Application Technology - pH: what does it mean? - CEBE Reinigungschemie GmbH

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Application Technology - pH: what does it mean?

Nobody who is occupied in the cleaning sector dares ask, even if they do not know the answer.

The pH is the negative logarithm of the hydrogen ion concentration. The hydrogen ion (H+) is in equilibrium with the hydroxyl ion (OH-), which may be represented in the following manner:

H2O <=> H+ + OH-

This simply means that water will divide (dissociate)into these two components. Depending on other substances present that may be dissolved in the water, this equilibrium may be shifted in one way or the other.

Pure water has a pH of 7. This means that the concentration of hydrogen ions as well as that of hydroxyl ions is 10(-7) g/mol. The product of both concentrations is always 10(-14).
 
The pH scale goes from 0 to 14. A pH of 0 means a very strong acid like 30% hydrochloric acid, for example, and a pH of 14 means a strong base or alkaline like 50% sodium hydroxide solution, for example.

A pH of 7 is classically known as pH-neutral, as this value lies in the middle of the scale. However, talking about neutral: there are differences. For example, human skin with a pH of 7 would no longer be regarded as healthy. A pH of 5.5 is commonly known as skin-neutral, implying that this value represents a healthy value for skin. This means that any hand soap being advertised as having a pH that is neutral for the skin, will have a pH of around 5.5.

It becomes even more interesting when we start talking about pH-neutral or the pH-neutral range in cleaning.Some will tell you that there is no such thing as a pH-neutral range, there is only a neutral point, namely 7.0. Generally speaking, however, one can say that the pH-neutral range lies from 6 to 8.

Since pH is a concentration expressed in logarithmic scale, this means that in order to increase the pH from 1 to 2, the solution has to be diluted with a tenfold amount of water. In order to attain a neutral pH, it has to be diluted with a millionfold amount!!

Turning this thought around, one can say that one can reduce the pH of water with only a very small amount of concentrated acid.

If biological systems were to react in this manner when small amounts of concentrated acid or base were added, this would have a fatal result for plants and animals.

This effect is prevented by buffers. Buffers are not only vital for humans, animals, and plants, they are also elemental parts in many chemical and industrial processes, where the pH may only change in very small fractions.

A buffer solution usually consists of a weak acid or base and one of its salts, for example ammonia solution and ammonium chloride.

In order to understand how a buffer works, let us imagine a weak acid, HA and its salt MA, in solution. The acid, as it is weak, will not dissociate completely while the salt will have dissociated completely. This may be expressed in the following manner:

HA <=> H+ + A-

MA <=> M+ + A-

Expressed in this way it means that the solution contains a relatively high share of not dissociated acid, HA, and also contains a large share of A- (a base, meaning alkalinity).

If one were to add acid to this solution, the H+ ions will associate with the A- ions in order to form non-dissociated acid, HA. Under the condition that there is a large enough supply of A- the largest part of H+ ions are removed. This means that the concentration of H+ ions has changed only marginally and therefore, too, the pH-value.

What happens to the system if the opposite happens, that is, base is added to it?

In this case the OH- ions (we recall the first part of this series, where the self dissociation of water, H2O, into H+ and OH- was explained) unite with the H+ ions. This has the effect of reducing the concentration of the H+ ions. However, since the system contains the weak acid, HA, this begins to dissociate until the balance has been regained - under the condition that enough weak acid is present - enough H+ ions are generated to maintain the concentration of same and therefore the pH unchanged.

These examples demonstrate how one can use a buffer system in cleaning products in order to ensure that the pH will change only marginally upon dilution.

The pH plays an important role in cleaning: it gives the user an indication whether a product is suitable for an application or not. For example a cleaner with a low pH value would not be suitable for synthetic carpet and the opposite would be true for a wool carpet.

In the same manner it is possible to discern the suitability of a cleaning product for calcium-based stones. If the pH lies in the acidic range (pH < 6), then the product should not be used on these materials.

Just the same one can determine whether a natural soap may be suitable for use on a wood or parquet floor, by measuring its pH. If this lies in the alkaline range (pH > 8) then one should avoid using this on these types of floors.

Of course there are always exceptions. But if you only have a limited amount of information at your disposal, then you can do a lot with something as simple as the pH value. It may save you the time of searching for further information.

For this reason the professional cleaner should always have some pH-paper or a battery operated pH-meter with him/her.
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