Three special Reaction in Leather chrome tanning - Olation, Polymerization, Oxolation and The terms oil and lipid

Olation: 

The name olation has been given to the combination of two or more molecules of basic chrome compounds with one another producing a bigger molecule.The mechanism of simple olation of 33.3% basic chrome chloride is represented as follows -
In this olation OH is combined with one chromium atom with the primary and with another chromium atom with the secondary valency. Thus Cr - OH - Cr. Two molecules of water have been eliminated and the positive charge of the olated molecule has been doubled. OH group held between the chromium atom is called the Ol group, as in alcohol, phenol. OH group or groups therefore held between the chromium atom are not simply titratable with acid, as in un-olated OH groups within the complicated can be titrated directly by acid. The method of olation is favoured by heating, increasing concentration, increasing basicity or storing wet 'chromed leather' for a length of time.

Polymerization: 

The increase in the size of the molecules of basic chrome salt is not restricted by their olation as a result of they'll also grow to a larger size by polymerization subsequent to olation.
There is a difference between olation and polymerization. In the former water is eliminated from the molecule but in the later there is no such water elimination but the molecules simply join up forming bigger molecules.
Both olation and polymerization are affected with a similar factors, viz. increase in basicity, ageing, temperature etc.

Oxolation: 

Under certain conditions olated chromium compounds are become into oxolated ones. The modification is represented as follows-
In this the OH link of the olated compounds is modified into oxygen link of the oxolated compounds with the liberation of acid. Oxolated compounds could also be formed by the continuous heating of the olated compound.

The terms  oil and lipid 

Oils: Generally refers to a  fat with  short or  unsaturated fatty acid chains that  is Liquid at  normal temperature. Oils have a high carbon and  hydrogen  content and are normally flammable slippery.

Fat: Fat may refer to fats which are solids at  normal temperature. Fats that are saturated  fats don't have any double bonds between  the carbon within the chain. Unsaturated fats have  one or additional double bonded carbons within the aliphatic chain.

Lipid: Lipid is the normal term as a lipid is  not necessarily a triglyceride. Fats as like other, lipids  are usually hydrophobic  and  are soluble in organic solvents and insoluble in water.

Triglyceride and phosphates  are esters which form the major components of most commercial oils  and fats, derived from vegetable and animal  kingdom.

Esters which in oils and fats are combination products  of alcohols (glycerol, compounds containing  alcoholic -OH groups) and  acids  (fatty carboxylic acids).

            RCO₂H  + R'OH = RCO₂R' + H₂O

In glycerol each of thee  three molecule -OH groups can react  with single molecule  of  an acid (fatty) to give an ester. When all three  -OH groups off glycerol molecule have  been  esterified  with fatty carboxylic  acids, triglycerides (fats and oils) are formed.

Fatty acids  of  higher molecular weight, such as solid saturated steric  acid, C12H35-COOH; pamitic acid, C15H31-COOH; or  liquid unsaturated  oleic acid, C17H33-COOH are widely disseminated  in animal  and plant organisms as esters. 

The chemical reactions  which takes place  to form say, stearin by  esterification of stearic  acid and glycerol  may be  written as:

Chrome tanning and Objectives of chrome tanning and Confirmation test of chrome tanning

Chrome tanning: 

The process of tanning hides and skins into leather with basic chromium salt is known as chrome tanning. It is a quick process.

Objectives of chrome tanning:

1. To convert the putrescible hides/ skins into non - putrescible leather.
2. To raise the shrinkage temperature ( Tₛ)/ hydrothermal stability.
3. To reduce the power to swell once wet back.
4. To increase the strength property of leather.  
5.  To protect from microbial attack.

Tanning

Confirmation test of chrome tanning: 

Chrome tanning process is usually considered to be completed when a piece of tanned leather does not shrink in boiling water for 3 or 5 minutes.
For this a sample is cut from the thickest part of the tanned skin or hide, smoothed out on a piece of wooden board and its boundaries outlined with a pencil. It is thus placid in a bowl of water kept at boiling point for three minutes. The sample is then taken out, cooled, ironed out once more and if no shrinkage or any curling lip on the sides and corners be noticeable the tanning should be considered as complete.
The test is followed by the tanners who follow one bath chrome tanning process.