the soap, they form an insoluble soap scum. This is once again quite unpleasing to look at and stains

many surfaces.

The sole reason for all these problems arising from hard water is because hard water tends to have higher

than normal concentrations of these minerals, and hence it leaves a considerable amount more residue

than normal water. The concentration of these minerals is what is known as the water's Total Dissolved

Solids or TDS for short. This is merely a way of expressing how many particles are dissolved in water.

The TDS vary from waters of different sources, however they are present in at least some quantity in all

water, unless it has been passed through a special distillation filter. The relative TDS is easily measured

by placing two drops of water, one distilled and one experimental on a hotplate and evaporating the two

drops. You will notice that the experimental drop will leave a white residue. This can be compared to

samples from other sources, and can be used as a crude way of measuring the relative TDS of water from a

specific area. The more residue that is left behind, the more dissolved solids were present in that

particular sample of water. The residue that is left, is in fact, the solids that were in the water.

Another, perhaps more quantitative way of determining hardness of water is by calculating the actual

concentrations of divalent ions held in solution. This can be done one of two ways. One is by serially

titrating the water with increasing concentrations of indicator for Mg++ and Ca++ (we will be using

EDTA). This will tell us the approximate concentration of all divalent ions. This method of serial

titrations is accurate to within 10 parts per million (ppm) .

Another possible method for determining the hardness of water is by using Atomic Absorption

Spectrophotometry or AA for short. AA is a method of determining the concentrations of individual

metallic ions dissolved in the water. This is accomplished by sending small amounts of energy through

the

water sample. This causes the electrons to assume excited states. When the electrons drop back to their

ground states, they release a photon of energy. This photon is measured by a machine and matched up to

the corresponding element with the same E as was released. This is in turn is related to the intensity of

the light emitted and the amount of light absorbed and based on these calculations, a concentration value

is assigned. A quick overview of how the atomic absorption spectrophotometer works follows. First, the

water sample is sucked up. Then the water sample is atomized into a fine aerosol mist. This is in turn

sprayed into an extremely high intensity flame of 2300 C which is attained by burning a precise mix of

air and acetylene. This mixture burns hot enough to atomize everything in the solution, solvent and solute

alike. A light is emitted from a hollow cathode lamp. The light is then absorbed by the atoms and an

absorption

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