Is sulfur dioxide basic or acidic

PH value

From Lederzentrum Leather Lexicon

The PH value(Latin: potentia Hydrogenii, "power of hydrogen") of a substance indicates, in logarithmic intervals, how acidic or basic this substance is. Distilled water is defined with a pH value of 7, so that - in comparison - a substance with a pH value of 8 (such as sea water) is ten times more basic and a substance with a pH value of 5 (e.g. coffee) is 100 times more acidic . Cola has a pH of 2, making it a hundred thousand times more acidic than distilled water.

In leather production and leather care, the pH value is important in that the tanning in the leather basically creates an acidic environment with a pH value of approx. 4.5 to 5.5. This pH value sometimes ensures that the fat and tannins bound in the leather are retained.

However, as the leather ages, the acid-base ratio shifts to the alkaline level (above pH 7), which makes the leather unstable. The leather darkens, becomes hard and brittle. The shift occurs through age, humidity and sun, but also through sweat (skin and hair contact in cars and furniture, sweat through feet in shoes (daily between 100-200 ml), hands in gloves or "bacon collars" in jackets).

Typical damage from skin and hair contact with furniture leather (headboards and armrests)


Steering wheels also react to hand sweat over time. The pH shift also makes the leather more susceptible to mold growth


Car armrests damaged by skin contact, a gear knob and a bus headrest


Another reason are alkaline cleaners. Most household cleaners are alkaline because they do a good cleaning job without any problems. Leather cleaners should not be alkaline, which is technically possible, but not all manufacturers take into account.

The care products should ideally restore or maintain the acidic state. In the case of mold growth on old leather, for example, vinegar essence is an excellent pH regulator for removal and prevention. In the general restoration of old leather, acidification is just as important as refatting in order to make the leather more resistant to environmental influences.

At the same time, there is of course a limit to how much acid a leather can withstand. A characteristic acid damage to book covers that can occasionally be observed in libraries is, for example, the red decay when sulfur dioxide from the air in the leather reacts to form sulfuric acid, which eats the cover beyond repair. However, sulfuric acid is also one of the strongest acids and is far below the maximum tolerable pH value of 3.5 for leather.

substance PH value Art
Battery acid <0 angry
Stomach acid (empty stomach) 1,0–1,5
Lemon juice 2,4
cola 2–3
Morello fruit juice 2,7
vinegar 2,9
Orange and apple juice 3,5
Wine 4,0
Sour milk 4,5
beer 4,5–5,0
acid rain < 5,0
coffee 5,0
tea 5,5
Rain (natural precipitation) 5,6
Mineral water 6,0
milk 6,5
Water (depending on hardness) 6,0–8,5 acidic to basic
Human saliva 6,5–7,4
blood 7,4 basic
Sea water 7,5–8,4
Pancreatic juice (intestinal juice) 8,3
Soap 9,0–10,0
Household ammonia 11,5
Bleach 12,5
concrete 12,6
Caustic soda (caustic soda) 13,5–15

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