What is a PH value

Everyday life human soil chemistry

pH value in A.everyday

Every aquarium enthusiast needs to know the pH value at which the fish feel comfortable. Depending on the type of fish, it requires a very specific pH value that must be adhered to. The pH value indicates how acidic or alkaline a solution is. The concentration can easily be measured with a pH test paper. The pH value can be read off on a comparison scale:
Measure the pH with a test strip
For example, lemon juice contains citric acid. Curd soap and water form an alkaline solution that turns the paper blue. A dye mixture is applied to the pH test paper, which acts as an indicator depending on the pH value. If the universal indicator paper turns red, it contains an acid. With caustic solutions, the paper turns blue. The gradation of the color can be shown on the pH scale:
The starting point of the scale is pure water, which at 25 ° C always has pH = 7, it reacts neutrally. Values ​​below indicate acids, values ​​above caustic solutions or alkalis. As the acid concentration increases, the pH value of a solution decreases and it becomes more acidic. As the alkali concentration increases, the pH value of a solution rises and it becomes more alkaline.
Approximate pH values ​​of common substances
Hydrochloric acid 32%
pH = −1
pH = 5
Hydrochloric acid 3.6%
pH = 0
Skin surface
pH = 5.5
Hydrochloric acid 0.4%
pH = 1
Mineral water
pH = 6
Stomach acid
pH = 2
Pure water
pH = 7
Lemon juice
pH = 2
pH = 7.4
Vinegar essence
pH = 2
Small intestine juice
pH = 8.0
pH = 3
Clean lake water
pH = 8.3
pH = 3
Detergent solution
pH = 10
pH = 4
Caustic soda 3.9%
pH = 14
Sour milk
pH = 4.5
Caustic soda 30%
pH = 15
When washing laundry with heavy-duty detergents, a high pH value of pH = 10 to pH = 11 is necessary so that the surfactants and the active ingredients contained in the detergents can fully develop. When washing gently with mild detergents, the pH value is lower, but this also reduces the washing performance. Aqueous soap solutions have an alkaline effect, while alcoholic soap solutions are neutral. The phenolphthalein pH indicator shows the alkaline effect of the soap in water:
Soap with phenolphthalein in alcohol and in water (right)

pH in the human body

The pH value plays an important role in the human body. in the blood the pH value is directly related to the oxygen absorption capacity. The hemoglobin in the red blood cells can bind more oxygen, the higher the pH value. The blood is very slightly alkaline (= basic): The pH value in the blood in healthy people is between pH = 7.35 and pH = 7.45.

The Saliva is neutral to very slightly basic with pH = 7.0 to pH = 7.1. The enzyme amylase working there splits the starch into smaller components. The Gastric juice is very angry. The respective enzymes for digestion only work at certain pH values. The pepsin in the stomach needs about pH = 2 so that it can break down the proteins. The enzymes in the Small intestine but only work in the neutral to slightly basic range. The Pancreas from the pancreas contains sodium hydrogen carbonate, it has a pH value of 8.0 and neutralizes the acidic digestive pulp from the stomach. It also supplies other digestive enzymes for the small intestine, as well as hormones. The bile with pH = 7.1 from the liver is temporarily stored in the gall bladder. It is released when food is consumed and is used for fat digestion Duodenum, the first section in the small intestine. There the optimal pH value is between pH = 6 and pH = 8. The Colon likes it more acidic again with pH = 5.5 to pH = 6.5.

The digestive system works with several buffer systems that are used to remove excess acid from the body. Because of this, the urine (or urine) from the kidney in contrast to the intestines acidic. The acidic components in the urine are uric acid, oxalic acid or citric acid. It also contains urea, creatinine, phosphates, and other salts like sodium chloride. The normal pH value in the urine is between pH = 4.8 and pH = 7.6. It fluctuates during the day. In the morning, the urine is more acidic as a result of acid degradation overnight. If this is not the case, then there is a disease in which the acid cannot be broken down. The pH level in the urine is also dependent on the diet: Meat and dairy products result in a lower pH level in the urine, while vegetables and fruits produce a higher pH level. They are called "basic food".

In the cells and in the Musculature the pH value is rather low, pH = 6.9, because the combustion in the cells produces carbon dioxide and carbonic acid. Exercise and sport lead to a faster deacidification of the body.

PH valueDigestive functions
pH = 7.0 to 7.1
Starch splitting and liquid supplier
Gastric juice
pH = 1.2 to 3.0
Protein cleavage
Pancreas from the pancreas
pH = 8.0 to 8.4
Enzyme supplier for the digestion of carbohydrates, fats and proteins in the small intestine, neutralization of gastric acid, hormone production
Bile from the liver
pH = 7.1
Enzyme supplier for fat digestion in the duodenum
Small intestinal juice
pH = 6.0 to 8.0
Colon JuicepH = 5.5 to 6.5
Absorption of nutrients and thickening of the residues (fiber)
Urine from the kidney
pH = 4.8 to 7.6
Deacidification, elimination of toxins and metabolic products
Muscles and cells
pH = 6.9
Energy generation
connective tissue
pH = 7.1 to 7.3
Support function, water storage, defense against disease
Skin surface
pH = 5.5
Acid mantle
The human skin surface is relatively acidic with pH = 5.5. This natural Acid mantle the skin inhibits the growth of bacteria and prevents them from entering. Normal soap is in the alkaline range with pH = 9. When washing with soap, the acid on the skin is therefore neutralized. The skin is then exposed to a higher risk of infection. After each wash with soap, it takes two to three hours for the natural protective acid mantle to be restored. For this reason, soap should only be used on very dirty hands or, in general, a “pH-neutral” soap should be used.


Recommendation: use hand washing detergent with pH = 5.5.

pH value in B.oden

The pH value in the soil influences the nutrient balance of the plants. Normally, ammonium ions and nitrate ions are in equilibrium in the soil at pH = 7. In acidic soils the ammonium ions predominate and in alkaline soils the nitrate ions (> nitrogen cycle). Certain plants such as spruce produce acid in the soil and lower the pH value to pH = 5. Some plants like peat moss or wood sorrel love acidic soils. A beech forest or limestone soil is almost neutral or slightly alkaline with pH = 7 to pH = 7.8. Coltsfoot, for example, likes to grow there.

Lime can buffer acid in the soil, which is created when humic substances form or when acid rain is brought in. The plants usually thrive at a pH between 5 and 7.8, depending on the requirements. A lime-poor soil can be washed out more quickly and therefore tends to over-acidify and to leach out nutrients. A very low pH value in the soil or in the lakes promotes the dissolution of metal ions in the groundwater. This can seriously damage plants or the forest.

pH in chemistry

The pH value is a number that indicates how high the concentration of hydronium ions [H3O+] is in an aqueous solution. It is given by the negative decadic logarithm of the H3O+-Ion concentration expressed:

pH = - log [H.3O+]

Example: There are always 10 in pure water−7mol / l of H3O+Ions (and also at OHIons). This results from the ionic product of the water. So water itself always acts to a small extent as a Brönsted acid and as a Brönsted base. This property is known as autoprotolysis of the water. Now the concentration of the H3O+-Ions in the water are inserted into the equation for calculating the pH value:

pH = - log [10th−7]  =  −(−7)  =  7

This results in a pH value of 7 for water. The pOH value indicates the concentration of OHIons in the water. A pH value and a pOH value can therefore always be calculated for all acids and alkalis. The sum of the two values ​​always results in 14:

pH + pOH = 14

The table below shows the relationship between pH and pOH as a function of the concentration present. Between two pH values, for example between pH = 14 and pH = 15, the concentration of the ions decreases or increases by a factor of ten. In alkaline solutions, the concentration of hydroxide ions exceeds that of hydronium ions. In acidic solutions it is the other way round.

PH value
10 mol / l acid
10 mol / l
10−15 minor
1 mol / l acid
1 mol / l
10−14 minor
Pure water
10−7 minor
10−7 minor
1 mol / l lye
10−14 mol /
1 mol / l
10 mol / l lye
10−15 minor
10 mol / l

Modern pH measuring devices consist of an electrode and the central unit. A voltage is generated at the electrode that depends on the pH value. The central unit converts the voltage into the pH value.
Metrohm AG pH measuring device with central unit and electrode
The device must be calibrated before a measurement. Usually a lab technician does this once every day at the beginning of the laboratory. Two buffer solutions with different pH values ​​are used for this. First you measure the pH in the first buffer solution (pH = 4). The electrode is then rinsed with distilled water and the pH value is measured with the second buffer solution (pH = 7). The displayed pH value is corrected using a key until it matches the pH value of the buffer solutions. It must be taken into account that the displayed pH value is temperature-dependent. For precise measurements, one therefore works with precisely set water temperatures.

Create an individual book: Basic text pH value


> Table of Contents