2)MASKING OF CHROME SALT IN POWDER

2)MASKING OF CHROME SALT IN POWDER

1.AIMS

·         The aim is to observe the effects of masking and basicity on chrome salts and, provided that it is possible, to observe the effects on the hide treated with chrome salts of different basicities and degrees of masking. We looked for differences of masking effects by time and temperature.

·         The proportions of products used in this practical will sometimes be increased a lot so that the effects can be observed more clearly.

2.THEORETICAL BASIS

·         Masking agents;

The tanning action of basic chrome sulphate can be modified by adding weak acids and their salts. The stypticity of the tanning substance can be increased or decreased by selecting the substances to be added that will after the chrome complex accordingly. In particular, masking acids or their salts modify the sizes of the particles of the tanning substance and their affinity for the hide, which could increase or decrease according to the type and quantity of the aggregate masking material.

The main masking substances used are:

-          Formic acid or sodium formate

-          Sodium phthalate

-          Sodium sulphite

-          Various commercial products consisting of mixes of organic acids and their salts.

The amount to use of the various products is between 0.5 and 1.5% by pelt weight, and this amount should be added on the basis of the particular characteristics of the compound.

·         The result of investigations against some of the ions than other chromium atoms
 greater interest (affinity) have been shown.  Masking some of the ions are listed as follows according to Stiasny affinities.

Nitrate < klorur < sulfat < formate < asetat < sulfit < ftalat < oksalat

·         Masking is defined as the incorporation of certain reactive groups, i.e. ligands, intochrome tannin complexes. The purpose of masking by mono basic salts is to enhance the chrome penetration rate and permit basification to higher pH: i.e., to increase the pH value at which the chrome complex precipitates. Cross linking  masking  salts can enhance chromereactivity, but reduce penetration rate.

   Dicarboxylic acids are well-known masking agents. Short chain dicarboxylic acid salts will produce cyclic, chelate complexes if the ring size is 5-7 membered; this will reduce the reactivity of chromium (III). Longer-chain dicarboxylic acid salts cross-link two chromium (III) molecular  ions; this makes  the chrome species bigger, thus  increasing  the  reaction rate but decreasing the penetration rate. By including an organic acid, the complexes are less cationic, thus  reducing the astringency or affinity  to the  collagen. The masked chromium salt penetrates the substance more easily and chrome distribution over the cross-section is more uniform. Today masked chrome  tannins are  commercially available. Tanning liquors with a low degree of masking are produced when acid dichromate is reduced with molasses.

   Several other  tanning auxiliaries are available on the market, usually based on somecross-linking function, e.g. aliphatic dicarboxylates, low molecular weight polyacrylates and syntans. They may be applied  at  the start of the tanning  process or even  form  a part of the basification system. Another example refers to the use of a mixture of aliphatic dicarboxylicacids in the pickle and sodium aluminium silicate for basification (6). A more recent development is to increase  the  complexity of  poorly bound or unbound chrome by applying  low molecular weight polyacrylates during  tanning or re-tanning (8)

3.MATERIAL AND PRODUCT

·         Beakers ;glass rod ;pH meter;a 15 mL graded pipette or a 50 mL burette,thermometer.

·         Chrome salt 33% and 42% basicity, sodium carbonate, formic acid, sodium bicarbonate, etilen-diamine-tetra-acetate (EDTA), hydrogen peroxide, oxalic acid, glucose, sodium dichromate, sodium formate.

4.METHODOLOGY

·         We had prepaired 100 mL of a chrome salt dissolution with 33% basicity at a concentration of 10%. After it was dissolved in cold water. We had measured its pH and kept half of the dissolution and had left it until the following day. We had heated up the other half of the dissolution to boiling point, then it was cooled end we had measured its pH.

We had repeated the same process for chrome salt with 42% basicity.                  

5.CALCULATIONS, RESULT AND ASSESMENTS

·         There are in the table.                                  

6.CONCLUSIONS

·         33% Basicity Chromium Salt

pH	Values	Explanation
pH1	2,87	At First time
pH2	2,47	By Temp.
pH3	2,40	24 H later (No Temp.)
pH4	2,63	Final (No Temp.)
pH5	2,67	Final (By Temp.)

               

               

·         42% Basiticy Chromium Salt

pH	Values	Explanation
pH1	3,65	at first time
pH2	2,84	by temp
pH3	2,95	24H later
pH4	2,99	final (no temp)
pH5	3,05	final (by temp)

                               

7.BIBLIOGRAPHY

·         The tanning engineering notebooks Edited by Assomac

·         http://tr.wikipedia.org/wiki/

·         forum.europa.eu.int/Public/irc/env/ippc_brefs/library

·         http://bp3.blooger.com.

·         santanaderi.com

·         http://www.scribd.com/doc/56917266/10/Masking-in-chrome-tanning