Download - CHEM 111 Lab Writeup

The Chemistry of Natu-ral Waters

Michael MartoglioApril 6, 2009

Chem 111Section 104

TA: Shradda SurveGroup: Jun Lee, Hayley Miller, Skyler

Marzewski

I. Introduction

In this lab, we were instructed to set out to explore different ways to test characteristics of

a sample of water and its water hardness. Hard water is evaluated as the type of water that con-

tains high concentrations of the divalent cations Ca2+ and Mg2+. These two cations enter streams,

rivers, lakes and other water bodies by the dissolution of minerals as the water flows through

rocks and soil.(2) The Calcium comes from sedimentary rocks such as limestone and dolomite,

while Magnesium comes from rocks like olivine and pyroxenes. (11) A water sample with high

concentrations of both of these are said to be hard, while lower concentrations are soft.

Because hard water is natural, it can be found basically anywhere and its uses are endless.

Hard water can be used in showers, garden hoses, and even drinking water. Although soft water

is preferred as a drinking water, hard water will not harm you. (12)

Hard water is important because of its use in industrial boilers and evaporators. Also un-

treated waters that are still hard are better in cleaning. When used with soaps and detergents,

they help reduce the chemical bonds between dirt and the surface make it easier to be cleaned.

(11)

Water hardness is measured in units of parts per million (ppm). The higher the ppm, the

harder the water is considered. (5)

In this lab we explored two primary ways of testing water hardness, EDTA and AA. The

EDTA is a titration can only be used when there are significant amounts of Ca2+ and Mg2+. The

EDTA is an inexpensive lab experiment. To use this experiment you take a known amount of

natural water and bring its pH to 10 by use of a buffer. Next you add EBT indicator which turns

the solution blue because of the high pH. This indicator will soon turn a red wine color with the

presence of Mg2+ but does not test for the presence of Ca2+.(11) Then a known concentration of

EDTA solution is added which reacts first with Calcium and then the Magnesium. Once all of

the Magnesium has reacted to form a colorless chelate and is removed from the indicator, it will

return to its blue color. The end of the titration comes when the color is now a definite sky blue.

(11)

The use of an Atomic Absorption Spectrophotometer is a large machine that although is

easier to use it works in a more complicated manner. In region 1 of the machine, you select what

you are testing for (Ca2+ or Mg2+). A beam of light leaves region 1 with the same energy level re-

quired to dissociate a molecule of what you’re testing for. In region 2, about 10% of your sample

is picked up and turned into an aerosol. The aerosol is then released in the form of a flame. The

light passes through the flame and since it has the energy to dissociate the element you’ve been

selected it is absorbed by the aerosol in the flame.(1) The light passes into region 3 through a slit

which only allows the wavelength for the element you selected in region 1. Region 3 contains a

photomultiplier tube (PMT) which reads and determines how much of Ca2+ and Mg2+ is in the

light. This amount is proportional to the amount in your water sample. (11)

These two methods differ because the titration is a very inexpensive experiment that

takes a few minutes to perform. The use of AA is a very large, expensive machine which takes

merely seconds. Also the AA is only capable of calculating the absorption of Ca2+ and Mg2+. The

titration can calculate all polyatomics such as Chloride and Sodium.

Water softening techniques work by using softeners such as sodium carbonate to reduce

the dissolved calcium and magnesium ions in the water. Large scale softening techniques use

negatively charged resin to attract the positive Ca and Mg ions. When the ions attach to the resin,

potassium, hydrogen or sodium ions are released into the water. (5)

I expect all of the water samples to, at some level, be considered hard. I think the water

sample from Whitmore to be the hardest because it is tap water from the bathroom. The sample

from the apartment on Pugh street is also tap water but because it is not on campus I think it will

be softer. The samples from the HUB and the on-campus dorm are from water fountains. Be-

cause these are supposed to be suitable drinking waters, I think they should be softer.

II. Procedure

The procedure for this experiment came step-by-step from the PSU Chemtrek book. (1)

In the first section we used the AA method to determine the level of hardness for our water sam-

ples. Because this is a machine that uses extremely advanced technology, the work on our part

was limited and it’s process was mentioned above. The intensity of the light from region 3 in the

spectrophotometer can be found using the Beer-Lambert Law. Our results will be discussed later.

Next, we dissolved a small amount of our sample and compared it to a drop of distilled water and

1x10-3 M Ca2+. My water sample left a dark circle where it previously was while the distilled wa-

ter left no trace behind and the Calcium left a light ring. This is because the white solids that re-

main are the original nonvolatile salts present in the water.(7) The amount of nonvolatile salts is

known as the total dissolved solids (TDS). (11)

We then performed a serial EDTA titration with Calcium, EBT indicator, buffer, and

EDTA solution. After close observation to find the well where the color changed we used the

formula MEDTA VEDTA = MCa2+ VCa

2+. In this equation M is the molarity of the substances and V is

the volume. For our experiment, the volume was represented by the number of drops in the well

where the initial color change took place. This equation, when solved for MCa2+ will give us the

concentration of the original Ca2+. (11) Again, our results will be discussed later.

Using the same concept as before, we found the hardness of a mixture of 20 mg of a com-

mercial conditioning product in our water sample. We then compared our results.

III. Results

Table 1 shows the absorbance rates for the water samples obtained by using the spectrophotome-

ter.

Table 1- AA Absorption Rates (6,7,8,9)

Sample Name Ca2+ Ab-

sorbance

Mg2+ Ab-

sorbance

Dilution

HUB Skyler 0.2926 0.3214 1:1

Apartment (Pugh St)

Hayley 0.2051 0.1671 1:1

Dorm (Packer Hall)

Mike 0.3186 0.3222 1:1

Whitmore Jun 0.4834 0.5419 1:1

Graph 1 shows the absorbance value versus the concentration of calcium. These numbers were

presented to us when conducting the AA experiment.

Graph 1-

Graph 2 shows the absorbance value versus the magnesium of calcium. These numbers were pre-

sented to us when conducting the AA experiment.

.

Also for section A, after producing these graphs and returning with our Calcium and

Magnesium data we are able to determine the hardness. By using the equa-

tions of the trendline on the graphs and plugging in our values for Ca2+ and Mg2+ respectively we

Graph 2-

can find the light absorbance values. For Calcium, my trendline equation was: y = 0.0111x +

0.0513

Now when I plug in the Calcium concentration from the AA for the y value and solve the

equation for x, I get x= 24.08 ppm Ca2+. I then use the following equation to convert to water

hardness. (11)

100 g CaCO3

1 ppm Ca2+ x 1 mole _ = 2.5 ppm CaCO3 = 2.5 ppm hardness 40.0 g Ca 2+ 1 mole

For example my conversion would be:

100 g CaCO3

24.08 ppm Ca2+ x 1 mole _ = 60.2 ppm CaCO3 = 60.2 ppm hardness 40.0 g Ca 2+ 1 mole

Table 2 shows the results doing the same with my group’s numbers.

Table 2- AA Water Hardness (Ca2+ ) (6,7,8,9)

Sample Name Hardness (ppm)

HUB Skyler 54.3

Apartment (Pugh St) Hayley 34.6

Dorm (Packer Hall) Mike 60.2

Whitmore Jun 97.3

We would do the same of the Magnesium value with the Magnesium trendline equation.

My magnesium equation was: y = 0.0182x + 0.0635

Now when I plug in my Mg2+ concentration into the equation for y and solve for x I get

14.2 ppm Mg2+ . We then must convert this number into water hardness using the following

equation:

100 g CaCO3

1 ppm Mg2+ x 1 mole _ = 4.12 ppm CaCO3 = 4.12 ppm hardness 24.3 g Mg 2+

1 mole

For example, my water hardness would be

100 g CaCO3

14.2 ppm Mg2+ x 1 mole = 58.4 ppm CaCO3 = 58.4 ppm hardness 24.3 g Mg 2+

1 mole

Table 3 shows my groups data going through those same steps.

Table 3- AA Water Hardness (Mg2+ ) (6,7,8,9)


HUB Skyler 58.4



Whitmore Jun 108.2

The total hardness is shown in Table 4.

Table 4- Total Hardness (6,7,8,9)


HUB Skyler 112.7




Whitmore Jun 205.5

Table 5 shows the absorption rates for my group’s water samples obtained by using the EDTA

titration

Table 5- EDTA Titration (6,7,8,9)

Sample Name Drops of Sam-ple

Drops of EDTA Dilution

HUB Skyler 1 39 none

Apartment (Pugh St)

Hayley 1 24 none

Dorm (Packer Hall)

Mike 1 23 none

Whitmore Jun 1 12 none

In section D, we had to determine the hardness of our water samples by counting the

number of drops of EDTA it took to change the color of the well. We use the following equation

with the following variables: MEDTA VEDTA = MCa2+ VCa

2+

“MEDTA”: (2.0x10-4 M)

“VEDTA”: number of drops of EDTA (differs for each sample)

“MCa2+”: What we are solving the equation for

“VCa2+”: 1

For example, the equation for my water sample would be:

(2.0x10-4 M) (23) = (MCa2+) (1)

MCa2+ = 4.6x10-3 M

Table 6 shows the concentration of the samples after plugging the number of drops in.

Table 6- Calcium Concentration (6,7,8,9)

Sample Name Concentration

HUB Skyler 7.8x10-3 M

Apartment (Pugh St) Hayley 4.8x10-3 M

Dorm (Packer Hall) Mike 4.6x10-3 M

Whitmore Jun 2.4x10-3 M

Instead of expressing the concentrations in molarity, we then had to convert it to ppm.

1x10 -3 M 100.0 g CaCO3 1000 mg CaCO3 100 mg CaCO3

1 liter of solution x 1 mole CaCO3 x 1 g CaCO3 = 1 liter of solution

The final number of this equation is equal to ppm

Table 7 shows the concentration of the water samples using the above equation to convert to

ppm.

Table 7- Calcium Concentration in Parts per Million (ppm) (6,7,8,9)

Sample Name Concentration (ppm)

HUB Skyler 780

Apartment (Pugh St) Hayley 480

Dorm (Packer Hall) Mike 460

Whitmore Jun 240

We then took the concentration in ppm and converted to grams/gallon. This was done by the fol-

lowing:

100 ppm x 1 grain per gallon = 5.8 grain per gallon 17.1 ppm

For example my conversion would be:

460 ppm x 1 grain per gallon = 26.9 grain per gallon 17.1 ppm Table 8 shows the Calcium concentrations in grain per gallon.

Table 8- Calcium Concentrations in grain/gallon (6,7,8,9)

Sample Name Concentration (grains/gal-lon)

HUB Skyler 45.6



Whitmore Jun 14.0

In Section E we experimented water softening with our water sample and a commercial

water-conditioning agent. This commercial product was designed to make water softer. We

mixed in 20 mg of the product and then completed an EDTA titration with our water samples.

Our results are displayed in Table 9. The conversions into ppm have already been completed.

(11)

Table 9- Concentration with Commercial Water-Conditioning Agent (6,7,8,9)


HUB Skyler 380



Whitmore Jun 120

The concentration of everyone’s water sample went down with the use of the water softener.

In Section F, we explored the use of cation exchange resins in the removal of Ca2+ and

Mg2+ from our water samples. Again using the EDTA titration we determined the hardness of our

treated water sample. Table 10 shows the concentrations of the samples after they were treated.

Table 10- Concentration after Ion Exchange (6,7,8,9)


HUB Skyler 580



Whitmore Jun 180

IV. Discussion

The AA method resulted in Jun’s sample from the Whitmore lab having the highest level

of water hardness. Haley’s sample from an off-campus apartment had the softest water. The sam-

ples from the HUB and the dorm were similar in numbers. When doing the EDTA Titration, the

Whitmore sample took the least number of drops to change colors. Because of this, it resulted in

the lowest concentration of calcium. The sample from the HUB required the most drops and

therefore had the highest concentration.

The two methods clearly do not agree with each other. They almost show exact opposites.

However the AA method somewhat agreed with my hypothesis whereas the titration did not. It is

most likely possible that while doing our titrations we made significant errors at some point. I

would trust the results from the AA because it is a machine that does the work and there is little

room for error. Because the machine is so expensive and complex, I trust the data more than my

own “do-it-yourself” experiment. During the titration, several things could have gone wrong.

The number of drops could have been counted wrong, not enough sample was used, or my judg-

ing of where the color changed could have been wrong.

During my research, I came across this table of typical values that determine water hard-

ness. Table 11 shows these values.

Table 11- Typical Hardness Guidelines (5)Grains Per

GallonMiligrams

Per Liter (mg/l)or Parts Per

Million (ppm)

Rating

less than 1.0 less than 17.1 Soft1.0 - 3.5 17.1 - 60 Slightly Hard3.5 - 7.0 60 - 120 Moderately Hard7.0 - 10.5 120 - 180 Hardover 10.5 over 180 Very Hard

Looking back, my numbers from the EDTA Titration are very high. The lowest is 240

and the highest is 780. Judging by my research it tells me that these water samples all fall in the

“very hard” category. A justified conclusion cannot be made from these numbers.

The AA hardness levels fall more into the ranges of the rating system I found. The HUB

sample and the sample from the dorm appear to be “moderately hard.” The sample from Whit-

more lab falls into the “very hard” category and the apartment sample is “slightly hard”. I pre-

dicted the Whitmore sample to be the hardest, but thought the drinking waters from the HUB and

the dorm would be softer. Perhaps the apartment water is so soft because it is a very large apart-

ment building and would want to decrease the chance of buildup in pipes as much as possible.

From my tables we also determined that the hardness levels after the commercial condi-

tioning agent was added dropped by half. After the Ion Exchange section, the hardness levels

were decreased by a factor of about 3/4ths.

V. Conclusion

The purpose of this experiment was to gather different water samples and test their hard-

ness levels. Hard water is a type of water that has high concentrations of salts, specifically Cal-

cium and Magnesium. Hard water is not necessarily damaging to your health however it is rec-

ommended to use softer water if possible.

My group gathered four samples each from different places located around our campus.

The samples came from the HUB, an on-campus dorm, Whitmore Lab, and an off-campus apart-

ment on Pugh Street. Prior to the experiment, we hypothesized which sources of water would be

the hardest. We then underwent two tedious experiments to test our hypothesis.

We concluded that the use of a very expensive, complex machine was the most reliable

and effective way to determine water hardness. Using an atomic absorption spectrophotometer

gave us values that we were able to apply to previous experiments and judge their hardness by.

The method that we found did not work as well was the EDTA Titration. This is a very tedious,

experiment where many things can go wrong. However if done correctly and successfully, the

titration provides data which can lead to the same conclusions.

After careful calculations, we received our water hardness values in parts per million

(ppm) and were able to rank them according to previous standings. Our values corresponded to

the rankings in which we predicted in the beginning indicating we had done something right.

VI. References

1) “A Microscale Spectrophotometric Determination of Water Hardness” http:// www.central -

methodist.edu/employee/jgordon/J%20Chem%20Ed%20-%20Spectro%20 Water

%20Hardness.pdf (accessed April 2009)

2) Boyd, Claude E. Water Quality An Introduction; Kluwer Academic Publishers: Norwell,

2000; pp126-128.

3) Guidelines for Chem 111 Formal Report. Chemistry Lecture Text. pp 58-60

4) “Drinking Water: Hard Water”. http://www.ianrpubs.unl.edu/epublic/live/g1274/build/

g1274.pdf (accessed April 2009)

5) “Household Water Quality- Water Hardness” http://www.ext.vt.edu/pubshousing356-4903

56-490.html (accessed April 2009)

6) Lee, Jun, Chem 111 Laboratory Notebook, pp 18-22

7) Martoglio, Michael, Chem 111 Laboratory Notebook, pp 25-30

8) Marzewski, Skyler, Chem 111 Laboratory Notebook, pp 34-38

9) Miller, Hayley, Chem 111 Laboratory Notebook, pp 31-36.

10) “Natural Water”. http://www.science.uwaterloo.ca/~cchieh/cact/applychem/waternatu-

ral.html (accessed April 2009)

11) Thompson, Stephen. PSU Chemtrek; Hayden-McNeil Publishing Inc.: Plymouth, 2006-2007;

pp 10-1 to 10-22

12) “Water Quality and Waste Management”. http://www.bae.ncsu.edu/programs/extension

publicat/ wqwm/tc1.html (accessed April 2009)

http://www.centralmethodist.edu/employee/jgordon/J%20Chem%20Ed%20-%20Spectro%20Water%20Hardness.pdf



http://www.bae.ncsu.edu/programs/extension/publicat/

http://www.bae.ncsu.edu/programs/extension/publicat/

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/waternatural.html

http://www.science.uwaterloo.ca/~cchieh/cact/applychem/waternatural.html

http://www.ext.vt.edu/pu

http://www.ianrpubs.unl.edu/epublic/live/g1274/build/g1274.pdf

http://www.ianrpubs.unl.edu/epublic/live/g1274/build/g1274.pdf