pH Control in the Tannery

Dr Clive Jackson-Moss

The measurement of pH in the tannery is one of the most important quality control measures that can be carried out. It is one of the few checks that can inform the tanner of whether a process is running according to plan or not. Many tanneries only measure pH once or twice during a process. However, this is not the recommended option and if possible, as many pH checks as possible should be taken during a process. This will provide the tanner with better information as to how well a process is running, and whether there are any significant fluctuations in pH during processes.

pH can be measured using either a pH meter, pH indicator paper strips or pH indicator solutions. Most tanneries use a combination of these methods. The pH meter and the pH indicator paper strips are used to give a pH value of the float in the drum. The pH indicator solutions usually give an indication of the pH inside the skin. This is achieved by applying some of the indicator onto a cross-section of skin and taking note of the colour that is produced. The colour is indicative of a pH value or range.

The pH changes that take place during processing are indicated below.

Soaking

Most tanners would use a soak pH of between 7 and 9. The modern trend has been towards using a higher pH during soaking to reduce the difference between the soak pH and the liming pH. This reduces the potential of drawn grain caused by uneven swelling of the hide as it enters the liming process. Usually only the float pH is checked at this stage.

Liming / Unhairing

pH values are usually between 12 – 12.5. Usually the float and the skin pH are checked. The skin pH is checked with phenolphthalein pH indicator solution. It is bright pink at pH values greater than 9.0, and hence it should show a bright pink colour throughout the skin cross-section after liming.

Figure 1. Pink colour obtained after applying phenolphthalein pH indicator solution to the cut edge of a limed hide. If the lime had not penetrated all the way through the hide, the phenolphthalein will not be bright pink in the centre of the hide

Washes after Liming

During the washes after liming, it is common to add a small quantity of a deliming chemical to start with the removal of lime from the skin. This will have the effect of reducing the pH slightly and it would be common to find a pH value of 10.5 – 11.5 at this stage of processing.

Deliming

Deliming of the hide or skin usually results in a pH of between 8.0 – 9.0. This pH is checked using both the pH meter for the float, and phenolphthalein for the skin cross-section. It is important that the skin pH is below 9.0 for the bating to be carried out effectively. Since phenolphthalein is colourless below pH 9.0, a properly delimed skin cross-section will be colourless.

Figure 2. Photograph showing a hide that is only partially delimed. The pH in the centre of the hide is above pH 9.0 and thus shows pink when the pH indicator phenolphthalein is applied to the cut. On the outer edges of the cut where the phenolphthalein shows clear, the pH is below 9.0.

Bating

Bating is usually carried out at a pH between 8.0 – 9.0 which is the pH at which most bates are active.

Pickling

During the pickling stage, the hides or skins are placed in a solution that contains salt, sulphuric acid, formic acid, and in some cases acid sulphones. At this stage, the pH is drastically reduced, with common pickle pH’s being in the 2.0 – 2.5 range. The pH of the skin is checked using the pH indicator bromocresol green. It has a yellow colour at the pickle pH values.

Figure 3. Photograph showing the pH indicator bromocresol green after applying it to a cut that has only been partially pickled. The yellow colour on the outer edges shows that the pH is below 3.0, but the blue in the centre of the cut hide indicates that the pH is still high, probably greater than pH 6.0. Scud is visible in the grain layer.

Figure 4. Photograph showing the pH indicator bromocresol green after applying to a cut that is fully pickled. The pH is less than 3.0 throughout the cross section and thus the colour of the indicator is yellow.

Tanning

The pH of the tanning float is usually between 2.5 – 3.0, which is low enough to allow penetration of the tanning chemicals. If the pH was not low enough, there is the possibility that the outer layers of the skin will be over-tanned, with not enough tanning chemicals reaching the centre of the skin structure.

Basification

The basification process is the stage at which the tanning chemicals bind to the collagen protein. This binding is due to an increase in pH, usually to an end pH in the range 3.6 – 4.2. It is best to have a gradual increase in pH over time during basification rather than spikes in pH.

Once the wet blue or wet white has been processed, it is usually sammed, shaved and then moves to the dyehouse for conversion into crust leather. The dyehouse processes are very dependent on small changes in pH during processing.

Figure 5. pH changes during the basification process. Note the gradual increase in pH over time which is important during the basification process (Source: Clariant)

Wetting Back / Washing

If wet blue is to be rechromed, a small quantity of acid will be used during the wetting back stage to reduce the pH to allow penetration of the rechroming chemicals into the skin. The use of formic acid is common with a final pH of about 3.0 being optimal.

Neutralization

The neutralization of wet blue or wet white involves removing acidity from the skin, with the result that the pH of the skin will increase. The degree to which the acidity is removed will have a significant impact on how well chemicals will penetrate into the skin during the subsequent dyehouse processes. Typically, shoe upper leathers are neutralized to pH values of between 4.2 – 4.8, while softer upholstery type leathers will have neutralization pH’s in the range 5.4 – 5.8. The pH of the skin is checked with the pH indicator bromocresol green. The colour of the indicator is dependent on the pH of the skin, which means that for shoe upper the colour of the indicator would be a yellow- greenish colour, whereas for upholstery the skin cross-section will be a blue colour.

Figure 6. Colour changes of pH indicator bromocresol green at various pH’s (Source: www.alchetron.com)

Retanning, Dyeing, Fatliquoring

Most tanners assume that the pH during these stages remains the same as that achieved after neutralization. However, this is not true as the chemicals used during these process stages affect the pH. Many tanners do not check the float pH during these processes. However, if they did they may be surprised to see just how much the pH can vary. Stricter process control is possible if pH values are monitored during these process stages.

Fixation

The addition of formic acid reduces the pH to a final value of 3.4 – 3.6 during this stage, and is critical to ensure that the dyehouse chemicals are bound tightly to the collagen protein. High end fixation pH values can lead to all sorts of problems such as dye bleeding and migration and poor fogging values.

Leather Finishing

Many tanners do not think it is important to measure the pH during the finishing of leather. It is useful to check the pH of resins, penetrators, finish mixtures etc as these often give an indication of the stability and age of products. For example, the coagulation of some mixtures is often due to a sudden change in the pH value of the mixture when another chemical is added.