What is pH?
pH is a measure of how acidic or alkaline a substance is in comparison to water. It is one of the very common parameters we talk about in Chemistry, especially in analytical chemistry. The reason is that the chemical equilibrium of the majority of chemical reactions is affected by the pH of the aqueous medium. This could be extended to other media too. At several conditions, pH also affects the kinetics of the reactions, their rates and catalyst action. Let's have other information related to pH such as what is pH value, pH value of acids and other substances etc.
Concept of pH
The pH is defined mathematically as the negative log (base 10) of the H+ ion concentration in mol/Litre. Since the numbers are less than 1 and also vary over a wide range, a negative logarithmic scale is chosen to represent the pH. Also, as the scale is in negative logarithm, lower pH means a higher hydrogen ion concentration (acidic nature) and similarly a higher pH means lower hydrogen ion concentration (basic nature). Also, the base 10 logarithm represents a 10 fold change of hydrogen ion concentration as a unit change on the pH scale.
The pH of human tissues normally falls between 7.36 and 7.44. Although the neutral pH at 20°C has a value of 7.4, the dissociation of water molecules is relatively higher at physiological temperatures. Hence, at 37°C, the neutral pH is represented by the value of 6.8. This is the reason why body fluids are slightly alkaline.
A pH value of 7.7 represents a Hydrogen ion concentration of 0.00000002 mol/L. Let’s try to understand this mathematically:
pH = 7.7
H+ = 10-7.7 mol/L
= 10-8 x 100.3 mol/L
=0.00000001 x 2 mol/L
= 0.00000002 mol/L
= 0.00002 mmol/L
(1 mol/L = 1000 mmol/L)
What is pH Value?
The pH range goes from 0 upto 14, where 7 represents neutral (pH value of water). If the pH is less than 7, it shows the acidic nature of the substance. If it is greater than 7, it shows the alkaline or basic nature of the substance. pH is actually a measurement of relative amounts of hydrogen and hydroxyl ions (H+ and OH-). Solutions having more free hydrogen ions are acidic and solutions having more hydroxyl ions are basic. The addition of chemicals alters the pH of water. Hence, pH is an important factor to consider in order to know what is changing chemically.
What is The pH Scale?
The pH scale is a scale that quantifies the acidic or alkaline behaviour of a substance. The range of the pH scale is mapped from 0 to 14 for highly acidic to highly alkaline. pH 7 denotes a neutral (neither acidic nor basic) substance.
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Every unit decrement of the pH scale reading signifies the increase in the concentration of hydrogen ions by a factor of 10 times. For instance, pH 3 denotes ten times stronger acidic than pH 4. A similar relationship holds for the pH values above 7 and the alkaline nature of the compound. A unit increment in pH above 7 denotes a ten times stronger base. For instance, a pH of 9 denotes a ten times stronger base than the pH of 8.
pH Value of Acids and Bases
Whether a solution is an acid or a base is decided by the hydrogen ions present in the solution. A very small fraction of pure water molecules also dissociates at room temperature, forming hydrogen and hydroxide ion pairs. Some of the hydrogen ions get combined with water molecules to form hydronium ions (H3O+). For simplicity, if we consider hydronium ions as hydrogen ions, there will always be equal numbers of hydrogen and hydroxide ions present in water. Hence, it is neither acidic nor alkaline.
In an acidic solution, a hydrogen ion donating substance is present. This shifts the balanced condition or equilibrium between the hydrogen and hydroxide ions. The solution now contains hydrogen ions more than hydroxide ions and hence behaves as an acid.
In a basic solution, just the opposite happens. A hydrogen ion accepting substance is present in such a solution. Or in another way, we can say that the number of hydroxide ions becomes more than that of hydrogen ions. Thus, the equilibrium is shifted in a direction just opposite to that of acids. Hence, the solution behaves as a basic solution.
pH of Food
Being a direct measure of acidic content, ph becomes one of the most common analytical measurements in industrial food processing. The pH of the food influences the amounts and types of preservatives to be used, and it becomes the most decisive factor in the selection of the container in which they will be packed.
Among various reasons for measuring pH in food processing, some are listed below:
To keep the production properties consistent between different batches
To optimise the efficiency of production and keep the cost-optimal
To keep the food safe to consume without causing any health problems to consumers
To meet the standards and regulation conditions set by regulatory bodies
As the pH scale is logarithmic, even a small change in its reading cannot be avoided. A change of 0.3 on the pH scale reflects the double acidic concentration. Hence, even a small variation of pH is significant and has impacts on the flavour, consistency and shelf life of packed foods.
The departments of health and other regulatory bodies suggest a certain allowed value range of pH of the sanitation solutions be used. For instance, a pH between 8 and 10 is suggested from chlorine concentration. Similarly, the iodine solution is supposed to fall below five on the pH scale.
The pH of some common things that we see in our daily lives are shown below:
The pH of Some Common Substances (pH Table)
Did You Know?
Only aqueous solutions can be classified into acids, bases or neutral solutions. Other non-aqueous liquids such as oils cannot be acid or base.
Acids and bases have the property of reacting with each other in a process called neutralisation. The reaction ends up producing salt, water and leaving the pH between those of the reacting components and closer to neutral.
As they split into ions which behave as charge carriers in aqueous solutions, acids and bases both conduct electricity.
Conclusion
pH scale information is the measure of the acidic or alkaline nature of aqueous solutions. The range of pH scales varies from 0 up to 14. The midpoint of the scale, i.e. 7 denotes a neutral solution (equivalent to pure water). pH less than 7 reflects the acidic nature,, and greater than 7 reflects the basic nature. pH basically is the measure of hydrogen and hydroxyl ions present in a solution.
FAQs on pH
1. What are the applications of pH?
The measurement of pH is done for a variety of reasons in science, research and water treatment plants, etc. The pH level of water that we consume directly or indirectly must be kept within a certain range. As the water gets shifted towards the base, it starts tasting bitter and also starts accumulating deposits in pipes and appliances. On the other side, as it becomes acidic, it starts corroding metals. It may also harm aquatic life. Various lab experiments require water at particular pH levels to give accurate results. The applications of pH are wastewater treatment, aquaculture, food and beverage, pool and spa water, aquariums and fish tanks, research, hydroponics etc.
2. Why are pH values restricted to the range of 0-14 only?
It is indeed possible to have a pH that falls outside the range specified by the pH scale. However, such values are insignificant on the basis of their meaning and the limitations of the instruments to measure such values. In the labelling of highly concentrated solutions, it is more convenient to use their concentration (such as molarity) as a measure of their acidic strength rather than their pH. The main objective behind the concept of pH is to bring concentrations (very small to moderate) of hydrogen ions on a more compact pH scale range. In comparison, it becomes irrelevant or even counterproductive as the concentration grows higher.
