The Chemistry of Natural Water

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 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

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 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 which 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 concentration 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++.

For more detail, refer to full in depth procedure as directed by: Penn State Version of...

Chemtrek

August 1996 - July 1997; Stephen Thompson; Prentice Hall; Englewood Cliffs, NJ

07632; © 199

RESULTS

Molarity x (100g CaCO3 / 1 mole CaCO3 ) x (1000 mg / 1g) = Xmg/1000g = ppm

Grains/Gallon = ppm /17.1

Example:

(1.6 x 10 -3 moles / 1 Liter) x (100g CaCO3 / 1 mole CaCO3 ) x (1000 mg / 1g) = 160

ppm

160 ppm/17.1 = 9.35 grains/gallon

Serial Titration Results

Name: # Molarity Parts Per Million Grains Per Gallon

Samir Sandesara 1 1.6 x 10 -3 160 9.35

Andy 2 1.6 x 10 -3 160 9.35

Ben 3 1.2 x 10 -3 120 7.01

Tom 4 1.8 x 10 -3 180 10.5

Table #1: This table displays the values obtained by serial EDTA titration of the water

samples.

Conversion Factors Given by AA operator: Ca++ = 2.5

Mg++ = 4.2

Ca++ x 2.5 = CaCO3 hardness ppm value

Mg++ x 100 x 4.5 = Mg CO3 hardness ppm value *NOTE: the Mg++ is x 100 because

it was diluted

before it was

put into the AA.

Example:

Ca++: 27.52 x 2.5 = 68.8 ppm 4.02 g/gal

Mg++: .251 x 100 x 4.2 = 105.42 6.16 g/gal

Atomic Absorption Values

Name

: # Abs

Mg++ Abs Ca++ AA ppm

Mg++ AA ppm

Ca++ ppm

Mg++ ppm

Ca++ g/Gal

Mg++ g/Gal

Ca++

Samir 1 0.2270 0.5923 0.251 27.52 105.42 68.8 6.16 4.02

Andy 2 0.2041 0.5493 0.225 25.10 92.40 62.75 5.40 3.67

Ben 3 0.3633 0.5800 0.401 26.83 168.22 67.07 9.88 3.90

Tom 4 0.2673 0.5589 0.295 25.65 123.90 64.11 7.24 3.75

Table #2: This table displays the values obtained from AA analyzation, and shows

the hardness of the

water as contributed by each

individual element.

Absorbency Values

Parts Per Million

0.000 0.0

0.125 0.1

0.403 0.5

0.716 1.0

Absorbency Values

Parts Per Million

0.0000 0.000

0.0142 0.493

0.0262 0.985

0.0536 1.970

0.2360 9.850

0.4540 19.700

0.9230 49.250

Floor Number Hardness (ppm)

1 174.3

2 159.1

3 235.5

4 188.0

DISCUSSION

The final hardness values were obtained by graphing the AA Standards on the

previous page

and then plugging

in the absorption values give by the AA (Table #2). This is the grey line that appears

in both graphs.

When this

line was extended down from the point of intersection, it was able to read the ppm

value at that point.

The ppm

value for both Ca++ and the Mg++ were then summed to attain the final hardness of

the water.

The other numbers above reveal much about the water in Hamilton Hall. Looking at

the final hardness

values

that were attained, it is clear that the two upper floors had harder water than the lower

floors. However,

table

#2 shows that the concentration of Ca++ decreased overall as the water climbed

higher in the dormitory.

What

was unexpected was that the concentration of Mg++ actually increased as it climbed

higher. As of

present, I have

no rational scientific explanation for this. The only possible explanation I could

possibly think of is

perhaps there

is something within the plumbing that contains Mg and the further the water travels in

it, the more

dissolves of

the Mg dissolves. Aside from that, there does not seem to be any possible

explanation. What is also

interesting

is that with the exception of the #3 sample, the hardness values attained from the AA

were very similar to

those

attained by serial EDTA titration. These indicates a low source of error and gives

support to my numbers.

Even more support is added to the numbers when the ppm values are added up in

Table 2. These values,

for

the most part, also seem to be in a relatively tight "ball park" of the final AA values.

Given that the

accuracy

of serial titrations is ± 10 ppm, it is extremely safe to say that my numbers are correct.

A brief overview of the numbers seems to show that there is indeed a trend, and the

more in-depth look at

the

numbers shows that they all seem to back each other up. This seems to imply a that

most if not all of the

results

are quite accurate and precise.

CONCLUSION

Upon completion of this lab, it can be said that the data supports only half of the

original

hypothesis. Yes, the Ca++ did seem to decrease as the water got further from the

source and climbed

higher in the dormitories. However, the Mg++ did not. Instead it did quite the opposite

and showed a

general trend of increasing in concentration as it got further away from the source and

higher in the

dormitories. Perhaps a viable explanation could be attained if studies were done on

the plumbing inside

the building. Perhaps there is a high concentration of magnesium in the solder used

to hold the pipes

together. Perhaps it is not in the pipes but rather perhaps the people on the upper

floors get up later and

therefore at the time of collection, the water in the upper floors had been sitting longer

than that on the

lower floors. In either case,. More investigation would have to be conducted in order

determine what

caused the unexpected results.

In light of this discrepancy, the overall accuracy of the lab was very good. The

numbers all seem to back

each other up and correlate very well. As was mentioned in the previous section, the

precision and

accuracy with which this lab was carried out seems to indicate that there is very little

source of error. The

only one that was possibly flawed was sample #3. This could have been due to an

error in the dilution or

any other factor. Since I personally did not carry out that portion of the experiment, I

cannot be sure.

However, the other 3 samples provide more than ample ammounts of accurate

information. Overall, it

seems that the lab was quite well done.

The hypothesis would have to be revised and as of this point, without further

investigation, it would have

to be reformulated to say that only the Ca++ would decrease in concentration

whereas the Mg++ would

increase.

REFERENCES

1) Brown, Theodore L. et al; Chemistry The central Science; Sixth Edition; Prentice

Hall, Englewood

Cliffs, NJ; ©1994

2) Stephen Thompson; Penn State Version of...Chemtrek; August 1996 - July 1997;

Prentice Hall;

Englewood

Cliffs, NJ; © 1990

3) Internet Resource; http://www.kinetico.com/hard.htm