17 votes

Why does ethylamine have two infra-red spectra? Are both correct?

When looking at IR spectra of hydrogen-bonding groups, always check how IR spectra was recorded. It appears that NIST spectra is recorded using gas phase, while the first one used liquid film. In ...
permeakra's user avatar
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14 votes
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Why does tryptophan absorb UV light?

Any molecule can absorb UV light. What the question is probably going for is why those three molecules absorb at longer wavelengths than other amino acids. This has to do with the conjugated pi bonds ...
Blaise's user avatar
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14 votes
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Looking for a dye which emits around 680 nm

There are two dyes commonly used in biochemistry research with $\lambda_\mathrm{Ex}$ of around $\pu{650 nm}$. They are Alexa Fluor 647 from ThermoFisher and Cyanine5 (Cy5) from Lumiprobe. Two example ...
Mathew Mahindaratne's user avatar
13 votes

How does molecular structure contribute in the property of fluorescence?

Keep in mind a few things that must happen for an absorption process to result in fluorescence: (1) the initial transition is to an excited electronic state that observes certain rules regarding the ...
Buck Thorn's user avatar
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10 votes
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Lifetime components in phosphorescence decay

Phosphorescence should be at a longer wavelength than any fluorescence so carefully using a filter or grating should remove any fluorescence. Also measuring at different wavelengths will change ...
porphyrin's user avatar
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8 votes

How does absorption spectroscopy work?

In an absorption experiment a collimated beam of light is passed through the sample. The intensity of this is measured before and after the sample. The absorption is measured as optical density which ...
porphyrin's user avatar
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8 votes

Why is absorbance the negative logarithm of transmittance?

Absorbance is useful because it is additive. That is, it's absorbance which is used in Beer's law: $$A = \epsilon \cdot c \cdot l$$ While you can certainly make a version of this law which uses ...
R.M.'s user avatar
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8 votes

If Beer's Law appears to apply to a high concentration (>0.01M) of a solute, is it valid to use for concentration calculation?

Internet consensus seems to define "high concentration" for Beer's Law as >0.01M Keep in mind that Beer's law is an approximation. Look at the more advanced version. Beer's law-advanced version. ...
AChem's user avatar
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How does a spectrophotometer measure absorbance based on the intensity of detected light?

There are two possibilities: If there is only one optical pathway, you record i) the blank sample for all wavelengths of interest and store the detector's intensity information in function of $\...
Buttonwood's user avatar
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7 votes

Absorption coefficient for liquid water

What the plot shows is the reciprocal path length needed for a given absorbance to apply, for example slightly less than 100 m in the visible to $10^{-8}$ m in the uv, as you point out, but this is ...
porphyrin's user avatar
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7 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 ...
AChem's user avatar
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Why decarbonate soda for spectroscopy experiment?

The primary reason is that when soda warms up, the $\ce{CO2}$ would escape slowly as bubbles. These bubbles would significantly scatter light affecting the absorption spectra. Usually, the effect of ...
ankit7540's user avatar
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6 votes

Why is absorbance the negative logarithm of transmittance?

Beer's law relates transmitted intensity to concentration $[c]$ of the solution at each wavelength $\lambda$ as $$ I_{trans_\lambda}=I_0e^{-\epsilon_\lambda [c]L}$$ where L is the sample path-...
porphyrin's user avatar
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When making a calibration curve, do we have to use the wavelength at maximum absorbance?

No it doesn’t. Measurements can in principle be made at any wavelength. If there are two colored specimens in the same solution, it is actually better to measure at the wavelength where the ...
FrankS's user avatar
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6 votes

Why does ethylamine have two infra-red spectra? Are both correct?

Honestly I have doubt about your textbook version of IR spectrum. However, I can suggest by the experience I have gained that the shapes of the peaks depends on the method you have used to obtain the ...
Mathew Mahindaratne's user avatar
5 votes
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Adding non stoichiometric amount of NH3 to copper solution

Both your conclusions are absolutely correct. I'll point out that you'll never get "all" the $\ce{Cu^{2+}}$ to the amine complex. You're trying to get greater than 99% of the $\ce{Cu^{2+}}$ to the ...
MaxW's user avatar
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Absorption coefficient for liquid water

The plot is from Wikipedia, where important/helpful links to the source information can be found, and is worth a look. A similar and more detailed plot can be found on this page of Martin Chaplin's ...
uhoh's user avatar
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Does a calibration curve of absorbance vs concentration need to pass exactly through the origin?

Good answers, but it is almost never a good idea to force the intercept to zero. Unless there is a clearly justifiable and compelling reason to do so, it is better to let the data set 'vote' for ...
Ed V's user avatar
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5 votes

Determining Kc for the equilibrium involving iron thiocyanate using spectrophotometric data

The problem is a bit strange since the details of the absorbance measurement aren't detailed very well. I'll assume the book defines the relationship as: $$A = \epsilon bc$$ where: A=absorbance $\...
MaxW's user avatar
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Determining Kc for the equilibrium involving iron thiocyanate using spectrophotometric data

To start with, its often helpful to make an ICE table when dealing with problems of chemical equilibrium. $$\begin{array}{cccc} \begin{array}{c|ccc} \hline & \ce{Fe^3+} & \ce{SCN-} & \ce{[...
Tyberius's user avatar
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4 votes

What makes a species "fluorescence quencher"?

A fluorescence quencher is any species that causes your fluorophore to stop fluorescing. There are several ways this can happen, but there are two broad categories: chemical reactions and energy ...
Ben Norris's user avatar
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4 votes

Predicting 260/280 of DNA or RNA from sequence?

You could try a different pH to get rid of secondary structure. It might change the absorbance spectra as well, though. Here is a paper where they measured spectra of DNA and its constituents: https://...
Karsten's user avatar
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Why does the reduction of NAD+ to NADH change the absorbance so much, and cause fluorescence?

Essentially NADH has an additional pair of electrons. These additional electrons which are oftein drawn as a lone pair of electrons alters the molecule in a similar way to the way in which many pH ...
Timothy Olsen's user avatar
4 votes

Why does a smaller bandgap correlate to increased absorption of visible light?

In your case, absorption takes an electron from a valence band and in to a conduction band. At energies below the narrowest gap between the valence and conduction band, the photon can't give enough ...
Jon Custer's user avatar
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4 votes

Is it possible for a substance to absorb visible light but still appear white?

Your eyes are not sensitive to UV light so any absorption there will be invisible to you. The visible spectrum you give has fairly uniform absorbance across the visible. The whiteness you see is ...
porphyrin's user avatar
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Is a spectrophotometric method of measuring the rate of a complex reaction valid?

This is a perfectly valid method of determining rates of chemical reactions. It is obviously limited to those reactions where either the product or the initial reactant can be monitored easily using ...
jheindel's user avatar
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4 votes
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Using Nanodrop for UV/Vis analysis of compounds at limited amounts

In short, i would say that you cant use Nanodrop for such work. Here is why: From a quick google search it appears that a nanodrop is a UV-Vis spectrometer that can measure very small amounts of ...
Outlander's user avatar
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4 votes

A Spectrophotometric technique to evaluate Vitamin C concentration in Citrus Fruit

It is not quite correct of the statement that "ascorbic acid cannot be determined by Beer Lambert's law, because it does not absorb visible light." Although the second part of the statement ...
Mathew Mahindaratne's user avatar
4 votes

How to determine chlorophyll content from spectrophotometry data (absorbance vs wavelength)?

How to determine chlorophyll content from spectrophotometry data (absorbance vs wavelength)? You have to consider two pigments, chlorophyll A and chlorophyll B, along with other substances that ...
Karsten's user avatar
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4 votes
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Why is the entire 3p sublevel peak shifted to the right on a PES of Sulfur vs Phosphorus?

My question was rooted in a misunderstanding with the process of Photoelectron spectroscopy. Originally, I thought that photons were beamed until all electrons were lost from an atom, which would ...
jb0's user avatar
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