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Determination of Calcium Concentration in Various Water Samples Using ICP-AES

Savereide, Anne

Department of Chemistry, Concordia College

Moorhead, MN 56562

Abstract

Various water samples were collected and then analyzed for calcium concentration using

ICP-AES. Samples from the Red River had significantly higher concentrations of calcium than

those from Long Lake, possibly due to a larger amount of sediment. Furthermore, water samples

taken from taps were much lower in calcium concentrations because they had been treated at a

water plant. Analysis using ICP-AES gave reasonable concentrations for all of the samples and

was determined to be a good method of determining calcium concentration.

Introduction:

Calcium is found in almost every water source. It is an important element in

determining water hardness, or the concentration of dissolved magnesium and calcium

ions in water. Water hardness has negative effects at both the domestic and industrial

level. Hard water requires that more detergent or soap be used to get things clean.

Clothes that are washed in hard water may feel rough, and spotting may result on dishes

and glassware.1 The hardness of water also affects the way water tastes. Furthermore, it

can result in lime scale buildup which can ruin plumbing. At an industrial level high

concentrations of calcium (or other metal ions) can lead to galvanic corrosion, an

oxidation-reduction process that occurs when there is a difference in charge between the

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metal piping and the ions in solution.2 For these reasons calcium is usually removed

through water softening in municipal water treatment plants.3 In water softening, lime

(CaO) is added, increasing the pH and causing Ca+2 ions to form calcium hydroxide

which precipitates out of the water, along with many other compounds such as

magnesium hydroxide. Because of this softening process, city water has a lower calcium

concentration than water taken directly from a lake, river or underground source. City

water is a combination of groundwater and whatever surface water is available. For

example, in Moorhead, 80% of the water treated at the Moorhead Water Treatment Plant

comes from the Red River, while only 20% is pumped up from underground.

Groundwater tends to have a higher calcium concentration than water found in lakes or

rivers. It is purified as it percolates through rock and soil to an aquifer.4As it moves, it

picks up many mineral ions, including calcium. Houses that use well water get their

water solely from groundwater. However, most homes that rely on well water use a

residential water softening system to soften their water because of hard water’s

undesirable effects.

While calcium can cause problems with washing and plumbing, it should be noted

that calcium has no health detriments. It is, in fact, essential to one's health. Calcium can

reduce the solubility of toxic ions such as copper and lead. It also provides dietary

calcium, which is an important structural component of bones, vital to various cellular

processes, and is a component of the carbonate buffer system found in blood.5

It is recommended that adults get about 1000 mg of calcium each day.6 Even

hard water only has a calcium concentration of 100 ppm, meaning to get one’s

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recommended calcium solely from water, one would need to consume 10 L of water each

day. For this reason, people get the majority of their calcium through their diet or dietary

supplements. However, calcium from water still makes up a fraction of most people’s

daily calcium intake.

Calcium is the fifth most prevalent element of the earth's crust, making up

approximately 3.6% by weight.7 Calcium dissolves into the water system through

calcium-containing soluble compounds in an aquifer. Some examples of these

compounds would be limestone (CaCO3), gypsum (CaCO3-MgCO3), and dolomite

(CaSO4x2H2 O).

Figure 1: Calcium carbonate concentration in water around the United States (mg/L).8

The amount of sediment and the composition of that sediment are major factors

that influence the concentration of calcium. The composition of the sediment depends on

the geology of the region. Calcium concentration varies throughout the United States

because of the different mineral compositions, as seen in Figure 1. Most of the water

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found in the United States is hard water.8 This is partially because much of the water is

stored as groundwater, which has a higher mineral concentration because the water has

flowed through soil to reach an aquifer picking up minerals along the way. Looking at

the Red River Valley area, it would be expected to see a calcium carbonate concentration

of roughly 121-160ppm. Temperature is also a factor in determining the concentration of

calcium. Unlike the majority of solutes, calcium is more soluble at colder temperatures.7

Within an area of similar geology (meaning a similar composition of sediment),

concentrations should be found at higher levels in water with more sediment, or silicate

minerals. Theoretically, this would mean that there would be more calcium found in

rivers than in lakes because the more turbulent water keeps calcium suspended. There

may also be a greater concentration found closer to the bottom of the river or lake than at

the top because calcium enters the water source by dissolving from the sediment found at

the bottom of the river or lake.

Another source of calcium could be from agricultural lime, which is used to

increase the pH of soil. However, the soils of the Red River Valley are naturally alkaline,

so agricultural lime is not widely used in the area.9

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) can be used to

determine calcium concentration in water.10 In ICP-AES, high power radio frequency

creates a strong electromagnetic field which creates plasma (ionized gas). Plasma excites

the valence electrons calcium ions to an excited state. When they fall back to their

ground state, they release a characteristic wavelength of electromagnetic radiation.

Similarly, calcium concentration can also be determined by atomic absorption

spectroscopy using a nitrous oxide-acetylene flame.11,12,13 Calcium concentration can

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also be measured by titration with EDTA using Murexide or Eriochrome blue black R as

the indicator.14 To determine the calcium concentration in the various water samples to

be analyzed in this experiment, ICP-AES was used, because it is a more accurate and

efficient method than titration, and because nitrous oxide was not available for the atomic

absorption instrument. In this study the concentration of calcium was analyzed in water

samples collected from various water sources including the Red River, Long Lake and

several taps.

Experimental:

Standard Preparation: A stock solution of 10.00 ppm Ca+2 was prepared by

diluting 5.00 mL of 1000 ppm Ca+2 (Ricca) to a 500.00 mL using ultrapure H2O. Next,

using volumetric pipettes, 5 mL, 10 mL, 15 mL, 20 mL and 25 mL of the 10.00 ppm Ca+2

solution were added to 5 different 50.00 mL volumetric flasks and brought to volume

using ultrapure H2O resulting in five standard solution concentrations of 1 ppm, 2 ppm, 3

ppm, 4 ppm, and 5 ppm.

Sample Preparation: A total of ten water samples were run on the ICP-AES. Four

samples were from the Red River, two from upstream, two from downstream. One from

each location was collected close to the surface of the river, and the other at a deeper

depth. Three of the samples came from Long Lake. One sample was collected near the

edge of the lake and two from a deep part of the lake. One sample was taken at a shallow

depth (about 2.5 ft) and the other was taken at a deeper depth (approximately 50 ft). The

other samples consisted of water from a groundwater well in International Falls, city tap

water from International Falls, and tap water from the Ivers Science Building in

Moorhead, MN. The samples collected in the Red River and Long Lake were collected

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using a Van Dorn sampler. The bottles used for sample storage were rinsed several times

with water from the sample source. The samples from International Falls were collected

by Kassie Dotzler. The samples from the Red River, Long Lake, and International Falls

were stored at 4o C until analysis took place. The tap water from Moorhead was collected

just before it was run through the ICP-AES in a beaker that had been rinsed several times

with tap water. The first time the experiment was run, the intensities fell significantly

outside the range of the calibration plot. To solve this problem, each of the ten sample

solutions was diluted by a factor of 10 and run again.

Calibration and Set-Up of the Instrument: A Varian 715-ES ICP Optical Emission

Spectrometer was used in this experiment. The argon gas source was at 80 psi and the plasma

flow rate was 15.0 L/min. The power was set to 1.00 kW. To test for calcium, the

wavelength emission was set to 317.933 nm. First, a blank sample of ultrapure H2O was

run followed by the five standard solutions which calibrated the machine. Next the 10

samples were run and the intensities were obtained and a calibration plot was created

using Excel.

Results and Discussion: Figure 2 shows the calibration plot obtained from the five standard solutions.

Figure 2: Calibration plot used for determine Ca+2 concentrations by ICP-AES.

0.00 1.00 2.00 3.00 4.00 5.00 6.00 1.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

1.40E+06

Concentration (ppm)

Inte

nsity

(c/s

)

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The intensities given for each sample were recorded and converted to

concentrations using the line of best fit and then multiplied by 10 to take into account the

1:10 dilution. The results are summarized in the following table:

Table 1: Results of Analysis by ICP-AES

Analyzed Dilution Sample Sample (bottle #) Intensity (c/s) [Ca+2] (ppm) Factor [Ca+2] (ppm)Red River, Downstream, Deep(18) 1.78E+06 6.7736 10 67.736Red River, Downstream, Shallow (17) 1.71E+06 6.5059 10 65.059Red River, Upstream, Deep (14) 1.67E+06 6.3531 10 63.531Red River, Upstream, Shallow (13) 1.65E+06 6.2862 10 62.862Well Water, International Falls (36) 1.54E+06 5.8795 10 58.795Long Lake, Edge(1) 9.60E+05 3.6576 10 36.576Long Lake, Middle Deep (9) 9.16E+05 3.4914 10 34.914Long Lake, Middle, Shallow (5) 9.11E+05 3.4700 10 34.700Tap Water, Moorhead 5.59E+05 2.1295 10 21.295Tap Water, International Falls (32) 2.11E+05 0.8039 10 8.039

As expected, water taken from the Red River had the highest concentration of

calcium, with an average of 64.804 ppm. This is most likely because the water has not

been treated in any way to remove calcium and because the water is flowing, calcium

containing sediment is stirred up and suspended in the flow of the water.

Long Lake has a naturally low calcium concentration. This makes sense with the

conductivity data that was collected along with the water samples. Pure water does not

conduct electricity, any conductivity that water has is due to dissolved ions, such as Ca+2.

Long Lake samples had an average conductivity of 470 uS/cm while Red River samples

had an average conductivity of 769 uS/cm, which implies that the Red River has a higher

concentration of dissolved ions.

The highest calcium concentration found in Long Lake was found near to the

shore, but the calcium concentration did not vary significantly in different locations or

depths of the lake. This could possibly be explained by the turnover caused by wind

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currents and temperature change that occurs in the spring and fall between the stratified

layers of lakes. The Long Lake samples were collected on October 11th, which could

mean that they were collected during a period of fall turnover. This may have acted to

essentially homogenize the calcium concentration in the lake. To have a better

understanding of how depth affects calcium concentration, it would be necessary to take

more samples over a period of time, preferably during the summer when the lake would

be more stratified.

Relative to the tap water from International Falls, which had a concentration of

8.039 ppm, the well water from International Falls had a high concentration of calcium,

58.795 ppm. The most likely explanation for this large difference is that the tap water

was softened in a treatment plant and/or an at-home filter and the well water was not.

Furthermore, the well water came from an ion-rich groundwater source.

While the samples collected from a deeper depth in both Long Lake and the Red

River had marginally higher concentrations of calcium, the difference between the

shallow and deep depths is not significant enough to support the hypothesis that calcium

concentration is directly related to depth in a body of water.

Based on the data, ICP-AES was an effective and efficient way to determine

calcium concentrations in the various water samples. The concentrations found make

sense for what would be expected from each source. The 2010 Fargo Water Quality

Report stated the level of calcium found in Fargo tap water to be 41.7 ppm, which is

higher than the Moorhead tap water that was tested, which had a concentration of 21.295

ppm, but it is not unreasonably different.

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Conclusion:

Using ICP-AES the calcium concentrations of ten samples were determined.

Water from the Red River had the highest calcium concentration most likely because it

contained the highest amount of suspended particles and amount of dissolved ions in

general. In domestic samples, well-water had a higher concentration of calcium than city

water that was treated by a plant.

Acknowledgements:

Thanks to Kassie Dotzler for bringing back water samples from International Falls.

Thanks to Kara Eken for her support.

Thanks to Dr. Jensen for all of his help and especially for running the second set of ICP-

AES samples.

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13) EPA Method #215.1: Calcium (AA, Direct Aspiration)

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15) City of Fargo Water Treatment Plant 2010 Water Quality Report.

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http://www.cityoffargo.com/attachments/0da7a2b9-c7b4-4431-a0ff-d9e708d28b08/2010%20Water%20Quality%20Report.pdf (accessed December 15, 2011).