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Chemistry Of Natural Water

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 thru 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 n 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 revent 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 ary 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 ater. 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 thru 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 spectrum is obtained. This is matched with cataloged known values to attain a reading on concentration. Because there are so many problems with hard water, we decided that perhaps the water on Penn State’s campus should be examined. My partners and I decided to test levels of divalent ions (specifically Mg++ and Ca++ ) in successive floors of dormitories.

We hypothesized that the upper level dormitories would have lower concentrations of these divalent ions because seeing as how they are both heavy metals, they would tend to settle out of solution. The Ca++ should settle out first seeing how it is heavier than the Mg++, but they will both decrease in concentration as they climb to higher floors in the dormitories. PROCEDURE We collected samples from around Hamilton Halls, West halls. In order to be systematic, we collected samples in the morning from the water fountains near the south end of the halls.

We collected water samples from each floor in order for comparison. The reason we collected them in the morning was so that the Mg++ and Ca++ would be in noticeable quantities. We then went about and tested and analyzed via serial titrations and via Atomic Absorption Spectrophotometry. We also obtained a TDS sample merely for the sake of comparison, and to ensure that were in fact dissolved solids in our water samples (without hich this lab would become moot). For the serial titration, we merely mixed the water sample with EBT, and then with increasing concentrations of EDTA.

The EBT served as an indicator to tell us when the concentrations of the EDTA and the divalent ions in solution were equal (actually it told us when Mg++ was taken out of solution but that served the same purpose). This allowed us to find the concentration of the divalent ions dissolved in solution. Based on this, we calculated the parts per million and the grains per gallon for each water sample. Finally, we took an AA reading for each sample. This gave us absorption values and oncentration values for each of the two main metals we were observing; Ca++ and Mg++.

We then plotted a graph of Atomic Absorption Standards. These were values given to us by the AA operator. These values helped us to calibrate the machine. The parts per million that we find will be based on plugging in the reported absorption value into the resulting curve from the graph of these values. The resulting concentration was used as the final value for the hardness for that particular sample. All calculations and conclusions were done based on these final values obtained for the concentration of Ca++ and Mg++.

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