Lab 11 Mineral Analyses: Atomic Absorption and Titration

Question 1

Which elements can atomic absorption spectroscopy detect?

Answer 1

Note: it cannot detect noble gases (and a few other elements)

Question 2

There are more emission wavelengths than absorption wavelengths.
True or False?

Answer 2

True.

Question 3

There are more absorption wavelengths than emission wavelengths.
True or False?

Answer 3

False.
There are more emission wavelengths than absorption wavelengths.

Question 4

What is the strongest (i.e., most intense) absorption line associated with?

Answer 4

Usually associated with the transition from E0 to E1

Question 5

What is the difference between absorbance and emission?

Answer 5

• Absorbance: Atoms absorb discrete wavelengths of light to bring them from the ground state (E0) to an excited state (E1, E2, etc.)
• Emission: Atoms release light at discrete wavelengths as the atom moves from an excited state to a lower excited state, or the ground state

Question 6

What is the principle of atomic absorbance spectroscopy? [6]

Answer 6

1. Ash the food sample to eliminate organic material.
2. Produce free atoms by heating the sample in a flame
3. Excite free atoms to a higher electronic state, through absorption of radiation produced by a hollow cathode lamp
4. Use a monochromator to select wavelength of interest
5. Measure the change in light intensity

Question 7

How does an atomic absorbance spectrometer work?

Answer 7

• Nebulizer converts liquid sample into aerosol and directs it to the flame
• Light from lamp is focused onto aerosolized sample
• Due to wet ashing, there are free atoms
• Monochromator allows us to measure decrease in light at specific wavelengths
• Wavelengths that the sample’s atoms have absorbed
• Detector amplifies signal

Question 8

Describe the parts of a hollow cathode lamp.

Answer 8

• Cathode - made of element of interest
• Anode - consists of tungsten
• Inert gas - argon or neon

Question 9

Why is the ‘hollow’ component of a HCL important?

Answer 9

• Specific type of lamp that allows for light to be emitted at lower voltage (less hazardous)

Question 10

In a hollow cathode lamp, the anode is made of the element of interest.
True or False?

Answer 10

False.
The cathode is made of the element of interest.
The anode consists of tungsten.

Question 11

In a hollow cathode lamp, the cathode is made of the element of interest.
True or False?

Answer 11

True.

Question 12

In a hollow cathode lamp, the cathode is made of tungsten.
True or False?

Answer 12

False.
The cathode is made of the element of interest.
The anode consists of tungsten.

Question 13

In a hollow cathode lamp, the anode is made of tungsten.
True or False?

Answer 13

True.

Question 14

How does a hollow cathode lamp work? [5]

Answer 14

1. Voltage across anode and cathode causes inert gas to ionize.
2. Ionized gas (plasma) causes atoms to sputter off of cathode.
3. Sputtered cathode atoms then become excited when they collide with each other and the ionized gas.
4. When the excited atoms return to a lower energy level they emit spectral lines characteristic of the element of interest.
5. Characteristic spectral lines are absorbed by the analytes in the sample.

Question 15

What are the 3 sources of radiation reaching the monochromator in AAS?

Answer 15

• Hollow cathode lamp
• Sample atom emissions
• Combustion occurring in the flame

Question 16

What is being detected in AAS?

Answer 16

• Monochromator is set to select for the specific wavelength that is absorbed by the atoms in the flame and to exclude other wavelengths.
• Therefore, the detector measures the change in the hollow cathode lamp radiation reaching the detector.

Question 17

Why is back titration more appropriate than direct/forward titration in some cases?

Answer 17

• The end point is often clearer because you are titrating the reactant as opposed to the analyte.
• Great to use when a suitable indicator for the direct endpoint is not available (e.g., substance of interest is not an acid or a base)
• Better to use if direct titration reaction is very slow or has an unfavourable equilibrium.

Back titration is often used when the analyte is volatile and some might escape during the titration (e.g., NH₃), when the analyte is an insoluble salt (e.g., CaCO₃), when a particular reaction is too slow, or when a direct titration would involve a weak acid-weak base titration which makes it difficult to observe the end point.

Question 18

EDTA has a higher affinity for Mg2+ than Ca2+.
True or False?

Answer 18

False.
EDTA has a higher affinity for Ca2+ than Mg2+ and will bind calcium instead of magnesium as long as it is available.

Question 19

EDTA has a higher affinity for Ca2+ than Mg2+.
True or False?

Answer 19

True.
EDTA has a higher affinity for Ca2+ than Mg2+ and will bind calcium instead of magnesium as long as it is available.

Question 20

The affinity of EDTA for calcium and magnesium is higher than the affinity of the calcein indicator for calcium and magnesium.
True or False?

Answer 20

True.
The indicator will not change colour until all EDTA is bound to calcium or magnesium.

Calcein can bind to either mineral, causing a colour change.

Question 21

The affinity of EDTA for calcium and magnesium is less than the affinity of the calcein indicator for calcium and magnesium.
True or False?

Answer 21

False.
The affinity of EDTA for calcium and magnesium is higher than the affinity of the calcein indicator for calcium and magnesium.
The indicator will not change colour until all EDTA is bound to calcium or magnesium.

Calcein can bind to either mineral, causing a colour change.

Question 22

Describe the first endpoint in calcium and magnesium determination via back-titration.

Answer 22

• Add excess reagent (EDTA) to analytes of interest (calcium and magnesium); EDTA forms 1:1 complexes; extra EDTA left over.
• Add standardized CaCl2 to solution to bind excess EDTA
• Once all EDTA is bound, excess Ca2+ will displace Mg2+ (EDTA has a higher affinity for calcium)
• Mg2+ will then bind calcein; colour changes from pink to green.
• Therefore, the amount of calcium added is equivalent to the excess EDTA.
• The difference in the total EDTA and the excess EDTA is equal to the equivalent moles of magnesium and calcium.

Question 23

Describe the second endpoint in calcium and magnesium determination via back-titration.

Answer 23

• Adust pH to ~13.5 (basic); magnesium will release from calcein and precipitate Mg(OH)2; colour returns to pink
• Due to high pH, when more calcium is added, it will displace magnesium and more precipitate will form.
• Continue adding calcium until all EDTA is bound to calcium; once there is excess calcium, it will bind calcein; colour will change back to green again
• Thus, the amount of calcium added is equivalent to the amount of magnesium that was present in the analyte.
• Amount of calcium in the sample may then be determined by simple difference.

Question 24

How can AAS be used to quantify minerals in a sample?

Answer 24

• Atomic absorption spectroscopy (AAS) quantifies the absorption of electromagnetic energy by atoms in a gaseous state, at a specific wavelength.
• The atoms are created by heating a sample in a flame. Each element has a characteristic wavelength of radiation that it will absorb.
• By passing the radiation of a discrete wavelength through the atomized sample, the absorption can be determined and related to the concentration of the element in the sample.

Question 25

How can back-titration quantify minerals in a sample?

Answer 25

• Titration can also be used to detect calcium and magnesium in milk by using the organic chelating agent, EDTA (ethylenediaminetetraacetic acid).
• The differential detection of the two minerals depends on the lower affinity of EDTA for magnesium and the higher affinity of calcein dye for magnesium, relative to calcium.
• Titration with a chelating agent such as calcein (bis-N,N-di (carboxymethyl) amino methyl fluorescein) is a type of complexometric titration, wherein the coloured complex is used to determine the endpoint of the titration.

Question 26

Calcein has a higher affinity for magnesium than calcium.
True or False?

Answer 26

True.

Question 27

Calcein has a higher affinity for calcium than magnesium.
True or False?

Answer 27

False.
Calcein dye has a higher affinity for magnesium than calcium.

Question 28

How should the glassware that you will use for the back titration be prepared?

Answer 28

soaked overnight in ddH2O to remove contaminating minerals

Question 29

What colours will you observe at the start and at the end of the titrations of the milk samples?

Answer 29

Start: pink
End: green

Note: this is in the EDTA-calcein titration for Mg and Ca2+ determination

Question 30

What final concentration of La will be in your calcium standards?

Answer 30

0.5%

Question 31

What correlates to the amount of magnesium in the sample in an EDTA-calcein titration?

Answer 31

Volume of CaCl2 needed for second end point

Question 32

What correlates to the amount of calcium in the sample in an EDTA-calcein titration?

Answer 32

EDTA(total) - EDTA(excess) = equivalent moles of Ca + Mg
Ca(added in first titration) = EDTA(excess)
Equivalent moles of Ca + Mg - moles of Mg = moles of Ca
Ca added in second titration = Mg in sample

Question 33

What is the purpose of lanthanum oxide in atomic absorption spectroscopy for analysis of calcium in milk?

Answer 33

In atomic absorption spectroscopy, La2O3 (Lanthanum oxide) is used as a chemical modifier to improve the accuracy of calcium analysis in samples such as milk. The addition of lanthanum to the sample matrix can help to reduce chemical interference by forming stable and non-volatile compounds with phosphate and sulfate ions that would otherwise interfere with the calcium measurement.

Question 34

What is the purpose of nitric acid and hydrogen peroxide in atomic absorption spectroscopy?

Answer 34

• Sample preparation; converts sample into a form that can be analyzed by AAS
• Nitric acid is used as a digestion reagent to dissolve the sample and release the analyte into the solution.
• Hydrogen peroxide can be used as an oxidizing agent to help break down organic matter in the sample.

Question 35

What is the purpoes of the hydrogen peroxide in AAS?

Answer 35

It enhances the oxidation properties of nitric acid in the digestion of organics.

This is during the wet ashing of the sample.

Question 36

Why is the glassware soaked in distilled water overnight prior to back-titration for Ca and Mg determination?

Answer 36

The glassware for the titration has been specially soaked in distilled water overnight to remove mineral residues that could interfere with your titration.

Question 37

What is the purpose of the salicylic acid in the milk sample preparations prior to back titration for Mg + Ca determination?

Answer 37

• To precipitate proteins from the sample to remove any interference from proteins.

Question 38

What is KOH used for in back-complexometric titrations using EDTA?

Answer 38

To adjust the pH to maintain basic/alkaline conditions

The calcein indicator will change colour below pH 12. Plus, EDTA is fully deprotonated in basic conditions, so it can effectively complex with metal ions.