All chemical compounds have a pH



Template: DISPLAYTITLE: pH value Der PH value is a measure of the strength of the acidic or basic effect of an aqueous solution. As a logarithmic quantity, it is defined by the decadic logarithm multiplied by −1 (= "logarithm") of the oxonium ion concentration (more precisely: the oxonium ion activity). The term derives from pondus Hydrogenii or potentia Hydrogenii (Latin pondus, n. = weight; potentia, f. = force; hydrogenium, n. = hydrogen).

Based on the dissociation constant of water kDiss = c (H+) · C (OH) = 10−14 Mole2/Liter2 the value ranges are divided into:

  • pH < 7="" entspricht="" einer="" lösung="" mit="" saurer="" wirkung="">
  • pH = 7 corresponds to a neutral solution
  • pH> 7 corresponds to an alkaline solution (basic effect)

However, this only applies to pure water and dilute solutions at 25 ° C.

Measurement of the pH value

The pH value of a solution can be determined using different methods:

Galvanometry

Most commercially available pH meters are based on this principle. A glass membrane ball filled with potassium chloride solution is immersed in the liquid to be measured. Due to the tendency of the hydrogen ions to accumulate in thin layers on the glass surface, a galvanic voltage builds up inside the sphere. A galvanic cell is created, the electromotive force of which is measured relative to a reference electrode that is independent of the hydrogen ions (see pH electrode).

Measurement by ion-sensitive field effect transistors (ISFET)

Similar to the glass electrode, the hydrogen ions build up a potential on the sensitive gate membrane of the transistor, which then influences the current permeability of the transistor and can thus be implemented using measurement technology.

Colorimetry based on the reaction of an indicator dye

substance PH value Art
Battery acid 1,0 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

The evaluation is usually carried out using color comparison scales. Either the color change of a single dye can be used for a relatively narrow range of measured values, or dye mixtures ("universal indicators") are used, which show a series of different colors over a wide range of pH values. Fields with different dyes are often arranged next to one another on measuring strips, each of which has its optimal readability in a different value range. For special purposes, the color display of an indicator dye can also be measured with a photometer and thus evaluated more precisely.

Various substances are used to color the universal indicator, which change color with different pH values. These include litmus (pH < 4,5="Rot;" ph=""> 8.3 = blue), phenolphthalein (pH < 8,2="Farblos;" ph=""> 10.0 = pink), methyl orange (pH < 3,1="Rot;" ph=""> 4.4 = yellow) and bromothymol blue (pH < 6,0="Gelb;" ph=""> 7.6 = blue).

definition

For very strong acids (pks-Value