If samples (lignosulfonates or precipitated lignins) contain high content of ash, does the ash have a negative effect on the determination of UV spectroscopy? Are more purified samples are better?
On the other hand, the ash that is in addition to finding reflects the purity and amount of inorganic chemistry (Ca, Na, ..) which is incorporated in the compound (example: lignosulfonates C20H24CaO10S2).
Purification of samples results in the changes in the content of OH groups.
Example of Samples and content of ash:
Vanisperse 34.2 %
Boresperse N 23.3%
Orzan 10.8%
Precipiataed Kraft lignin 3.9%
Precipitated lignin II. 0.4%
UV spectroscopy
After being dried overnight at 80°C, precisely weighted amount (about 5 mg) of the lignin was dissolved in 5 mL of dioxane and 5 mL of 0.2 M NaOH. Some of the solutions were not quite clear and these were filtered using a 0.45 m PVDF membrane filter. From each lignin solution, 2 mL was further diluted to 25 mL using either a pH = 6 buffer solution (citrate — NaOH, Merck), and 0.2 M NaOH solution.19 This gave each solution a final concentration of lignin of about 0.08 g/L. UV-Vis spectra were recorded on a CECIL spectrophotometer in the absorption region 200 - 450 nm, scan speed 5 nm/s and resolution 1 nm. Lignin solution with the pH = 6 was used as a reference and the alkaline solutions measured against it. From the difference spectra, the absorbance values at 300 and 350 nm, measured against the solution containing 0.2 M NaOH were recorded. According to an original work of Gartner et al.six structural types of phenolic structures exist (Fig. 1). Maxima at 300 nm and 350-360 nm are assigned to unconjugated phenolic structures (I and III) and that at 350-370 nm to conjugated structures (II and IV). According to Zakis et al. the maximum at 360 nm is attributed only to IIa and IVa types of phenolic structures in lignin.
a) Non-conjugated phenolic structures (I + III)
OH(I + III) = {(0.250 x A300 nm(NaOH) + 0.0595 xA350 nm(NaOH)} x1/(cxd) (eq. 1)
b) Conjugated phenolic structures (II + IV)
OH(II + IV) = {0.0476 xA350 nm(NaOH)} x 1/(cxd) (eq. 2)
c) Total amount of phenolic hydroxyl groups
OH(I + II + III + IV) = {0.250 x A300 nm(NaOH) + 0.107 x A350 nm (NaOH)} x 1/(cxd) (eq. 3)
where Aλ – absorbance at a given wavelength divided by the corresponding molar absorptivity, c – mass concentration in g/L; d – pathlength through the sample in cm.
Hi Michal,
Of course the more purified samples are quite better. This due to the interferences between absorption peaks of UV absorbents species. In other words, The appeared peak in the UV spectrum may be is a sum of different absorbents species including the impurities. On the other side. If there is no interference within the peak, the impurities will affect the absorbance through molecular interactions. Furthermore, the molecular interactions also affecting the Uv-visible spectrum for single pure molecules which concentration depended (causing deviation from Lambert-Beer model).
Michal, nekladieš vôbec jednoduché otázky.
First of all, do not dry lignin preparations at elevated temperatures, otherwise certain condensation of its macromolcules in the lignin saccharidic matrix will occurre. The following text concerns sooner the steps that should be done before "native lignins" isolation from wood and plants.
In the case of MWL, Pepper and Freudenberg dioxane lignins and some other preparations, the best way how to remove mineral moieties from wood meal is in the first instance their removal from wood via a series of extractions.
The first extraction should be done with cold water in the presence of maby 0.5% EDTAcetic acid for several h. Then the meal should be washed with deionized H2O and followed with ethanol extraction at ambient temperature. In this second step, however, a small amount of so called Brauns lignin will be removed from the wood sample!
To remove interferring extractives from wood /depending on the wood species/ acetone extraction followed by toluene extraction ought be applied. Time of both organosolvent extractions may be 24 h /in a batch mode, wood to solvent ratio 1:50 at list/. Temperature should not exeed 20 to 30 oC.
After isolation of "native lignins" you may perform your ionisation differential UV-VIS
spectroscopy and determine the contents of non-phenolic, phenolic and conjugated phenolic structures with alpha-carbonyl, alfa - beta double bonds on a propane side chain of phenylpropane building unit of lignin.
In your case of measuring the UV spectra of kraft lignin and precipitated lignin II, you might remove Ca, Mg and other metalic cations linked to lignin via dissolving lignins in water solution of NaOH to which Complexon 2 or 3 is added. Then the samples should be precipitated by acidification of the solution, then centrifuge and dry the purrified preparations in a vacuum or over 98% H2S04, or better over P2O5.
Mnoho šťastia,
R. Solár
PS. Možno by stačilo vyzrážať z roztoku lignínov Ca2+ a Mg2+ ako oxaláty , resp. karbonáty, nie som si istý ako v prebytku NaOH prebehne tvorba oxalátov alebo karbonátov. Ak sa vytvorí zákal týchto nerozpustných solí v roztoku lignínov, je nutné sa ich zbaviť filtráciou, prípadne centrifúgou pred zrážaním lignínov.
Addition to my above answer: in the case of "native" lignins isolation, extraction should be performed in the rverse order /sequence/ starting with toluene and ending with water. Advantage of this is a much better extracgion of cations, and shorter total time
time of purification.
Note to the second part of my answer: the obtained, purified lignins /lignosulphonates/ have to be washed with deionized water. In this case you may dry them at elevated temperatures due to their previous thermal and chemical alterations at the temperatures of kraft pulping.
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