Figure shows the UV-VIS spectroscopic result of NaCl solution prepared in distilled water. The solution is transparent and it absorbs in the whole visible region with increase in its concentration but the absorbance peak is found to at 540 nm. To find the concentration of unknown NaCl in solution using Beer Lambert law in which the Molar absorptivity depends on wavelength therefore should I use a white led or a LED with peak emission wavelength at 540 nm. I searched the literature but I have not found why the peak absorbance occurs at 540 nm. Please friends help me out
The absorbance you are seeing is going down with increasing % of NaCl solution; therefore, I would guess the absorbance you are seeing is due to contamination in your distilled water (and it is definitely not due to your NaCl solution). Have these absorption spectra been background corrected, i.e. did you subtract the absorption spectrum of the 0% NaCl solution from each? Also, I don't believe an aqueous NaCl solution should have any absorbance in the visible region (400-800 nm) and very minimal in the UV region of a typical UV-vis instrument (200-400 nm).
First, I also think that this spectrum is a result of impurities because the optical density is very small. Second, It is need to show a background line with distilled water only. Third, the optical density increase can be result of some colloid presented or formed in water solution. In this case the absorbance will be not zero during all visible region. So, it is need to filtrate the solution through 0.25 mkm membrain. 540 nm it is red colour. Impurity can be present on the wall of cuvette.
I agree fully with the excellent answers of Marvin and Sergei. But , Dear Itak, prior to post a question , you must give a lot of infos as follows:
Typ of spectrometer :
- single beam or dual beam or diode arrays.
-Your UV-Vis spectrometer must be OK. (Lamp, filter , slit...)
- What a kind of cuvettes are you using ? Quartz (hellma p.e.) or PS (polystyrol) 340-900 nm or PMMA (polymethylmethacrylat ) 300-900 nm.
- You must clean very carefully your cuvettes (if you use quartz) (H2SO4/dist. Water/ drying with EtOH and aceton (all pro analysis and spectranal quality).
- It seems that you produce a "salting out" effect on the wall of your cuvette. Why ?
With increasing concentration of NaCl to 4 % , you obtain a lower absorption value than with 1% NaCl.
-It is a conjugated impurity .
-Last question :
Have you used your own cuvettes or the cuvettes of a colleague (if quartz)?
JRG
There seems to be some impurity in your sample. Just check i) if the cuvette is clean, ii) the distilled water you are using is clean and iii) finally ensure that NaCl does not contain any impurity.
I think this link is very useful,
http://www.instanano.com/characterization/theoretical/concentration-calculation-from-uv-vis-absorbance/
an optical technique for detection and identification of NaCl from satellite images was established and then calibration is conducted (radiometric and atmospheric correction)
Surely it cannot be a Nacl absorbance spectra. I also developed the same for my study. Nacl absorbance is NIL in the region of 400-800nm. The absorbance taking place and starting from 200nm-400nm only.
Dear Iftak, clean your cuvette properly and go for a baseline correction both The cuvettes containing DW. After this step start a scan containing same blank solution(DW) its absorption must be showing zero. If it shows zero then your machine is ok.
Now make a fresh stock solution of NaCl be sure it should be colourless if it is having any colour then definately you are taking wrong chemical, it got adultrated. The last thing dissolved air must be removed from DW.
purge your instruments with inert gas if your compound absorbs light near 200 nm and use quartz cuvettes only while 200 nm or below.
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