9 votes
Accepted

Why not use all the colours in colorimetry?

Why not send in white light (with all the wavelengths at once)? This is certainly and routinely done today. The main thing is how much price are we willing to pay? In a rigorous sense, colorimetry ...
  • 35.2k
8 votes

Visible Light Spectrophotometer Recommendations

Disclaimer: I'm not affiliated with any company However, Ocean Optics cuvette spectrophotometers are a good choice This model is used in some undergraduate labs and is good because the cuvette ...
8 votes

Visible Light Spectrophotometer Recommendations

Good spectrometers in "new, unopened" condition are out of your budget. Their prices start at about 2000 USD. This also includes the spectrometers specifically made for education—like the ones from ...
  • 975
7 votes
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Notation for excited states

As has already been mentioned in comments, those letters and numbers for each state correspond to their symmetry, specifically the symmetry of the (purely electronic) wavefunction. For more ...
7 votes

Why not use all the colours in colorimetry?

The previous answers by @Buck Thorn and @M. Farooq are very good, but not quite complete. The main advantages of FT UV-Vis spectroscopy would be 1) accuracy of wavelength (or wavenumber) determination ...
  • 4,470
7 votes
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Reason for usage of Metal chlorides and Platinum or Nichrome wires in flame test

Metal chlorides are usually much easier volatilized than oxides or sulfates. They melt or sublime at temperatures much lower than 800°C. So they are volatilized in any flame like Bunsen burners. ...
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6 votes
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pKa of phenols using UV-Vis spectroscopy

To quote the key points of an easy to read publication by Salgado and Vargas-Hernández (doi 10.4236/ajac.2014.517135, open access): All starts with the dissociation equilibrium between the acid $\ce{...
  • 23.9k
6 votes

Why is the graph of fluorescence vs. wavelength continuous?

There are two main reasons for what you're describing. The first is the fundamental properties of the molecules involved, as mentioned in the comments. Even for isolated molecules, the transition is ...
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6 votes
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De-excitation of a molecule

After radiation is absorbed and the electron is at an excited state in the molecule there are several pathways for de-excitation to occur (see fig.). The pathway of choice depends on its rate, ie how ...
  • 1,523
6 votes
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What is an exocyclic double bond?

Surely you are aware that the calculation of this sort is but a very rough estimate, yet still I feel the urge to stress that once more. Having said that, let's look at the double bond in the first ...
  • 30.2k
6 votes
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Why diethyl ether does not absorb in UV region?

We can measure the UV-Vis spectra for π → π* or n → π* electronic transitions (other transitions are forbidden according to selection rule) [1, pp. 577–578]. But in diethyl ether, being a saturated ...
6 votes

Why is 295 nm a good wavelength to check absorbance when testing for aspirin?

I think this was basically an educational experiment. Your analytical method assumes that the hydrolysis was complete (this is the key requirement of a successful calibration curve). Your experiment ...
  • 35.2k
6 votes

The reason for selection of wavelengths in the spectrophotometry of Quinoline Yellow SS

The experiment in your link pg 31 is "Path Length Dependence of Absorbance Values". As you stated, quinoline absorption spectrum has a dip near 337 nm. The reason for choosing the wavelength ...
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5 votes
Accepted

Why does benzene have 3 π–π* transitions in the UV range, but ethene, butadiene, and hexatriene each have only 1?

It seems that here we only need to consider the lowest energy transitions, i.e. HOMO–LUMO transitions. The difference between benzene and the linear polyenes is that both the HOMO and LUMO of benzene ...
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5 votes

Woodward Fieser rules for dienes - non conjugated double bond as exocyclic?

Woodward's rules are an empirical method for predicting the maximum absorption of a molecule based on the functional groups present (that is, conjugated functional groups which possess a distinctive ...
  • 16.9k
5 votes

Why not use all the colours in colorimetry?

An explanation is provided in this abstract (1): Fourier transform spectrometry in the UV-Vis region (FT/UV-Vis), because it is source shot-noise limited, has a signal-to-noise ratio (S/N) ...
  • 19.5k
5 votes

How to Combine an Emission spectrum into a colour?

It is not simple -- for example, to many people, red + blue spectral lines appear purple/violet, i.e., shorter wavelength than pure blue! Though you could assign a color name to a pure spectral line, ...
4 votes

Evidence for a chemical reaction

Most light doesn't result from chemical reactions While there are many chemical reactions that can emit light and many related reactions where the emission is related to chemistry (for example the ...
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4 votes

laser diode problem

I just did the experiment suggested by @porphyin. I used a 405 nm laser diode built into a toy "Space Gun" and the orange long pass filter shown in the figure below: The results are in the next dark ...
  • 4,470
4 votes
Accepted

Why do some molecules show non-symmetric absorption and emission bands?

Mirror image spectra are only observed in solution and then only if the ground and excited state potential energies have almost exactly the same shape. Thus in rigid molecules such as anthracene a ...
  • 27.8k
4 votes

How can I prove ascorbic acid degradation with Uv-vis spectroscopy?

Some of the experimental details are missing, but if you recall $$\mathrm{Abs} = \varepsilon \cdot c \cdot d$$ about the absorption depending on the molar extinction coefficient $\varepsilon$, the ...
  • 23.9k
3 votes
Accepted

Beer's law with a polychromatic light source

There are several ways to answer the question, so here are my three. The first answer involves using a simple integer wavelengths approximation, i.e., noticing that Figs. 1 and 2 have 1 nm point ...
  • 4,470
3 votes

Does broader peak mean stronger absorber in UV/VIS?

The classical concept of oscillator strength $f$ is useful here, but it should be used only in a qualitative way. This is defined as $f=a\int\epsilon_\nu d\nu$ where $a$ is set of units with value $4....
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3 votes

What is an exocyclic double bond?

The double bond B is exocyclic to ring 2 as it is attached to an atom which is shared between ring 1 and ring 2 while the double bond A is not connected to any ring 2 atoms and is within just one ring,...
3 votes

Is a hue indistinguishable from its equivalent frequency of light?

Certainly not The problem is not, strictly, one of physics but one of perception. The eye doesn't detect individual wavelengths of light but the relative amount of red, green and blue light (but the ...
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3 votes
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UV Absorption of iodine with some solvent

Iodine is highly polarisable and will form a (Mulliken) charge-transfer complex with many aromatics. Very many pairs of donor - acceptor pairs do this and have been studied for approx $100$ years, see ...
  • 27.8k
3 votes

Different force constant in ground vs excited state in spectroscopy

At the risk of providing a somewhat circular answer, in the harmonic approximation of an interatomic potential, the force constant is a measure of the force of attraction between the atoms (in a ...
  • 19.5k
3 votes

Why do some molecules show non-symmetric absorption and emission bands?

From the Jablonski diagram, The mirror image is only true if you are talking about transitions from $S_0$ to $S_1$ (absorption) and $S_1$ to $S_0$. Also read about Kasha's rule. Quinine is the ...
  • 35.2k
3 votes
Accepted

Is it false that the absorbance of two concentrations is the sum of the absorbances of the concentrations?

There is a fallacy in your experiment. Yes, absorbance is indeed additive tha low concentrations provided the reagents do not react. This sum property allows simultaneous determination or two or more ...
  • 35.2k
3 votes

How to Combine an Emission spectrum into a colour?

As I understand your issue, you need to convert spectra into RGB triplets. Quantitative colorimetry theory addressing that issue is quite thoroughly stated in this wikipedia article on CIE 1931 color ...

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