Is pH 5.5 Neutral

Cosmetics with a pH of 5.5 are well kuowu from advertisemeuts or dragstore shelves. Most of these products are shower gels, moisturizers or other creamy stuff that can be rabbed directly into the skin, and it is implied that a pH of 5.5 is an important and skin-friendly properly of these products. Unfortunately, this leads people to conclude that a friendly pH is a neutral pH. In first year university chemistry, a sizeable part of the students selects 5.5 as neutral pH despite the fact that they were supposed to absorb something else. 

Pure water does not simply contain molecules since ions are also present, as the following reaction also shows  

This process bears the respectable name of self-dissociation of water and is pre-smned to be an equihbrium reaction, because it concludes when the species indicated on the left and right side of this chemical equation (reactants and products) are present at the same time. This means the order of the reactants and products in the above chemical equation could even be reversed. The reaction itself is also called reversible. In pure water, the same amounts of hydrogen (H+) ions and hydroxide ions (OH ) form. The notation used for hydrogen ion is still a matter of debate in today s science. Some experts stubbornly stick to the notation HjO which is called oxonium or hydroxonium ion (— 4.10). It is difficult to formulate justice here, as reality itself is much more complex than any single chemical formula could express. But as far as notations go, using the simpler one seems... well, simpler. 

With this information, one can conclude that a pH of 5.5 is lower than neutral. When the concentration of hydrogen ion is larger than the concentration of hydroxide ion, the solution is acidic. Still, it must be remembered that the surface temperature of a person s skin is not exactly 25 °C. Table 4.4 shows the dependence of neutral pH on temperature. At 37°C, the neutral value is 6.81. Therefore, 5.5 must still be acidic. 

Aiyone interested in why pH values gained such prominence in the scientific world should also note that many of the properties of a solution do not depend on its actual constituents— pH is all that matters. A solution of pH 14 is highly harmful for the eyes and the skin, and it is not important what causes this basic nature. In Hungary in 2010, there was a major catastrophe of red mud In this disastrous waste spill, the highly-basic pH of the mud was the primary hazard, and not the metal content of the mud, as the media reported at the time (— 4.8). 

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