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What are pH indicators?

The pH indicators are substances used to determine the amount of pH that is in a specific medium. When talking about the concept of pH, it represents the concentration of hydronium ions (H 3 O + ) that is present in an aqueous solution.

Likewise, it is measured on a negative logarithmic scale between 0 and 14, where solutions with a pH lower than 7 are considered acidic, those with a pH greater than 7 are basic and those with a pH equal to 7 are considered neutral solutions. This parameter is expressed as follows with the Henderson-Hasselbalch equation: pH = pKa + log 10 ([A  ] / [HA]).

In the above expression, the pKa represents the negative logarithm of the acid dissociation constant, and the molar concentrations [A  ] and [HA] are those of the weak acid and its conjugate base, respectively. Knowing the pH allows to study the quality of water and food, and to be able to maintain the repeatability of an elaborated chemical product.

Indicator types

There are three main types of pH indicators: 

  • Acid-base liquid indicators, which work according to a certain pH range.
  • Papers, strips and other indicator materials that change color as liquid or gaseous sample is added to their surface.
  • Digital pH meters, which measure the potential electrical difference between two electrodes.

Liquid indicators

Liquid indicators are weak organic acids or bases that have varying colors depending on their acidic or basic form. These work within limited ranges, varying in color once this is reached, and ceasing to vary in color when the maximum level of the range is reached.

To work they should only be used in solutions where a color change can be observed (preferably colorless).

There are a large number of liquid indicators of different colors and pH ranges, including cresol red (red to yellow in a range of 0.2 to 1.8), methyl red (red to yellow in a range of 4 , 2 to 6.2), bromocresol green (pink to blue / green from 4.2 to 5.2), and phenolphthalein (colorless to pink in a range of 8.0 to 10.0).

These indicators are popular for degrees in analytical chemistry, although you must have a certain level of training to perform this practice accurately.

Indicator papers or strips

There are several types of paper used to measure pH, but the best known is called litmus paper, which is made from a powder that comes from lichens.

Litmus paper is used to know if a liquid or gaseous solution is acidic or basic (without knowing what its exact pH will be or an estimate of it), and it comes in two presentations: blue and red.

Blue litmus paper turns red under acidic conditions, and red litmus paper turns blue under basic or alkaline conditions, and can be recycled for reverse testing once the paper has already changed color.

The known limitations of the paper – such as its inability to offer an exact or estimated pH value and the ability to change to other colors when it reacts with certain compounds – has caused it to be replaced by liquid indicators and / or pH meters.

pH-meters

The pH meters are born from the need of laboratory analysts to obtain exact values ​​of this parameter, something that was not possible with paper indicators or liquid indicators.

They are based on the measurement of the electrical potential differential between the pH of the electrode and the reference electrode.

The operation of these pH meters is explained in more depth in the next section, but in general these indicators are considered the most accurate par excellence, providing a precise number (to the accuracy of 0.01 pH units) of the parameter and counting with a sensitivity and speed superior to the other two methods.

In addition, they can also measure other characteristics, such as dissolved solids, electrical conductivity, and solution temperature.

The only disadvantage of this type of pH meters is that they are delicate equipment and, in addition to an initial calibration that must be carried out by an instrumentalist or connoisseur of the equipment, they will also need to be cleaned regularly to prevent the electrodes from accumulating material on them.

How do the pH indicators work?

Liquid indicators

Liquid indicators change color due to the action of protonation or deprotonation in their structure (depending on the basic or acidic nature of the indicator), which is based on the equilibrium of the reaction, as follows: HIn + H 2 O ↔ H 3 O + + In 

That is, once the indicator has been added to the solution, if the equilibrium of this medium begins to tend in the opposite direction due to an increase or decrease in the hydronium ion (again, depending on the nature of the indicator), it will change color until finally stay of a new invariable color.

Indicator papers

Indicator papers, specifically litmus paper, must come into contact with the solution to be analyzed discreetly in order to measure accurately.

That is, in a liquid solution it should not be completely introduced into it, but should be dripped with it or briefly touched a sample of the substance.

In the case of a gaseous solution, the gas must pass over the surface of the paper to allow it to make contact and change color.

pH-meters

PH meters work by the potential electrical difference between a pH electrode and a reference electrode.

The pH meter measures the voltage that exists between two electrodes in a solution and displays the result converted into a corresponding pH value.

The equipment itself consists of a pair of electrodes – one of which is metallic and insensitive to the pH of the solution – and a simple electronic amplifier. For its calibration, the equipment is calibrated with solutions of known pH.

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