The process of de-acidification of chrome leather is known as Neutralisation. Leather after leaving the chrome bath is in an acid condition with a pH of 3.6 to 4.2. The acid in chrome leather is present as free acid or as protein - bound or in the chrome complex itself. The basic chrome salt bound by hide protein in a wet condition hydrolyses and liberates further quantity of acid, getting itself more basic and fixing on to the hide fibers more firmly.

In wet chrome leather this process of hydrolysis is continuous and the presence of acid in the leather causes an excessive positive charge. The main object of neutralisation is to reduce this positive charge to as low a degree as possible so that in the subsequent reatanning, dyeing and fat liquoring agents do not combine prematurely with the surface of the leather.

This required extent of neutralisation can be achieved by repeated washing in plain water, a time consuming process, but can be speeded up with a chemical treatment. As the acid in the leather is to be removed only partially, mild alkalis or alkaline salts alone or in combination with synthetic tanning agents are introduced.

The choice of a neutralising agent has considerable bearing on the final leather produced. Various mild alkaline salts, such as sodium bicarbonate, ammonium bicarbonate, borax, calcium or sodium formate, hypo and sodium sulphite are generally accepted as neutralising agents.

The depth of neutralisation depends largely on the quantity, the manner of application and the right selection of a neutralising agent. An in appropriate neutralisation can deteriorate the quality of leather, which cannot be made good subsequently. This holds particularly true for over-neutralisation, which becomes irreparable even by adding acid.

The most poplar neutralising agent is sodium bi-carbonate used either alone or in an admixture of sodium or calcium formate or syntans. Sodium bi-carbonate reacts with the chrome complex and converts the strong mineral acid into carbonic acid- a weak acid and volatile acid. However, its also penetrates the loose portions more than the firmer parts of the leather. The varying pH conditions in the loose and the firm parts of the leather necessarily upset the subsequent retanning, dyeing and fat liquoring operations.

Calcium or sodium formate reacts with the chrome complex converting free sulphuric acid in leather into a weaker and less harmful formic acid. The rate of penetration of formates is much quicker than sodium bicarbonate and imparts a uniform pH in all of the different parts of the leather. The buffering action of formic acid keeps the pH more constant during the ensuing retanning and fat liquoring operations resulting in uniform uptake. The combination of bi-carbonate and formate gives satisfactory deacidification with uniformity in all parts of the leather.

Compared to sodium bi-carbonate, borax has surface reacting and gives a higher pH but a slower penetration.

As is already seen, due to hydrolysis the chrome complex in wet leather forms higher basic chrome compounds with liberation of free acid. After neutralising this free acid with weak alkalis, the chrome compound still remains unstable and can further hydrolyze, liberating more quantities of free acid. Addition of synthetic tanning agents like OROTAN SN, OROTAN L CONC, eliminates this drawback.

Syntans are ammonium or sodium salts of aromatic sulphonic acid. When syntans come into contact with basic chrome complex, its sulphonic acid radical (which has a tanning action) gets more firmly embedded in the chrome complex than the sulphate radical, thus forming a new complex which has a lesser tendency to hydrolyze. Neutralising with syntan involves no risk over-neutralisation associated with alkaline salts like sodium bi-carbonate. The tanning sulphonic acid will rather produce a moderate acid adjustment of the outer layer of leather which is considered favorable for grain tightness. On account of its tanning action, a syntan besides neutralising, initiate the subsequent retanning process.

The importance of neutralisation for tightness and other quality characteristics of the finished leather has now been given its proper place. Neutralisation as understood today is not only to cover the reaction of alkaline with the acid present in the skin but more important, to stablise the chrome complex by replacing the strong with the weak acid having tanning properties.

It is difficult to judge the acid reserve and its distribution in a cross-section of leather and all the more so, considering the differences in the structure with one hide and from hide to hide in one pack.

The use of synthetic tanning agents along with weak alkalis in the neutralising system ensures uniformity throughout the leather surface, without any danger of over-neutralisation and binds the chrome complex more firmly with the hide protein.

Neutralization is carried out in a drum. The efficiency of the process as pointed out earlier, depends on the right selection of a neutralising agent, its quantity, float and drumming time. Good results is achieved in a short float of 50% water (on shaved weight). Neutralising agents are added in either liquid or powder form and drumming is continued for 45 to 60 minutes depending on the thickness of the leather. The desired end-point can be tested in a cross-section of the leather with coloured pH indicators, like Bromo Cresol Green or Bromo Phenol Blue. A satisfactory neutralisation will put the leathers in right condition to receive the processes of reatanning, dyeing and fat liquoring.