6.2

pH Value

 The pH scale

• The pH scale is a set of number that determines the degree of acidity or alkalinity of a substance.
• The pH value would be:
  • 7 for neutral substance. 
  • > 7 for alkaline substance (\(OH^- > H^+\))
  • < 7 for acidic substance (\(H^+ > OH^-\)\(OH^-\))

 

\(\xleftarrow[]{\textbf{Acidity } \uparrow}\)							Neutral	\(\xrightarrow[]{\textbf{Alkalinity } \uparrow}\)
0	1	2	3	4	5	6	7	8	9	10	11	12	13	14

 

• pH: a logarithmic measurement of the concentration of hydrogen ions, \(H^+\) contained in an aqueous solution.

 

Formula for pH

\(pH = -log[H^+]\)

 

• The concentration of hydroxide ions, \(OH^-\) is also used to calculate the pOH value.

 

Formula for pOH

\(pOH = -log[OH^-]\)

 

Relationship of pH with pOH

\(\begin{aligned} pH + pOH &= 14\\ pH&=14-pOH \end{aligned}\)

 

Hubungan nilai pH dengan kepekatan ion hidrogen dan ion hidroksida 

 

Relationship of concentration \(H^+\) with pH

Concentration \(H^+\) \(\uparrow\), pH value \(\downarrow\)

 

Relationship of concentration \(OH^-\) with pH

Concentration \(OH^-\, \uparrow\),   pH value \(\uparrow\)

 

Measurement of the pH value of a solution

• The pH value can be measured by using:

Universal indicator

• The universal indicator will change colour, and it indicates what the pH based on the colour produced is.
• The universal indicator can be found in two forms; solution or paper strips. 

 

pH meter

• The pH meter is an instrument that is used to measure the pH value of a solution accurately.
• pH meter shows the number of the pH value in the screen.