The Chemistry Of Natural Water Biology Essay

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All water contains some level of hardness. One probably has heard the terms "hard water" or "soft water", but what do these terms mean, and what do they refer to? To answer the questions, we should closely pay attention to the cations with charges +2, or divalent cations, such as Ca2+ and Mg2+. Water hardness is a property of water that can be determined by its concentration of Ca2+ and Mg2+ 1. Water is considered to be hard, when it contains a large concentration of Ca2+ and Mg2+. Conversely, water is said to be soft when it contains a low concentration of such divalent cations.

Water hardness is important to households and factories since it measures the impurity of water. If the water is too hard, it could create precipitates and thus can create problems. In the household scale, hard water might create minor effects. Doing laundry with hard water may not be very effective because hard water decrease the cleaning capabilities of soaps and detergents2. Moreover, hard water causes the unsightly white rings on bathtubs, basins, and cooking utensils. In the industrial scale, however, water hardness could lead to a more significant problem. Accumulated precipitates from hard water clog up pipelines in factories and require more maintenance. In addition, scale buildup and mineral deposits reduce the heating efficiency in pipelines and boilers. This results in metal overheating, creating more pressure in the system, and leads to hazardous situation.

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To measure the water hardness, we must to do the titration. After an experiment is performed, the results were used to determine the water hardness by comparing them to the hardness scale provided by the U.S. Department of Interior and the Water Quality Association3.

Table 1. Classification of water hardness (hardness as calcium carbonate).

Classification

Mg/l or ppm

Soft

0 - 17.1

Slightly hard

17.1 - 60

Moderately hard

60 - 120

Hard

120 - 180

Very hard

180 and over

In this experiment, two scientific methods were used to determine the concentration of divalent cations dissolved in the solution, Atomic Absorption Spectrophotometry (AA) and ethylenediamietetraacetic acid (EDTA) titration. The two methods were used to prove that they should provide the similar results. The most distinctive difference between them is that AA requires less amount of time to process and obtain the result than EDTA because AA is performed conveniently by machine while EDTA is performed manually by scientist. However, because of the fact that AA uses machine to determine the hardness value, it is much more costly.

In general, AA method can also be used to determine the concentration of particular metal elements in the sample. Absorption occurs when the change in energy level of an atom is equal to that of the incident light, or hv = ΔE 4 since each particular atom has a unique energy level, The technique of AA relies on this principle, which is obtaining the absorbance value from an atom to calculate the concentration value. Because of the fact that the absorbance value is proportional to the concentration of metal ions in the sample, the concentration value can be obtained by using the Beer-Lambert law.

The instrument used for AA technique is called AA spectrophotometer. It can be separated into three main parts5. Fist part is where the light would be emitted. It contains hollow cathode lamps (HCL's), acting as light sources for the atom to absorb. According to the principle of AA, the type HCL must correspond with the tested element. For instance, if we were to find the concentration of Mg2+, the Mg lamps must be used.

Second part is basically functioned to turn the liquid solution to ash, or aerosol. It contains a sample chamber and a burner. In this step, the liquid is aspirated into the instrument and is burned with a very high temperature of approximately 2300 oC. At this high temperature liquid sample is turning into atoms. The atoms will then absorb the light from the first section when the energy of the light matches with their energy separation.

Third part contains three components including, a monochrometer, a detector, and an internal computer system. The monochrometer consists of a plane grating and a curved mirror. The grating is carefully adjusted so that it would allow only the wavelength of the light from section two to pass through the narrow slit. Because the atoms only absorbed some of the passing light through the flame, there will be a decrease in the initial signal. The detector which is a photomultiplier tub (PMT) would then detect this decrease. Since the decrease is proportional to the concentration of the metal dissolved in the sample, the concentration value could then be evaluated by the Beer-Lambert law, It = Io (10-abc), where It is the transmitted intensity, Io is the intensity of the initial light source, a is a constant, b is the path length of light, and c is the concentration.

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The EDTA technique can be simply explained as a method using titration as the principle. This principle is based on the reaction involving four solutions, Ca2+ and Mg2+, eriochrome balck T (EBT) indicator, NH3/HN4Cl MgEDTA, and EDTA5. To test for the concentrations of Ca2+ and Mg2+, first, an equal amount of EBT and NH3/HN4Cl/MgEDTA and the water sample must be combined using a small drop microburet. The next step is the titration. To do this, the experimenter would use the microburet to suck up some amount of the EDTA solution, and begin titrating. The experimenter would count every time he or she drops the solution. The EDTA will be dropping until the sample begins to change color from pink to blue, and the number of drops will be recorded. To calculate the concentration value, the conservation of moles principle is applied, mole of EDTA = moles of sample. This yields MEDTAVEDTA = MsampleVsample

After calculating the water hardness, one might find that the water sample is too hard, and might want to decrease its hardness. Decreasing the water hardness, or softening the water can be done simply by adding an amount of water-conditioning product that contains Ca(OH)2 or Na2CO3 5.These two compounds would chemically react with Ca2+ and Mg2+ in the water and cause precipitates, Mg(OH)2 or CaCO3. The precipitates will be collected, leaving us with the softened water. The chemical reactions involved in the softening process are shown in the following:

Adding Ca(OH)2; Ca(HCO3)2 + Ca(OH)2 → 2CaCO3(s) + 2H2O

Adding Ca(OH)2; Mg(HCO3)2 + 2Ca(OH)2 → 2CaCO3(s) + Mg(OH)2 (s) + 2H2O

Adding Na2CO3; CaSO4 + Na2CO3 → CaCO3(s) + Na2SO4

Four different water samples were obtained in order to compare the hardness of each sample: PSU dorm water from Shunk Hall, Aquafina, Brita filtered PSU water from Thompson Hall, Deer Park. It is expected Deer Park and Aquafina are soft water since they are the drinking water. On the other hand, I also expected that the Shunk Hall water is very hard since State College is mountainous and thus, has less well. Furthermore, it is also expected that Brita filtered water from Thompson Hall is less hard than Shunk Hall water since some of the cations would have been filtered by the filter. Thus, it is supposed that the water hardness would decrease in order of Shunk Hall water, Brita filtered Thompson Hall water, Deer Park and Aquafina. I conducted a research on Aquafina bottled water and I was able to obtain the procedure that the company uses to purify their water. The webpage Aquafina Company6 explains their steps to purify water and its scale is bigger and complicated than that of Brita Company7 does. The concentration of Ca2+ and Mg2+ may vary depend on the time the filter has been used. Between the two bottled water samples, we supposed that Aquafina would be slightly softer than DeerPark since it is a generic brand which the quality is usually low, and the water is from spring that makes us to think it could contain more mineral8. Furthermore, it is also expected that the values also expect that the values that will be obtained from this experiment will closely follow what The Water Encyclopedia9 predicts.

Procedure

Each of our group members prepared the four different water samples: Brita water sample was prepared by Maggie, Deer Park was prepared by Timothy, Shunk Hall water was prepared by Ryan and I prepared Aquafia. This experiment was performed in a group of four. The purpose is to find water hardness in each water sample, and draw a conclusion by comparing the results with the hypothesis. Each member did their own experiment using two methods, AA and EDTA titration.

For AA technique, the water sample was separated into to two containers, one for testing Ca2+ and another for testing Mg2+. It was made certain that the water is two-third full. The sample then was be aspirated into the instrument and the absorbance value was obtained for each sample. These values were kept to find the concentration in ppm, together with the calibration graphs of each element. Finally, the two converted values for Ca2+ and Mg2+ are summed up to give the total hardness value.

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For EDTA, One drop of water sample was added to each well of 1 x 12 well strip and one drop of EBT indicator, and one drop of MgEDTA buffer was added to each well. Then, the serial titration was carried out with EDTA solution - i.e., 1 drop of EDTA to the first well, 2 drops to the second well, etc. However, I found out that my sample is somehow too soft and thus, a new set of 1 x 12 well was made by putting 5 drops of Aquafina sample. Then the titration was continued by following the same procedure. Furthermore, the hardness of the water sample was calculated by using the formula below.

MEDTA VEDTA=Msample Vsample

For TDS experiment, a small piece of aluminum foil was prepared. Next, 1 drop of Aquafina sample, 1 drop of distilled water and 1 drop of 1 x 10-3M Ca2+ were dropped on the foil. Then the samples were allowed to evaporate to see what remained after the evaporation of the water samples. The result was recorded.

For Water Softening experiment, Aquafina water sample was mixed with water conditioning product, then the same procedure for EDTA titration was conducted. The result was also recorded for the further discussion.

Results

From the number of Drops of EDTA, hardness of water sample was calculated by using the formulas suggested in Chemtrek.

Sample calculation with Aquafina (EDTA method):

Table 1. Water Hardness by EDTA Titration

Water Sample

Drops of EDTA

Drops of sample

Dilution

factor

Molarity

(mol/L)

Water Hardness (ppm)

Grain per gallon

Brita

6

2

x

0.0006

60

3.51

Shunk Hall

12

1

x

0.024

240

14.04

Aquafina

1

5

x

0.00004

4

0.234

DeerPark

4

2

x

0.0004

40

3.51

Furthermore, it was able to plot the graph of light absorbance vs Calcium/Magnesium ion concentration by using the data set provided below:

Table 2. Ca2+ concentration and absorbance value data set

Ca2+concentration

(ppm)

absorbance value

(at 422.7mm)

Check standard

(ppm)

1

0.01621

1.02

5

0.07625

5.11

10

0.13471

9.66

25

0.31458

25.71

50

0.54286

50.52

Table 3. Mg concentration and absorbance value data set

Mg2+concentration

(ppm)

absorbance value

(at 422.7mm)

Check standard

(ppm)

1

0.02919

1.12

5

0.11884

5.08

10

0.22131

9.82

25

0.52823

25.75

30

0.60006

29.49

Chart 1. Light Absorbance vs. Calcium ion Concentration

Chart 2. Light Absorbance vs. Magnesium ion Concentration

From the relationship between light absorbance at 422.7 nm and metal ion concentration, it is able to find out the best-fit linear line, metal ion calibration curve for AA. The equation of the curve is used for calculating Ca2+ and Mg2+ concentration from AA values. For AA values I have 0.0011 for Ca2+ and 0.031 for Mg2+.

Sample calculation with Aquafina (AA method):

â…°) Ca2+:

â…±) Mg2+:

Total Water hardness = -3.4625ppm + 4.012ppm = 0.5495ppm

Table 2. AA Result and Water Hardness

Water Sample

Absorbance Value, Ca2+

Absorbance Value, Mg2+

Dilution

factor

Water Hardness(ppm)

Brita

0.0264

0.0864

2

17.69

Shunk Hall

0.2722

0.2821

2

114.14

Aquafina

0.0011

0.031

x

0.5495

DeerPark

0.0424

0.0685

x

17.70

Note: If there are too many metal ions dissolved in water sample, atomic absorption spectrophotometer cannot give the proper value, so Brita water and Dorm Water is diluted with distilled water (1 to 1 ratio) to avoid the problem.

Table 3. Water Hardness by EDTA Titration and AA

Water Sample

Hardness by EDTA(ppm)

Hardnessby AA(ppm)

Difference

(EDTA - AA ppm)

Brita

60

17.69

42.31

Shunk Hall

240

114.14

125.86

Aquafina

4

0.5495

3.451

DeerPark

40

17.70

22.30

Table 4. TDS Determination

Water sample

Observation

Distilled water

White ring was barely observed. It can be considered as nothing.

Brita

somewhat thicker ring than reference

Shunk Hall

thicker ring than reference

Aquafina

somewhat ring compare to reference

DeerPark

thicker ring than reference

1 x 10-3M Ca2+ (reference)

Faint ring

Water softening

Water hardness was calculated with same method used above for EDTA titration. 1 x 10-3M Ca2+ sample was used because Aquafina water sample was too soft. The color changed from the 5th well8. Thus, molarity can be calculated from the equation below:

Ion exchange

1 x 10-3M Ca2+ sample was also used because Aquafina water sample was too soft. The color changed from 2nd well8. Thus, molarity can be calculated from the equation below:

Table 5. Water softening results

Water softening method - 1x10-3M Ca2+

Molarity Ca2+

(M)

Water softening method - Brita Filtered Thompson Hall water

Molarity Ca2+

(M)

Water softening method - Shunk Hall

Molarity Ca2+

(M)

None

1.20x10-3

None

6.00x10-4

None

2.40x10-3

Commercial conditioning product

1.00x10-3

Commercial conditioning product

4.00x10-4

Commercial conditioning product

1.20x10-3

Ion exchange

4.00x10-4

Ion exchange

3.00x10-4

Ion exchange

2.00x10-4

Discussion

The hardness of water samples were determined by using two different techniques - EDTA titration and AA. However, there is a limitation for EDTA titration since we cannot determine the hardness of water if it contains too much metal ion in it since there are only 12 wells in a strip. EDTA needs more accurate and careful performance while AA does not relatively. Furthermore, since EDTA is able to react with any divalent cation content in the solution, the presence of the other divalent cation dissolved in water could higher the value of water hardness than that of AA. Therefore, AA hardness is more reliable. As I expected before I start the experiment, the water hardness determined by AA technique and EDTA titration decreased in order of tap water of the dormitory, water filtered by Brita, DeerPark, and Aquafina. The water sample obtained from Shunk Hall was extremely hard compare to other water samples since it was not drinking water while other three samples were. Moreover, Aquafina water sample was much softer than what I had expected; the measured water hardness for Aquafina water sample in this experiment was nearly zero.

Some differences between water hardness measured by AA and EDTA titration are found. Generally, water hardness resulted in EDTA titration has higher than that resulted in AA. The reason of the disagreement comes from how each technique derives to attain the concentration of Ca2+ and Mg2+. AA determines concentration of only Ca2+ and Mg2+ in the sample by light absorbance of each metal ion while EDTA titration uses reaction of EDTA solution and divalent cations in the sample to find concentration of Ca2+ and Mg2+. Since EDTA solution is able to react with cations other than Ca2+ and Mg2+, water hardness resulted from EDTA determination is higher. This meant that EDTA titration provides a more general and rough data and requires more accurate and careful performances. AA can be considered to be a relatively more reliable, precise, and accurate method, measuring atomic absorption spectrophotometer.

From water softening experiment, concentration of Ca2+ decreased from 1.20 x 10-3 M to 1.00 x 10-3 M and to 4.00 x 10-4 M by Commercial conditioning product and Ion exchange respectively. Furthermore, from Table 5, it is able to know that the concentration of Ca2+ also decreased for other 2 samples. This indicates that water hardness decreased, and those water softening methods are effective.

Conclusions

With the results of the experiment, the hypothesis was proved to be right. With the results from AA and EDTA titration, it was able to tell that the three water samples, Brita, Aquafina, and DeerPark, were much softer than Shunk Hall water sample. Even though there are some differences between water hardness values in two methods, both concludes water in Shunk Hall is hardest, and Aquafina is softest. Those determined water hardness of the water sample practically agree with outside sources. EDTA titration had generally higher water harness values than those determined by AA. Compare to EDTA titration, AA technique is more accurate and precise. Also, water softening techniques were practiced that proved its function.