3) CHROME MASKING
1.AIM
·
The aim of this practice is
determined to tanning effect of different rate chrome/oxalic and chrome/edta solutions in cold and hot environment.
3.THEORETICAL
BASIS
·
Masking and masked solutions:
Anions which are firmly held in a complex
retard penetration of OH ligands. Therefore they may prevent formation of large
olated and insoluble complexes. This action is known as masking. Entry of
masking agents into the chrome complexes in solutions of basic chromium
sulphate appears to depend on:
a)
relative amouns of masking agent and Cr
b)
absolute concentration of Cr
c)
presence of other competing ligands(sulphate, chloride, hydroxyl)
d)
whether competing ligands are added together or separately
e) pH
f) T
g) time
h)
whether ligand is added as free acid or salt
·
Masking of chrome tanning agents
Masking refers to the entering of acid
radicals (usually organic acids) into the mainly used chromium sulfate
complexes. The properties of the chrome liquors and that of the resultant leather
can be influenced by appropriate masking, e. g.:
With regard to the tanning agent:
-Reduction of astringency
-Increase of the resistance to alkalis
(flocculation)
-Increase of the penetration rate.
With regard to the leather:
-Improvement of fullness
-Increase of shrinking temperature
-Improvement of grain pattern and smoothness
-Modification of handle
-Improvement of vertical chrome distribution
-Lightening of leather colour.
·
Masking agents with increasing
complex affinity
Chlorate ← nitrate ← chloride ← sulfate ←
sulfite ← rhodanate ←
formate ← acetate ← adipinate ← sulfophthalate
←succinate ← tartrate ←
glycolate ← phthalate ← sulfosalicylate ←
maleinate ←
malonate ← lactate ← citrate ← resorcylate ←
oxalate ← hydroxide
(Each succeeding anion displaces the preceding
one from the chromium complex).
·
Laboratory applications(EDTA)
In the laboratory, EDTA is widely used
for scavenging metal ions: In biochemistry and molecular biology, ion depletion is commonly used to
deactivate metal-dependent enzymes, either as an assay for their
reactivity or to suppress damage to DNA or proteins. In analytical chemistry,
EDTA is used in complexometric titrations and analysis of water hardness or as a masking agent to sequester metal ions that would
interfere with the analyses. EDTA finds many specialized uses in the biomedical
laboratories, such as in veterinary ophthalmology as an anticollagenase to prevent the worsening ofcorneal ulcers in animals. In tissue culture EDTA is used as a chelating agent that
binds to calcium and prevents joining of cadherins between cells, preventing clumping of
cells grown in liquid suspension, or detaching adherent cells for passaging. In histopathology, EDTA can be used as
a decalcifying agent making it possible to cut sections using a microtome once
the tissue sample is demineralised. EDTA is also known to inhibit a range of metallopeptidases, the method of inhibition occurs via the chelation of the metal ion required for catalytic
activity.
Oxalic
acid
EDTA
3.MATERIAL AND PRODUCTS
·
Beakers; glass rod; a 100mL
burette; pH meter.
·
33% basicity chrome, 42% basicity
chrome and oxalic acid and EDTA
4.METHODOLOGY
·
At the same time prepare about
20mL of masking dissolutions at concentrations of 1% and 5%. Measure the pH of
these dissolutions an adjust them to the approximate value of 3,5 by adding
formic acid.Firstly, we had two goups. One group used 33% basicity chorme with
EDTA and the other group used 42% basicity chrome with oxalic acid.Then each
group prepared 6 different concentrations of solutions(2.5/7.5 ml,5/5
ml,7.5/2.5 ml).We measured their pH and taked notes.After that we boiled
solutions.When the solutions was getting cold about 40 C , we measured their pH
again.In adition we observed their colours and precipitates.Finally, we filled
our tables with that results.
5.CALCULATIONS,
RESULT AND ASSESMENTS
·
There are in the table.
6.CONCLUSIONS
·
There are in the table.
7.BIBLIOGRAPHY
·
Leather Technology, Ege University
·
BASF
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