GCCCD Concentration of Calcium in Seawater Lab Report

Determination of calcium in seawater by flame atomic absorptionspectroscopy (FAAS)
Background and goals:
Calcium, like other constituents that we have previously explored in CHEM 100A (F–,
Cl , Br–) is a conservative component, i.e. the ratio of its concentration to other components
varies insignificantly, even if total salinity varies. The concentration of Ca2+ in standard mean
sea water (S=35) is 10.3 mmol/kg.{Pilson, 2013} The percentage of calcium in the total salt
content is approximately 1.18% by mass.{Pilson, 2013} The quantification of calcium in the
seawater collected off the Scripps Pier is one goal of the present lab.
You will use one of the most powerful techniques in analytical chemistry, namely atomic
absorption (AA) spectroscopy, for this lab. You are expected to become familiar with the
principles and operation of an AA spectrophotometer by attending the lecture, and reading the
relevant pages in Harvey (Section 10D, pages 599-612).{Harvey, 2009} Note that the specific
type of instrument in your lab is a flame-absorption AA spectrophotometer.
As with any method, calibration is important. In this case, you will employ a series of
external standards. You should compare and contrast this method of calibration relative to the
method of standard additions.{Harvey, 2009}
–
Safety and PPE:
Lab coat, Safety glasses
Instruments and Materials:
AA spectrophotometer, Perkin Elmer PinAAcle 500
Balances, top-loader and analytical
CaCl2 dried (in a weighing bottle, desiccator)
5% La203 solution
SIO Pier Sea Water
Media Bottle, 500 ml
Plastic Bottles: 1 x 250 ml
6 x 60 ml
3 x 30 ml
Before day 1 of the calcium lab: Excel Spreadsheet
1. CaCl2 will be dried for you. Use Solution Prep spreadsheet from your Excel Assignment to plan
for preparation of stock solution, as well as the dilutions for sub-stock, standards, blank and
unknown sea water samples.
2. Concentration range for calcium standards is 1-5 ppm. Calcium concentration in diluted sea water
samples should fit within this range.
3. Lanthanum will be added to all standards and samples (acts as a complexing agent that reduces
chemical interferences and may increase instrument sensitivity).
1
Procedure: Solution Preparation (solvent DI water)
1. Prepare 500 g of a ~1000 ppm Ca2+ stock solution. How much dried CaCl2 do you need? CaCl2 is
hygroscopic, so limit the exposure to air until you have weighed the CaCl2. Refer to the “Notes on
solution preparation” under “Materials needed for Calcium lab report”.
2. From the stock solution, prepare a sub-stock solution. Think about what would be the “best” concentration
of the sub-stock considering concentration range for your standards, and how much sub-stock solution is
needed in total.
3. Lanthanum will be added to all standards and diluted sea water samples. Its final concentration should be
0.1% in all solutions. Lanthanum will be provided as 5% solution, so use dilution equation in your spreadsheet
calculations. Check your calculations with your TA before start of solution prep.
4. From the sub-stock, prepare five Ca2+ standard solutions in 1-5 ppm range. In addition, prepare 0.1%
Lanthanum blank (in DI water) as well.
5. The concentration of Ca2+ in seawater is ~400 ppm. This is too high for direct measurement by the AA
spectrophotometers in lab, so it will need to be diluted. Dilute your sample of seawater to an appropriate
concentration. Is sea water sub-stock needed? Add enough 5% Lanthanum solution to your sea water
samples to make them 0.1% in Lanthanum. Try to optimize the concentration of the diluted sample to
minimize error. Your approximate concentration of calcium must fall within the range of your standard
solution concentrations.
6. You should have total of 8 samples: DI water blank, Lanthanum blank, 5 standards and diluted sea
water sample.
Sample measurement with a PinAAcle 500 Spectrophotometer: Notes on the Instrument
• Be gentle with the aspirator tube and avoid pulling it near the instrument. It is important not to disturb
the nebulizer.
• Air should not be drawn into the sample for excessive lengths of time. It is best to keep the aspirator
tube submerged in a liquid; the instrument is continuously drawing sample while the flame is ignited.
1. Remove the aspirator tube from the instrument beaker and submerge it into your DI water blank.
Auto-zero the instrument on this sample by pressing “Analyze Blank”. Then read its absorbance by
pressing “Analyze Sample”. What absorbance value do you expect for DI water?
2. Remove tube from the DI water blank, dry gently with a Kimwipe, and submerge into Lanthanum
blank. Measure the average absorbance by pressing “Analyze Sample”.
3. Dry the tube gently with Kimwipe and transfer the tube back to the DI water bottle. Purge with DI
water for at least 10 seconds.
4. Recommended order of samples is: DI blank, Lanthanum blank, standard solutions in increasing
concentration, diluted seawater samples, DI blank. Always return the tubing to your DI blank bottle
between runs of standard solutions and samples to clean the tubing.
5. Return the aspirator tube to the instrument beaker at the end of your measurement.
6. Record all your data.
2
Data analysis and questions for consideration:
•
You will use linear regression to calculate the best linear fit to your calibration data. Use Abs
2+
La blank and Abstd vs actual Ca to construct a calibration curve.
•
After you construct your calibration curve and apply linear regression, determine the
concentration of your diluted sea water samples using the slope and x-intercept. Based on your
dilution of the sea water sample, back calculate concentration of calcium in undiluted, original
sea water sample. Report your result with the uncertainty determined by the error propagation.
•
Compare your calcium level to an expectation based upon the salinity determined in your
previous work, and the known mass ratio of calcium relative to the total dissolved salts. How
does your result compare with the expected accuracy of the instrument? What impact does the
dilution present?
•
When sea water samples were analyzed on the instrument, flame had a different color than
your standards. Where does this color come from? Is the detector recording or “seeing” this
light? If the Lanthanum does not absorb any energy from the lamp, what role does the
Lanthanum play?
References:
Harvey, D. Chapter 5, “Standardizing Analytical Methods” in Analytical Chemistry 2.0,
Electronic Textbook, 2009.
Harvey, D. Chapter 10, “Spectroscopic Methods” in Analytical Chemistry 2.0, Electronic
Textbook, 2009.
Pilson, M. Chapter 4, “Major constituents of seawater” in An Introduction to the Chemistry of the
Sea, Second Edition, Cambridge University Press, 2013.
Based on Calcium Lab Procedure R. Pomeroy
rev Sam Doyle, Michael Tauber W16
rev RP, MV F21
rev Li, S22
3
Theories Behind Atomic Spectroscopy
Quantum vs Classical
Length
Classic
Quantum
meters
nanometers
Quantum vs Classical
Classic
Length
scale:
Rules:
meters
Newton’s
law
Quantum
nanometers
Schrodinger
equation
Quantum vs Classical
Classic
Length
scale:
Rules:
Energy:
Probability:
Quantum
meters
Newton’s
law
nanometers
Schrodinger
equation
Continuous
Discrete
Discrete
Continuous
How far away is the quantum scale?
To see atoms and molecules, you need 50,000,000x Magnification
Galileo, 1992
Geisel Library
3,900,000 Miles
UCSD
Quantum Mechanics in Modern Chemistry
In Quantum scale, energy levels are discrete:
• Electrons travel in defined orbits around the nucleus.
• The orbits are labeled by an integer, the quantum
number n.
• Electrons can jump from one orbit to another by
emitting or absorbing energy.
• Photon energy is the most common energy involved in
the excitation/relaxation.
Bohr Model
Hydrogen Atom Spectra
Atomic Spectra
• When the atoms/molecules are
excited, they prefer to absorb/emit
specific energy that matches the
energy difference between the
electron orbitals.
• Each atom/molecule has a distinct
set of absorption/emission peaks,
which serve as their fingerprints.
• The intensity of the
absorption/emission peak is
related with the population of the
chemicals – allow use to quantify
their concentration
Atomic Absorption Spectroscopy
AAS: Analytical technique that is utilizes the absorption of light (photon) by atoms within
the sample to measure the concentration of elements that may be present in a solution
Na
How do we relate concentration of an
element to the absorption of light?
Elements detectable by atomic absorption (pink)
Absorption vs Concentration: Beer’s Law
•
As light passes through a sample it’s power (P) decreases as some of it is absorbed
•
This attenuation of radiation is described quantitatively by transmittance (T) and absorbance (A)
•
Pt defined as radiant power transmitted by sample
•
P0 defined as radiant power transmitted by blank, accounts for
loss due to scattering, reflection, and absorption by matrix.
Absorption vs Concentration: Beer’s Law
•
As light passes through a sample it’s power (P) decreases as some of it is absorbed
•
This attenuation of radiation is described quantitatively by transmittance (T) and absorbance (A)
Concentration of Ca2+ in Sea
— Flame Atomic Adsorption Spectroscopy
Calcium in Seawater
•
Calcium is a conservative element in seawater
•
Seawater contains ~ 0.01 mol/L (~400 ppm; 1.2% of the dissolved salt)
•
Concentration of Ca in seawater is much higher than rivers (1~2 ppm)
•
Food for corals: constituent of reefs
How to measure Calcium: Absorption Spectroscopy
•
Calcium an element detectable by Atomic Absorption
•
The molar absorptivity is relatively high (3.32 × 104 L·mol−1·cm−1)
•
Signature lines in visible regime ~420 nm, ~580 nm, ~615 nm,
Elements detectable by atomic absorption (pink)
How to measure Calcium: Absorption Spectroscopy
Components needed for Absorption Spectroscopy
Light source
Sample
Light detector
FAAS: Flame Atomic Absorption Spectroscopy
Components: Light Source
• The light source is usually a hollow
cathode lamp of the element that is
being measured
• It contains a tungsten anode and a
hollow cylindrical cathode made of the
element to be determined.
• These are sealed in a glass tube filled
with an inert gas (neon or argon) at ~ 15 Torr
• Each element has its own unique lamp
which must be used for that analysis
FAAS: Flame Atomic Absorption Spectroscopy
Components: Light Source
• When a potential (~ 500 V) is applied between
the anode and the cathode, gas is ionized and
the cations are accelerated towards the cathode
FAAS: Flame Atomic Absorption Spectroscopy
Components: Light Source
• When a potential (~ 500 V) is applied between
the anode and the cathode, gas is ionized and
the cations are accelerated towards the cathode
• Cations strike the cathode with enough energy
to “sputter” metal atoms from the cathode into
the gas phase– ion bombardment
FAAS: Flame Atomic Absorption Spectroscopy
Components: Light Source
• When a potential (~ 500 V) is applied between
the anode and the cathode, gas is ionized and
the cations are accelerated towards the cathode
• Cations strike the cathode with enough energy
to “sputter” metal atoms from the cathode into
the gas phase– ion bombardment
• Gaseous atoms in the excited state emit
photons as they fall back to the ground state
FAAS: Flame Atomic Absorption Spectroscopy
Components: Light Source
• When a potential (~ 500 V) is applied between
the anode and the cathode, gas is ionized and
the cations are accelerated towards the cathode
• Cations strike the cathode with enough energy
to “sputter” metal atoms from the cathode into
the gas phase– ion bombardment
• Gaseous atoms in the excited state emit
photons as they fall back to the ground state
• The emitted photon energy matches the
absorption lines – cathode made of the element
to be determined.
FAAS: Flame Atomic Absorption Spectroscopy
Components: Sample — Atomizer
• Elements to be analyzed need to be
in the atomic state
• Atomization is separation of particles
into individual molecules and
breaking molecules into atoms.
• This is done by exposing the sample
analyte to high temperatures in a
flame or graphite furnace .
FAAS: Flame Atomic Absorption Spectroscopy
Components: Sample — Atomizer
reducing environment
Ca2+ +
2e- 
Ca
Reducing environment: no free oxygen
e.g. center of the flame
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Monochromator
• The atom could have multiple
emission/absorption line
• The absorptivity of each line is
different.
• We use monochromator to choose
the specific line with large absorptivity
for the most accurate measurement.
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Monochromator
• The atom could have multiple
emission/absorption line
• The absorptivity of each line is
different.
• We use monochromator to choose
the specific line with large absorptivity
for the most accurate measurement.
• For Ca, we choose the 422.7 nm line.
Strong Lines of Calcium ( Ca ) (nist.gov)
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Photomultiplier tube
• Incoming electron hit the focusing
electrode and generate a photoelectron
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Photomultiplier tube
• Incoming electron hit the focusing
electrode and generate a photoelectron
• The photoelectron is accelerated by the
voltage between the focusing electrode
and the next dynodes
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Photomultiplier tube
• Incoming electron hit the focusing
electrode and generate a photoelectron
• The photoelectron is accelerated by the
voltage between the focusing electrode
and the next dynodes
• The accelerated photoelectron hit the
dynode, and generate more electrons
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Photomultiplier tube
• Incoming electron hit the focusing
electrode and generate a photoelectron
• The photoelectron is accelerated by the
voltage between the focusing electrode
and the next dynodes
• The accelerated photoelectron hit the
dynode, and generate more electrons
• After many dynodes, it become an
electron pulse and collect by the anode
FAAS: Flame Atomic Absorption Spectroscopy
Components: Photo detector – Photomultiplier tube
• Incoming electron hit the focusing
electrode and generate a photoelectron
• The photoelectron is accelerated by the
voltage between the focusing electrode
and the next dynodes
• The accelerated photoelectron hit the
dynode, and generate more electrons
• After many dynodes, it become an
electron pulse and collect by the anode
A modern photomultiplier can
detect individual photons
FAAS: Flame Atomic Absorption Spectroscopy
Hollow Cathode Tube
Photomultiplier
Atomizer
Ca concentration with FAAS
Procedures
• Make a dilution series from a calcium chloride
dihydrate salt
• Use the A & C of the standard solutions to construct
a calibration curve — linear regression
• Important: Dilute your sample to 1-5 ppm (Linear
dynamic range is below 5 ppm for Ca. This is
element dependent).
• Use the calibration curve to find out the
concentration of the diluted sample, and convert it
to the concentration of the original sample.
Ca concentration with FAAS
To minimize error
•
Smallest Error
Try to minimize it
For the most accurate result: The concentration of your
unknown solution should be at the middle of your
calibration curve.