What is pH?

Attractive female chemical assistant work with test tubes

Most living things depend on a proper pH level to sustain life. Your body is continually working to maintain the appropriate pHs. The blood flowing through our veins must have a pH between 7.35 and 7.45. Exceeding this range by as little as 1/10th of a pH value could prove fatal. pH is a measure of acids and bases that plays an important role in our life from cooking to cleaning to showering. Many foods must maintain a certain pH during their manufacturing processes to ensure they are of a certain quality. Plants will grow best if they are planted in a soil which is maintained at an optimal pH. The pH of wastewater leaving chemical plants, wastewater purification plants and municipal drinking water plants must be within a specific pH range as set forth by state or federal regulatory agencies. This value is typically between 5 and 9 pH. Almost all processes containing aqueous solution have a need for pH measurement. Put simply, pH is an integral part of our life.

A functional definition of pH is the measurement of acidity or alkalinity of a solution which is measured on a scale of 0 to 14. A pH value below 7 implies an acidic substance, while a pH above 7 indicates the material is alkaline. Water is often thought of as “neutral,” meaning it has a pH of 7 and is neither acid nor alkaline. However, this is only true for pure water and only at a specific temperature*

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When working with a liquid, chemists use the concentration of hydrogen ions to calculate pH levels. They measure pH as the concentration of hydrogen ion with the given equation:

pH = -log[H+]

Log is a base-10 logarithm and [H+] is the concentration of hydrogen ions in moles per litre of solution. The square brackets around the H+ automatically mean “concentration” to a chemist. What the equation means is just what we said before: for each 1-unit change in pH, the hydrogen ion concentration changes ten-fold.

The higher the concentration of hydrogen ions, the lower the pH value and higher the acidity. The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10-7 M.A pH of 7 is considered “neutral”, because at this pH, the concentration of hydrogen ions is exactly equal to the concentration of hydroxide ions, produced by dissociation of the water. Conversely, a concentration of hydrogen ions more than 1.0 × 10-7 M   mean that the substance has low acidity and the solution is considered “alkaline” or “basic”.

It’s important to note that a solution must be aqueous to have a pH. You cannot, for example, calculate pH of vegetable oil or pure ethanol.

Table 1 given below has examples of substances with different pH values**

pH Value H+ Concentration
Relative to Pure Water
Example
0 10 000 000 battery acid
1 1 000 000 gastric acid
2 100 000 lemon juice, vinegar
3 10 000 orange juice, soda
4 1 000 tomato juice, acid rain
5 100 black coffee, bananas
6 10 urine, milk
7 1 pure water
8 0.1 sea water, eggs
9 0.01 baking soda
10 0.001 Great Salt Lake, milk of magnesia
11 0.000 1 ammonia solution
12 0.000 01 soapy water
13 0.000 001 bleach, oven cleaner
14 0.000 000 1 liquid drain cleaner

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