CHEM PH108 Experiment 5A (Revised December, 2020) – Background

Experiment 5A
Calcium Analysis in Drinking Water by Flame Photometry

Spectrophotometry refers to any method of analysis that uses electromagnetic radiation, or light,
to measure concentration. Three basic types of spectrophotometry are used in laboratories around
the world: Absorbance, Emission, and Scattering or Nephelometry.

Absorbance. In absorbance methods, the amount of light absorbed by the molecules or atoms
in a sample is, in general, proportional to the concentration of the molecules or atoms of interest.
The general form of the equation is known as Beer Lambert’s (or simply as Beer’s) Law,
represented by the equation

A = 𝜀𝜀𝜀𝜀𝜀𝜀 𝑜𝑜𝑜𝑜 𝐴𝐴 = 𝑎𝑎𝑎𝑎𝑎𝑎

Where:
• A is the amount of light absorbed;
• ε or a is a constant;
• b is the path length of the light travelling through the sample;
• c is the concentration of the absorbing species in the sample; and
• εb or ab is the slope of the linear calibration curve. 1

Figure 1. This figure demonstrates Beer’s Law: the more

concentrated the solution, the darker the solution and
the more light it absorbs, Reference:
https://www.sciencebuddies.org/

Emission. In emission methods, the molecules or atoms in the sample emit light after being
excited by light, chemicals or heat. In general, the amount or brightness of the light emitted is
proportional to the concentration of the molecules or atoms of interest. The general form of the
equation is

𝑃𝑃𝑒𝑒 = 𝑚𝑚𝑚𝑚
Where:
• Pe is the radiant power or the intensity of the emitted light (it is relative value);
• m is the slope of the linear calibration curve; and
• c is the concentration of the species in the sample emitting the light.

Emission is a bit less common than absorbance, but it still occurs frequently in nature and in our
lives; for example, in the bright colours produced when fireworks explode in the sky; light emitted

1
For details on the units of ε, a, b, and c, refer to Experiment 4A.
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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

from fluorescent tubes; light emitted from welding; or on CSI (a TV show) when black light is
used at a crime scene. The brighter the emitted light, the higher the concentration of the species
in the sample emitting the light.

Li Flame Na Flame Ca Flame

Figure 2. Emission flames for Li, Na and Ca. Li has a red flame; Na has a yellow flame; and Ca
emits a yellow/orange flame. Photos by Fiona Winterton and Dr. Jay Mycroft

Scattering or Nephelometry. In this method, the light is neither absorbed nor emitted, but its
direction is changed by interacting with some type of particles in the sample. As the concentration
of the particles present in the sample increases, more light is scattered. The general form of the
equation is

𝑃𝑃 = 𝑚𝑚𝑚𝑚
Where:
• P is the radiant power or the intensity of the scattered light (it is a relative value);
• m is the slope of the linear calibration curve; and
• c is the concentration of the species in the sample scattering the light.

Humans, even small children, intuitively understand the relationship between the scattering of
light and the concentration of particles that scatter the light. An example is driving during a snow
storm. The more snow that falls, the more light is scattered by the snow flakes, and the harder
it is to see because less light is reaching the eye. Another example is a glass of water that is not
clear because it is suspended with particles. The suspended particles in the water scatter light
and therefore, less light reaches the eye.

Figure 3: Scattering of light by particles in a tube of water.

www.umass.edu/rso/sciout/blue-sky.html

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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

In this experiment, the calcium concentration in drinking water will be measured by emission
spectrophotometry. The instrument or spectrometer used is called a flame photometer. 2

Flame Photometry
A flame photometer is, in its simplest form, a Bunsen or Meker burner with some attached optical
filters/lens, a photocell and a digital readout. See Figure 4. The liquid sample is aspirated and is
mixed with air and natural gas in a small chamber called a nebulizer. Most of the liquid sample
drips from the nebulizer drain, but a small amount of the liquid sample forms an aerosol and
travels with the combustion gases into the burner where it is burned in the flame at a temperature
of about 1,700 oC.

Figure 4. Block diagram of a simple flame photometer. Reference: http://www.bwbtech.com/

Figure 5 demonstrates that a Ca atom in drinking water absorbs the heat energy from the flame,
and an electron is promoted to a higher or excited level. The electron quickly loses energy as it
returns to its ground state and emits an orange-coloured photon of wavelength 622 nm which is
characteristic of calcium. The optical filters filter or remove unwanted or stray light and focus the
emitted orange-coloured photon onto a photocell, also known as a photomultiplier tube. The
photocell produces a voltage/current that is proportional to the radiant power of the light reaching
the cell from the calcium. The higher the concentration of calcium in solution, the brighter
or greater the radiant power or intensity of the calcium light at 622 nm, and the greater
the voltage/current.

Figure 5. Bohr model of a Ca atom showing

the thermal excitation of an electron in the
flame, from the 2nd energy level (ground
state) to the 4th energy level (excited
state). When the excited electron loses
energy as it returns to the 2nd energy level,
a photon characteristic of Ca (orange-
coloured light, 622 nm) is emitted.

2
A flame photometer is a type of spectrophotometer used for measuring the intensity of light in a part of the spectrum,
especially as transmitted or emitted by a particular substance. (Oxford Dictionary)

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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

In this experiment, the Jenway FPF-7 Flame Photometer will be used. This flame photometer can
analyze not only calcium, but also sodium, potassium, barium and lithium. See Table 1 below.

Table 1. Summary of Atoms and Emission Wavelengths Measured By the Jenway Flame
Photometer

Element Emission Wavelength (nm) Flame Colour

Sodium(Na) 589 Yellow
Potassium (K) 766 Violet
Calcium (Ca) as Ca(OH)2 622 Orange
Barium (Ba) 515 Lime Green
Lithium (Li) 670 Carmine Red

• Barium is measured at 515 nm to avoid interference with another Ca emission line at 554 nm.
• The main atomic emission line for Ca occurs at 423 nm; however, calcium is measured by
using the calcium hydroxide band emission at 622 nm.

Figure 6. The elements “boxed” on the Periodic Table above can be analyzed easily by emission
spectrophotometry using a flame photometer. Image scanned from Burriel et al., p. 144.

The Jenway FPF-7 flame photometer uses a low-temperature flame (≈1,700 oC) produced by the
combustion of natural gas and compressed air. Other flame photometers achieve much higher
flame temperatures (≈3,200 oC) by using different combustion gases and can therefore be used
to analyze many more elements. The elements highlighted in Figure 6 can be easily analyzed by
emission or flame photometry.

As simple as flame photometry sounds, in the real world, numerous complications can arise. Some
reagents interfere with the analysis of various metals. For example, the emitted radiation from
the Ca line at 622 nm can be difficult to separate from the light emitted by high concentrations
of Na (emits at 589 nm) and Sr (emits at 605 and 666 nm) in the same sample. In addition, high
concentrations of polyatomic ions like SO42–, C2O42–, and PO43– can result in the precipitation of
CaSO4, CaC2O4 and Ca3(PO4)2; the calcium ion will therefore not be detected. To help reduce
interference, the chelating agent EDTA is often added to the samples to facilitate the detection of
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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

Ca. An understanding of flame photometry and chemistry is therefore required to produce a

meaningful analysis and data.

Preparation of 1,000. ppm and 100.0 ppm Ca2+ Standard Stock Solutions and Calibration
Standards
A set of calibration standards must be prepared precisely and accurately from a pure and stable
form of calcium. In this experiment, the primary standard 3 calcium carbonate (CaCO3) will be
used to prepare the calibrations standards.

Preparation of Stock Solution 1 (1,000. ppm Ca2+)

This solution will be provided. It will be prepared by dissolving the primary standard CaCO3 in
dilute HCl.

The mass of primary standard CaCO3 required to prepare a 1.000-L solution of 1,000. ppm Ca2+
is 2.4973 g. This amount can be calculated as follows:

1,000. mg Ca2+ g mol Ca2+ mol CaCO3

× 1.000 L sol′ n × × ×
1.000 L sol′n 1000 mg 40.078 g Ca2+ mol Ca2+
100.087 g CaCO3
×
mol CaCO3

= 2.4973 g CaCO3

Preparation of Stock Solution 2 (100.0 ppm Ca2+)

Pipette a 25.00-mL aliquot of the 1,000. ppm Ca2+ stock solution into a 250.0 mL volumetric flask
and dilute to the mark with distilled water. This stock solution will also be used as a calibration
standard.

Preparation of Calibration Standards Containing 75.00, 50.00, 25.00 and 10.00 ppm
Ca2+
Each of these calibration standards is prepared by pipetting an appropriate volume of Stock
Solution 2 into a 100.0-mL volumetric flask and diluting to the mark with distilled water.

Calibration Curves
Flame photometry can detect calcium in aqueous solutions over a wide range of concentrations
(known as a dynamic range) at 622 nm. Concentrations can range from about 10 ppm (mg/L) to
about 100 ppm. In this experiment, five Ca2+ calibration standards will be prepared as described
above. The exact concentration of each standard will be corrected for the actual
concentration of the provided Stock Solution 1. An unknown water sample will then be
analyzed. If the concentration of the unknown water sample is outside the 10 – 100 ppm
detection range, then it will be diluted accordingly (see Table 3).

The instrument will be zeroed by using the blank (distilled water); the 100.0 ppm Ca2+ calibration
standard will also be used to manually set the maximum emission signal to ≈100. The emission
signal for each calibration standard will be measured in increasing order of ppm Ca2+ beginning
with 0.000 ppm (Blank). The unknown or diluted unknown water sample will then be analyzed.
Typical data are presented below.

3
A primary standard is a reagent that is pure and stable enough to be used directly after weighing. The entire mass is
the primary standard (which therefore assumes a 100% assay – see Module 3). Examples of primary standards include:
CaCO3; pure metals like Au, Cu, Fe, and Ni; potassium hydrogen phthalate or KHP; KCl; KIO3; benzoic acid (C7H6O2); and
sodium oxalate (Na2C2O4).
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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

Table 2. Sample Flame Photometry Data for Ca Collected During the Development of the
Method 4

Target Ca Actual Ca. Raw Power Corrected Power 5

Concentration Concentration (no units) (no units)
ppm Ca2+
Blank 0.000 002 0
10.00 ppm 10.00 010 8
25.00 ppm 25.01 024 22
50.00 ppm 50.01 060 58
75.00 ppm 75.01 092 90
100.0 ppm 100.0 120 118
Water sample 1 --- 014 12
Water Sample 2 --- 031 29
Water Sample 3 --- 013 11
Blank - Check --- 007 5
50.00 ppm Check 6
50.01 062 60

Ca Analysis (622 nm) by Flame Photometry

140

y = 1.1713x
120
R² = 0.9936

100
Emitted Power, Pe

80

60

40

20

0
0 20 40 60 80 100
ppm Ca2+

Figure 7. Sample emission calibration curve for Ca2+ obtained by using the Jenway FPF-7 Flame Photometer

Water Samples
Actual water samples will be provided for analysis. To assist with the analysis and to save time,
the water samples will be labelled as shown in Table 3.

4
Your data should look similar, but not identical, to that shown in the Table.
5
The Corrected Power is obtained by subtracting the Raw Power of the first Blank reading from each subsequent Raw-
Power reading.
6
The 50.00 ppm Check is used as a quality control measure. This is explained on pp. 9 – 10.
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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

Table 3. Hardness Categories, Ca Concentrations and Dilution Requirements

Water Hardness Calcium Carbonate Calcium Dilution Requirements

Category Concentration Concentration Range
Range 7 (ppm Ca2+)
(ppm CaCO3)
>Very Hard >400 >160 Required
Very Hard 180 – 400 72 – 160 Required
Hard 120 – 180 48 – 72 No dilution
Moderately hard 60 – 120 24 – 48 No dilution
Slightly hard 17 – 60 7 – 24 No dilution
Soft <17 <7 No dilution

Dilution of some water samples is required as indicated above to ensure that Ca concentration
falls within the detection range (10 to 100 ppm Ca) of the instrument. For example, if a water
sample from Category Very Hard (72 – 160 ppm Ca) is analyzed, then it must be diluted to bring
its concentration within the 10 – 100 ppm detection range. Otherwise, the analysis will not be
accurate. The easiest dilution is by a factor 2 (2-fold dilution), which is 1 part in 2, or 50.00 mL
in 100.0 mL, etc. The concentration of the sample prepared from the 2-fold dilution should then
be 36 – 80 ppm Ca, which is within the detection range.

Remember: if the measured sample is prepared by a 2-fold dilution, then the concentration of
the diluted sample obtained from the calibration curve must be multiplied by 2 to calculate
concentration of the original, undiluted sample correctly.

Calculation of Ca2+ Concentration in Unknown Water Samples

The calibration curve is the key piece of information that relates the concentration of the Ca to
the emitted power from the flame photometer. Without the calibration curve, the analyst cannot
quantify the ppm Ca in the unknown water sample but can only estimate its concentrations from
the calibration data.

To find the concentration of Ca2+ in the water samples, the equation of least squares must be
used.

7
Water hardness is reported as ppm CaCO3. However, the flame photometer measures ppm concentration Ca2+. To
convert ppm Ca2+ to ppm CaCO3:

g
molar mass CaCO3 �100.087 � 1 mol CaCO3
mol
ppm CaCO3 = ppm Ca × ×
molar mass Ca2+ �40.078
g
� 1 mol Ca
mol

= ppm Ca × 2.4973

The value 2.4973 is known as the Gravimetric Factor (GF). A GF is a conversion factor that is derived from the
stoichiometry of the reaction associated with the analysis. Specifically, the GF is calculated from those unit factors in the
stoichiometric calculation that remain constant (molar masses and mole ratios). A GF is therefore a convenient and quick
way to perform repetitive stochiometric calculations if the reaction is the same. The value of GF changes as the
reaction changes.

For this experiment, the reaction can be written as Ca2+(aq) + CO32–(aq) → CaCO3(s) because the flame measures ppm
Ca2+, but the results must be reported as ppm CaCO3. The GF for this analysis is therefore calculated from the molar
masses of CaCO3 and Ca2+ and from the reaction stoichiometry between CaCO3 and Ca2+ (1:1) as shown above.
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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

Sample Calculation for Unknown Water Sample 1 (from Table 2)

A 10.00-mL aliquot of Water Sample 1 was diluted to 100.0 mL (10-fold dilution). The Pe value
of the diluted water sample was 12. Calculate the ppm Ca2+ in the original sample. Use the
calibration curve shown in Figure 7.

Solution.
First, use the equation y = 1.1713x or Pe = 1.1713c to determine the concentration of the diluted
sample:

𝑃𝑃𝑒𝑒 12
𝑐𝑐 = = = 10. ppm
1.1713 1.1713
Now, use the dilution factor to calculate the ppm concentration of the original water sample:

ppm Ca2+original sample = 10. ppm × 10 = 1.0 × 102 ppm

Alternatively:

100.0 mL
ppm Ca2+original sample = 10. ppm × = 1.0 × 102 ppm
10.00 mL
Quality Control Checklist of the Analysis
Before you blindly clean up and go home, you should always stop and check the quality of your
data. Let’s subject the sample data collected from the development of the method (Table 2) to
the following checklist.

1. Do the data form a straight line or are the data points mixed up in some way?

• Yes, the data form a straight line as demonstrated by the corresponding calibration curve
(Figure 7), which has the least-squares equation Pe = mc where m = 1.1713. Furthermore,
the linear least-squares line has an R2 value close to 1. This is a good sign that, statistically
speaking, the data appear reasonable and follow a linear trend.
• The data pass through the point 0,0 as demonstrated by the equation Pe = mc. The data
was therefore plotted in Excel correctly, and the Trendline was formatted correctly. This
also indicates that the Blank was a true blank and is representative of the calibration
standards analyzed. Notice that the emission equation, Pe = mc, is of the form y =
mx and not y = mx + b. In the latter formula, one might expect a real, nonzero intercept.

2. Does the Emission Power from the two readings of the 50.00 ppm calibration standard fall
within the acceptable ±10% relative error required for this analysis?

• A small relative error indicates that the calibration of the instrument did not change
significantly during the experiment. This really says nothing about accuracy because we
do not know the true ppm value of the unknown sample but suggests good precision.

Pe1 ― Pe2
% Relative Error = × 100%
Average Pe

60 ― 58
= × 100% = 2%
59

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CHEM PH108 Experiment 5A (Revised December, 2020) – Background

This value is well within the acceptable range of ±10% relative error.

3. Does the Emission Power of the water sample fall within the range of Emission Powers for the
10 and 100 ppm Ca standards?

• Yes, the emission signals for the three water samples (12, 29 and 11) fell within the range
of the calibration curve emission signals of 8 to 118. However, ideally, it would be better
if the emission values for the sample fell more in the middle of calibration curve.

If any of the three conditions above are not met, then you must try to figure out why and redo
the analysis. Here are a few possibilities:

• Was the 100.0 ppm stock solution prepared correctly?

• Were the calibration standards prepared correctly?
• Were the unknown water samples that required dilution diluted correctly?
• Were any sample labels mixed up?
• Were the samples analyzed in the correct order?
• Was the aspiration tube rinsed with the distilled water wash before analyzing a
sample with a lower or unknown concentration?
• Was the data plotted correctly in Excel?
• Was the Trendline formatted correctly in Excel by setting the intercept to (0,0)?

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