Description of a Method for Finding the pH of a Solution in the Laboratory

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    Definition of Acids and Bases

    • Acidic compounds are those which readily split apart or dissociate when dissolved in water to form form positively and negatively charged ions. The positive ion is a hydrogen ion, also called a proton, which is why acids can also be described as proton donors. For example, hydrogen chloride literally donates a proton to the solution. The strength of the acid depends on the stability of the negative ion formed when a proton is donated. Hydrochloric acid is very strong because the chloride ion formed in dissociation is very stable.

      Bases are called proton acceptors, meaning they will accept protons from water, resulting in the formation of hydroxide ions. The strength of the base depends on the stability of the positively charged ions formed by the acceptance of the proton.

    Definition of pH

    • The pH is a measure of the concentration of hydrogen ions (protons) in a solution. Acids have a high concentration of protons; bases have a low proton concentration. The value of a pH is defined as the negative logarithm of a hydrogen ion. Hydrochloric acid has around 10^-1 moles per liter of hydrogen ions, meaning it has a pH of 1. Potassium hydroxide has around 10^-14 moles per liter of hydrogen atoms, meaning it has a pH of 14.

    pH Measurement With Color Change

    • Certain types of organic dye will change color depending on the concentration of hydrogen or hydroxide. This happens because either hydrogen or hydroxide ions will bond with the dye molecules and affect the wavelength of visible light that they absorb. Such dyes are called indicators. Litmus is a common indicator and will change from blue to red when a solution is acidified. The reaction of acids and bases with indicators is reversible. In the case of litmus, the addition of more base will change its color back to blue. Universal Indicator is another very common laboratory indicator, which will exhibit a range of colors depending on pH. Strong acids are red, weaker acids orange and neutral solutions yellow. Weaker bases turn Universal Indicator green; stronger bases turn it blue, then purple as the base gets stronger.

    Electrical Measurement of pH

    • The electrical conductivity of a solution depends on the concentration of ions in that solution and this property is exploited by a pH meter. A pH meter is a type of electrode that is calibrated using a buffer solution which has been prepared to a specific pH, typically 7.0, meaning that it contains 10^-7 mole per liter of hydrogen ions. Any solution containing more hydrogen ions will carry a current greater than that of the buffer; solutions with hydrogen ion concentrations lower than the buffer will carry a lower current. Indicators are good at establishing an approximate pH; however, much more precise values can be found using a pH meter.

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