- ALL groups will continue their film synthesis from the last week. And ALL groups will start new film syntheses (PVA only and PVA with 10% sodium montmorillonite)
- Groups A* and B* will be using the UV-Vis to make calibration curves for malachite green absorbance and to determine the equilibrium of their malachite green/dye samples
- Groups C* and D* will take a powder X-ray diffraction of one (or two) of their films. They will take Fourier Transform Infrared Spectra (FTIR) of all of their films. They may (time permitting) perform differential scanning calorimetry (DSC) of all of their films.
Our group was assigned to do the UV-Vis portion today, and to use the FTIR, DSC, and PXRD on Friday.
Note: On Friday Groups A and B will perform the tasks Groups C and D perform on Wednesday and vice versa.
- Film preparation
- See previous protocols: 1 and 2
- Calibration Curves
- Take UV Vis spectra for Malachite green concentrations: 0ppm, 0.20ppm, 0.50ppm, 1.1ppm, 1.4ppm, 1.7ppm, and 2.0ppm Malachite Green
- Find the two absorbance maxima in the spectra. Plot the absorbance value vs concentration at each ppm for each maximum you find.
- For each absorbance max, determine the slope of your A vs concentration plots. This will give you the extinction coefficient for the wavelengths you are analyzing
- Malachite Green Equilibrium
- We will use UV-Vis spectroscopy to determine the amount of Malachite green absorbed into a film
- Upon determining the amount of Malachite green in the film, we can determine the equilibrium of free Malachite green vs bound malachite green by creating an I.C.E. table. (Remember those from Gen Chem II??)
- We can also determine the mass of malachite green absorbed per mass of film
- To properly determine MG concentration, we will need to dilute our samples so that their absorbance is similar to the absorbance of our calibration curve samples.
- You can see from the intense color of your MG/film solutions that they are much darker (and will absorb more light) than those you used for your calibration curves (except for the 2ppm).
- Before you get to lab, figure out how you are going to need to make your dilutions.
- Hint 1: Use a 10mL volumetric flask (only 1 per each group)
- Hint 2: Transfer your dilutions to a glass test tube and label (you'll only need 3mL for your absorbance measurement)
- Hint 3: The two groups who are running UV Vis should take turns running 1 sample at a time. (At the end of the day, each group will have run 6 calibration samples and 8 diluted samples)
- Hint 4: Delegate responsibility among your group members for:
- Running Spectra
- Cleaning Cuvettes
- Making Dilutions
- Analyzing Spectra and making calibration curves
- Hint 5: Acetone is fantastic for really cleaning MG out of volumetric flasks and cuvettes.
- Powder X-ray diffraction
- Sample prep
- To be discussed in class
- Instrument Settings
- 2o start angle
- 40o stop angle
- 1o/second scan speed (this seems too fast, will check on this)
- 0.5o sampling width
- We will be using attenuated total reflectance (ATR) mode for our samples
- Place the film in the sample holder. Execute measurement
- Save measurement data onto thumb drive
- Will update this section soon
Below are the graphs of the results of the UV-Vis spectroscopic analysis. The first graph shows the Beer's Law plot derived from data gathered from the UV-Vis specs of the stock solutions at a wavelength of 423 nm. The second graph is a Beer's Law plot of the absorbance values of the various stock solutions at a wavelength of 617 nm.
The graph indicates that the molar abosorptivity of Malachite Green at a wavelength of 423 is 0.0283 cm-1*M-1.
This standard curve indicates that the molar absorptivity of Malachite Green at a wavelength of 617 nm is 0.1193 cm-1*M-1.
The table below shows the data used to calculate the mgMG/gFilm after films were soaked in MG solutions.
The concentration of MG in the film was calculated in the following way:
(Initial ppm)(.010L) = (Initial mass of MG in 10 mL of solution, mg)
[(Solution Absorbance post-experiment)/(molar abosorptivity = 0.1193/ppm)]x(Volume = 0.01L) = (Final mass of MG in solution, mg)
[(Initial mass of MG in Solution)-(Final mass of MG in solution, mg)]/(initial mass of film) = (Concentration of MG in film, mgMG/gFilm)