Lab 2 - Brass

Define brass

A generic term for alloys of copper and zinc (and also containing small amts of iron, lead, aluminum, and tin)

What properties does the amount of copper in brass affect?

Color, hardness, ductility, mechanical strength, electrical conductivity, corrosion resistance, etc.

What tool helps determine the percent copper in brass?

Spectroscopy

Define spectroscopy

Involves the interaction of electromagnetic radiation and matter

What does the absorption of electromagnetic radiation result in?

Different types of transitions in a substance, depending on the energy of the radiation

What does low energy microwave radiation cause?

It's converted to energy of molecular rotation

What does infrared radiation cause?

It excites vibrational frequencies associated with covalent bonds in a molecule

What does visible and UV light cause?

Electron transitions between different electron energy levels in a substance

What does the absorption spectra of substances consist of?

Specific lines or bands that can be used to identify a substance

Describe the orbitals of transition metal ions

Transition metal ions have filled/partially filled d orbitals

What does the presence of water molecules/ligands surrounding a transition metal ion in solution cause?

It leads to energy differences among the d orbitals.

What does the energy differences among d orbitals correspond to?

It corresponds to different wavelengths and energies of visible light, resulting in the varying colors of transition metal ions

How can the concentration of a colored transition metal ion solution be determined?

By measuring the color intensity

Purpose of a visible spectrophotometer

Used to measure the absorption of visible light

What is the relationship between absorbance and concentration?

Absorbance is proportional to concentration; the higher the concentration of a transition metal ion solution, the more intense the color, and the greater its absorbance

Beer's law

A=abc

A=absorbance

a=proportionality constant or molar absorptivity coefficient, with units 1/Mcm)

b=path length in cm

c=concentration (Molarity, moles/L)

How can Beer's law be used to determine the unknown concentration of a metal ion in solution?

Through measuring its absorbance and using Beer's law, as well as a calibration curve

Calibration curve

A graph that plots absorbance versus concentration for a series of standard solution, which gives rise to a straight line that passes through the origin

Calibration curve equation

y=mx+b, where y=absorbance, x=concentration, and b=0

Why are calibration curves used for quantitative analysis?

Because they even out fluctuations due to random error, and establish the range of concentration values over which Beer's Law is valid.

Purpose of the experiment

To design a procedure to analyze the amount of copper in brass using visible spectroscopy

How can brass be dissolved?

By reacting it with concentrated nitric acid

What happens when brass reacts with nitric acid?

Brass dissolves, and the metal components of the alloy are oxidized to copper ions, zinc ions, and iron ions (Cu2+, Zn2+ and Fe3+).

Purpose of the introductory activity

Determine the optimal wavelength for Cu2+ through measuring absorbance at wavelengths between 400 to 700 nm

Purpose of Part A

Generate the calibration curve

Purpose of Part B

Determine absorbance for Cu2+ of dissolved brass alloy

Equation between copper and nitric acid

Cu(s) + 4HNO3 (aq) → Cu(NO3 )2 (aq) + 2NO2 (g) + 2H2O (l)

Absorbed color vs. perceived/transmitted color difference

Absorbed color: the color that is absorbed by particular ions that are not visible

Perceived/transmitted colors: the color that is perceived or visible to viewers

What is the color of light that Cu2+ ions transmit and absorb?

They transmit blue/green colors, and absorb orange/red colors.

Safety caution of nitric acid

Concentrated nitric acid is severely corrosive, a strong oxidizer, and toxic by ingestion and inhalation. Reactions of nitric acid with metals generate nitrogen dioxide, a toxic, reddish-brown gas. Work with nitric acid only in a fume hood.

Describe the introductory activity procedure

1. Each team is given 0.1 Molar solutions of copper/iron/zinc nitrate, and copper/iron/zinc sulfate

2. Measure the absorbance for each solution at every 20 nm from 400-700 nm to generate a spectrum

3. Determine the best wavelength at which to measure the absorbance of the three solution

4. Record absorbance data to the data table

Describe how to use a spectrophotometer

1. Turn on the instrument and allow it to warm up for 15-20 minutes

2. Set the wavelength to the desired value

3. Handle cuvets at the top so no fingerprints are in the light path

4. Polish cuvets with a lens tissue

5. Place a cuvet that is 2/3 full of DI water in the sample holder and set the percent transmittance to 100% with the appropriate control; this is the blank sample

6. Fill a cuvet 2/3 full and place it in the assigned spectrophotometer and record wavelength and absorbance

When are absorbance measurements more accurate?

In the range of 0.1 to 1.0

For each salt solution, is the cation or anion responsible for the absorbance spectrum? Why?

The cation is responsible for the absorbance spectrum.

This is because when comparing two salt solutions with the same cation but different anions, there are no significant differences between their absorbance ratings, showing that the cations determine the absorbance spectrum of solutions and the absorption is not affected by the presence of the anion.

Do Zn2+ ions absorb visible light? Explain in terms of the color and appearance of Zn2+ aqueous solutions, and the electronic structure of Zn2+ ions

Zn2+ ions don't show any visible absorption of light, since Zn2+ aqueous solutions are colorless.

The absorption of light is dependent on exciting electrons to higher energy levels due to specific differences in energy levels between orbitals. Since Zn2+ ions have their d orbitals filled, and since absorption of visible light by transition metal ions is based around the excitement of electrons to the d orbitals, electrons cannot be excited to Zn2+ D orbitals. Since electrons can't be excited to a higher orbital and remain in their own orbitals, light energy can't be absorbed and Zn2+ ions cannot absorb visible light.

Identify a suitable wavelength to analyze Cu2+ ions in aqueous solution.

640 nm

If Cu2+ and Fe3+ ions are suspected of being present in the solution, will Fe3+ ions interfere with the analysis of Cu2+ at the wavelength selected? Why or why not?

At the wavelength of 640 nm that the team selected, the Solutions containing cupric ions ((Cu(NO3)2) and CuSO4) both have high absorption values of 0.25 and 0.263. For the solutions containing Fe3+, Fe(NO3)3 and Fe2(SO4)3, the absorption values are 0.02 and 0.08. Because of the large differences in the absorption values between the Cu2+ solutions and the Fe3+ solutions, with the Cu2+ solution having a significantly higher absorbance, the Fe3+ won't absorb many of the 640 nm light photons, allowing for the results to be calculated for the Cu2+ solutions with minimum interference.

Why are calibration curves often extended?

To accommodate the range of possible Cu2+ concentrations that may be obtained by dissolving brass

Equation for the volumes of 0.4 Cu(NO3)2 stock solution and water required to prepare 8.0 mL of each standard solution for the calibration curve

(8.0 mL)(1 L/1000 mL)(x moles/1 Liter) = a moles

(a moles)(1 L/0.4 mol)(1000mL/1L) = b mL Cu(NO3)2 stock solution

8.0 mL-b mL = c mL DI water

How to plot a calibration curve

1. Graph concentration (M) vs. absorbance of a particular metal ion at a specific wavelength

2. Generate a straight line and a y=mx equation that intersects through the origin

Purpose of calibration curve

To plot concentration vs. absorbance and create a line to describe the linear pattern of this data, allowing for the interchangeable calculations of the absorbance and concentration of a particular ion at a specific wavelength of light

Procedure for guided inquiry part B

1. Weigh the two brass pieces

2. Add 2 mL in addition to the amount of nitric acid that's required to react with all of the brass, assuming it's 100% copper

3. Place the beaker in a hot plate with the brass, pour inthe nitric acid, and cover it with a watch glass

4. After the brass dissolves, remove the beaker from the hot plate, and let it cool in the fume hood

5. Add 50 mL DI water, stir it, wash drops from the wash glass into the beaker

6. Transfer the contents to a 100 mL volumetric flask, rinse the beaker with 5 mL DI water, and add this solution to the volumetric flask

7. Dilute the final volume to 100.0 mL

8. Analyze the absorbance of the brass solution, and calculate the percent copper in the original brass sample

Why is additional 2 mL HNO3 added to react with the brass?

To ensure that HNO3 is an excess reactant and that the brass alloy completely dissolves

Purpose of rinsing the watch glass with DI water, and rinsing the beaker with DI water

To return any residual ions to the solution to ensure that the concentration measurement is correct

Calculation for the amount of concentrated nitric acid needed to react with the entire brass alloy sample

( x g copper)( 1 mol Cu/63.55g Cu)(4 mol HNO3/1mol Cu)(1L HNO3/15.8 mol HNO3)(1000 mL/1 L) = total mL required to react

Calculation for percent copper in brass alloy sample

1. absorbance of Cu2+ in sample = 2.1x ; X = a M Cu(NO3)2

2. a M Cu(NO3 )2 (1 L/1000 mL)(100.0 mL)(1 mol Cu(NO3 ) 2 /1 mol Cu 2+ ) = b moles Cu 2+

3. b moles Cu 2+ (63.55g/1 mol) = c grams Cu

4. (c g Cu/0.966 g alloy) x 100% = d Cu in alloy

How much energy does each light particle, or photon, contain?

Light particles contain discrete amounts of energy

How do molecules absorb or emit energy?

The absorption or emission of light by an atom/molecule occurs in discrete amounts called quanta; it can only occur if the energy of the photon is equal to the energy difference between the two energy levels in the atom or molecule

This means that molecules can only absorb photons of specific energies, frequencies, and wavelengths

Is absorbance of light, or transmittance measured?

Absorbance of light is difficult to measure, so transmittance is measured (the fraction of light that passes through a solution of molecules)

What do spectrophotometers do?

They measure the intensity of light entering a sample and compares this to the intensity of light emerging from the sample

Three items that affect the amount of light emerging from the sample

1. Concentration of molecules in solution

2. Length of the sample path

3. Specific properties of the molecules

How does the concentration of molecules in solution affect the amount of light transmitted?

- As more molecules exist in solution, the amount of photons absorbed increases, decreasing the transmittance as well

How does the length of the path affect the amount of light transmitted?

By increasing the pathway that the light must travel through, the number of molecules that will interact with increases, increasing the apparent concentration as well

How does the properties of molecules affect the amount of light transmitted?

Molecules absorb light at different efficiencies and different energies. Therefore, the transmittance depends on the specific molecule in solution and the wavelength of light being passed through the sample.

What is the relationship between transmittance and concentration?

A logarithmic relationship

Why is it a logarithmic relationship?

This is because as light travels throughout the sample there is a decrease in the intensity of light absorbed throughout the sample. Since the intensity of the light entering changes throughout the sample, the relationship is logarithmic rather than linear.

Absorbance equation

A = 2 - log(%T)

T = IE/IO

IE = intensity of exiting light

IO = intensity of incident light

Practical uses of Beer's law

When two of the three variables a, b, or c are known, the absorbance can be measured and the unknown variable determined

Do chemists usually determine concentration using Beer's law? Why or why not?

No, because it can result in answers that are incorrect due to differences in instrumentation and scientific errors.

If the instrument being used isn't the same as the one used to determine the constant a, the concentration may be calculated wrong. Cuvettes also might be scratched/dirty, and pipetting done incorrectly.

How are concentrations usually determined?

Using a standard curve

How are standard curves constructed?

- They're generated when a isn't known and/or to minimize experimental error

- The researcher determines the absorbance of several known solution concentrations, or standard concentrations

- This is then plotted as an absorbance vs. concentration graph, creating a standard curve

- It should be a straight line with a y intercept of zero

How can standard curves be used to determine concentration?

- The absorbance of the experimental solution can be found on the y-axis, and the corresponding point on the x-axis determined

- OR the linear equation of the line can be determined, and the absorbance plugged in as y to solve for x

Sources of error in beer's law measurements

Dirty cuvettes, poorly mixed solutions, poor pipetting techniques, and incorrect light source or wavelength

What kind of solutions is Beer's law used for?

Dilute solutions

Why is the absorbance reading unstable/nonlinear at values above 1.0?

When absorbance is above 1.0, this means the solution is too concentrated, and the solution should be diluted and data recollected.

When transmission is extremely low, the amount of light detected will be low as well, meaning there is a greater room for error - potentially leading to the curve deviating from a straight line.

Why is the absorbance nonlinear at values below 0.1?

Because the solution concentration contains too little solute and is very dilute, resulting in a high transmission.

When transmission values are extremely high, there is also potential for higher error in measurement, resulting in the curve deviating from a linear path.

What is the purpose of a blank cuvette?

A blank solution contains no solute, and is used to calibrate the spectrophotometer readings and to create a baseline response