IGUALADA TANNING SCHOOL
Igualada Technical Engineering School
Technical University of
Catalonia (UPC)
PRACTICAL
NUMBER 1:
SIMULATION
OF BUFFER IN DELIMING.
AIMS:
1. The
aim is to simulate the neutralization of the alkalinity of the hide after
liming by using a dissolution of sodium hydroxide.
2. The aim is to observe the possible formation of buffer
solutions and if so, the pH values that are obtained.
3. This can also be useful when comparing the deliming
power inherent to different products.
MATERIAL :
Beakers;
glass rod; a 50mL burette; pH-indicator paper (i.e. litmus paper). If possible
use a pH-meter.
PRODUCTS :
Sodium
hydroxide; ammonium sulphate; acetic acid; lactic acid; chloridric acid; sodium
bisulphite; commercial deliming (both solid and liquid); formic acid,
hydrochloric acid.
PROCEDURES:
- Prepare
a 250 mL solution with 3% of sodium hydroxide and 100 mL solutions with 10% of
each of the remaining products.
- Take
25 mL of the solution of sodium hydroxide and measure its pH and slowly add a
solution of ammonium sulfate (adding 5 mL at a time). Shake briefly after each
addition, observe and measure the pH. Repeat this operation until 100 mL of
ammonium sulfate have been added or until the pH values remain constant with
the new additions.
-Repeat
the same operation with the remaining commercial deliming products.
-
Graph the results by putting the pH values on the y axis
and the added mL of the acid-deliming products on the x axis.
DISCUSSION AND CONCLUSIONS :
-
Observe whether a buffer is formed and if so, the pH values at which it is
formed.
-Draw
a comparison between the values obtained when using different deliming products
both in relation to the formation (or non-formation) of buffer solutions and to
the amount that is needed in order to neutralize the same amount of sodium
hydroxide (25 mL in this experiment).
-Draw
conclusions.
PRACTICAL NUMBER 2:
CHROME SALTS
AIM:
1. 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.
2. The
proportions of products used in this practical will sometimes be increased a
lot so that the effects can be observed more clearly.
MATERIAL:
Beakers; glass rod; pH-indicator
paper (i.e. litmus paper) or pH-meter; a 10 mL
graded pipette or a 50 mL burette, thermometer.
PRODUCTS:
Chrome
salt with 33% basicity and another salt with higher basicity; sodium carbonate; formic acid; sodium bicarbonate; etilen-diamine-tetra-acetate-disodium
= (Edtna) or any type of industrial masking; hydrogen peroxide; oxalic acid; sucrose
or glucose; sodium or potassium dichromate; sodium formate.
1- PREPARATION OF A CHROME
LIQUOR:
Note:
This experiment must be carried out carefully. Only use the right amounts,
never use more than the given amounts. The amounts of both dichromate and sulphuric
acid should only be the ones given. On the other hand, the amount of sucrose
can be modified.
PROCEDURES:
-
Weigh 20 g of either potassium or sodium dichromate in a precipitate vessel of
500 mL or bigger, partially dissolve them in 40 mL of water.
-
Prepare 25 mL of a dissolution of sucrose or glucose at 30%.
-
Add 20 g of sulphuric acid slowly and shake in order to complete the
dissolution of dichromate (the heat that is released when the sulphuric acid
dissolves favours both the dissolution of the dichromate and the beginning of
the reaction with the reducing agent).
-
Before it cools down, add 20 mL of the dissolution of sucrose or glucose very slowly and carefully. If it all works out accordingly, there will be a
release of gases and vapor which are either a little toxic or uncomfortable and
the high temperature is maintained or is even increased (i.e. exothermic
reaction).
-
Once the addition is finished, leave it to one side until the following day.
- Check
the colour of the resulting liquor. The presence of a yellow shade indicates
that the dichromate has not been fully reduced to chrome sulfate. In order to detect this, an analysis can be carried
out determining the residual oxidation power compared to the potassium iodide
(IK) with a starch indicator, among others.
-
If chrome is detected in the form of dichromate, the reduction can be fully
completed by adding some more mL of a solution of sucrose or glucose and
carefully heating up the dissolution to boiling point.
-
If we wanted to obtain a liquor with 43% basicity instead of a chrome liquor
with 33%, then we should only use 18 g of sulphur acid instead of 20 g.
-
If we wanted a liquor with 53%, then we should use only 16 g of sulphuric acid
instead of the 20 g suggested in this experiment.
-
Note: Generally, in reductions at an industrial level, the dissolution does not
need to be heated up since the heat of the reaction is enough to fully complete
the reduction of the dichromate.
2- MASKING OF CHROME SALT IN
POWDER:
PROCEDURES :
-
Prepare about 100 mL of a chrome salt dissolution with 33% basicity at a
concentration of 10%.
-
After dissolving it in cold water, measure its pH, keep half of the dissolution
and leave it until the following day.
-
Heat up the other half of the dissolution to boiling point, let it cool briefly
and measure its pH.
-
Repeat the process with another type of chrome salt with different basicity
(42%) but always using chrome salt in powder form.
-
According to the pH values obtained from the results, try to draw conclusions
concerning the labile masking caused by the sulphates.
3- CHROME MASKING:
PROCEDURES :
-
Prepare about 100 mL of a chrome salt dissolution with 33% basicity at a
concentration of 5%.
-
At the same time prepare about 20 mL of masking dissolutions at concentrations of 1%, 3% and 5%. Measure the pH of
these dissolutions and adjust them to the
approximate value of 3,5 by adding formic acid.
-
Mix 2.5 mL of the chrome solution with 7.5 mL of the masking solution at 1%.
Then shake it and observe any changes in colour and /or precipitations and
measure its pH.
-
Divide the dissolution in two and leave half of it to one side until the
following day.
-
Heat up the other half of the dissolution until it boils and maintain for five
minutes. After that, leave it to cool down slightly and
watch out for any changes of colour or possible precipitations, and finally
measure its pH.
-
Repeat the whole process mixing 5.0 mL of the chrome salt solution with 5.0 mL
of the Masking solution at 1%.
-
Repeat the whole process mixing 7.5 mL of the chrome salt solution with 2.5 mL
of the Masking solution at 1%.
-In
order to further study this phenomenon, the procedures of the process will be
repeated using the the highest concentrations of the masking solutions.
- Compare
the results in changes of colour, pH values, possible precipitations among the
solutions that were analysed immediately, the ones that were left to one side,
the ones that were boiled and try to draw conclusions.
-
Repeat the whole process using oxalic acid instead of Edtna-2 as masking.
-
Compare the two types of masking and draw conclusions.
4-DETANNING SIMULATION:
PROCEDURES:
-
Prepare a 100 mL solution with 8% of chrome salt of 33% basicity by heating it
up and leaving it to cool.
- Likewise,
prepare a 50 mL solution with 8% of sodium carbonate, as well as a 25 mL
dissolution with 8% of oxalic acid.
-
Add about 25 mL of a chrome salt solution
and 25 mL of a sodium carbonate solution
(slowly and shaking). After the addition and when the CO2 is fully
released, measure the pH and observe precipitates.
- Slowly
heat up to boiling point, leave it to cool, then measure the pH and observe
precipitates if there are any.
- Repeat
the same steps with the sodium carbonate but instead of heating it up, very
slowly add 25mL of hydrogen peroxide (DO NOT HEAT). Observe any changes in
colour and any other phenomena.
-
To the 25mL solution of chrome salt, add a 25mL solution of oxalic acid, shake
and observe possible colour changes.
-
Slowly heat up to boiling point, observe again, measure its pH and leave it to
rest. After one day, check for any new changes.
-
The experiment with the sodium carbonate can be replicated using other alkali
products such as sodium bicarbonate, ammonium hydroxide, etc. (Always add the
alkali product to the chrome salt solution slowly and carefully).
PROCEDURES
TO FOLLOW CONCERNING THE WRITING AND PRESENTATION OF THE REPORTS ON THE
PRACTICALS
All
the reports on the practicals should follow these criteria:
1. Aim: A brief description of what the
student studies and attempts to do in each of the practicals in this module. (5%)
2. Theoretical basis: Research information
related to the topics of study for each of the practicals, both in the textbook
and from other sources. (35%)
3. Material and products: List all the materials and products that have
been used in all of the practicals.(Note the purity of each of the products
used). (5%)
4. Methodology: Provide a description of
how the practicals have been carried out. This implies the material and products
that have been used, the working procedures, etc. (15%)
5. Calculations, results and assessments: This
refers to the results that the student has come up with during the practicals. (15%)
6.Conclusions: Observations
regarding the final result. (10%)
7. Bibliography: Write a bibliography for
all the references that have been consulted while writing the report. (!5%)
AGENT, DELIMING: Acidic
salt or acid salt or acid used to neutralise the lime and/or other alkali in
limed pelt and so lower its pH
CAPACITY BUFFER: The
ability of a solution, e.g. of a vegetable tannin infusion, to resist charge of
pH on addition of acid or alkali
BRIDGE, OL: An
OH group joining two metallic ions in a playnuclear complex –Cr-OH...Cr–
BRIDGE, OXO: An
Oxigen atom joining two metallic ions in a polynuclear complex –Cr-O-Cr–
AGEING: The
growing-old of a chrome tanning liquor,during which it un dergoes changes in
chemical constitution, e.g. hydrolysis, migration of ions, olation, etc., or of
mineral-tanned leather during wich the tanning agent becomes more firmly fixed
BOND, CHELATE: A
more or less firm two-point attachment to or ion, with two or more active
groups, e.g. ethylene diamine, oxalic acid, ethylene tetramine, acetic acid,
thus forming a ring
AGENT, MASKING:
Acids or their salts, whose anions enter the chromium complex, thereby
hindering the penetration by the complex-active collagen groups and producing a
milder tanning action and a more uniform chromium distribution throughout the
leather thickness
BASICITY: The
relationship between the chromium present in a trivalent chrome agent liquor,
or a chrome-tanned leather, and the acid radicals associated with it. Measured
in various ways
BASICITY, SCHORLEMMER (percentage):
A method of expressing the basicity of a trivalent chrome
tanning salt in terms of the percentage of the possible number of hydroxyl
groups with which the chromium could combine, of those with which it is
actually combined; passes from 0 in a normal salt to 100 in chromium
trihydroxide
BASIFICATION: The
processo f rendering a chrome liquor more basic, either before use or during
tannage, by addition of a solution of an alkaline substance, e.g. sodium
bicarbonate or carbonate
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