It was Danish chemist Soren Sorensen who in 1909 proposed a more useful quantity called pH which gives the acidic or basic strength of a solution. The pH of a solution is defined as the negative logarithm of the 'hydrogen ion concentration (in mol/L)".
`pH = - log [H^+] = log 1/( [H^+] )`
`=>` pH is a measure of acidity. As the pH increases, the acidic nature decreases.
Acidic solutions : `[H^+] > 1.0 xx 10^(-7) M`, `pH < 7.00`
Basic solutions : `[H^+] < 1.0 xx I0^(-7) M`, `pH > 7.00`
Neutral solutions : `[H^+] =1.0 xx 10^(-7) M`, `pH = 7.00`
`=>` pH of the solution of strong or weak neutral salt in water is always neutral.
`=>` pH of the solution of acidic salt in water is always acidic (i.e. `<7`). pH of the solution of basic salt in water is always basic (i.e. `>7`).`=>` On dilution, `pH` of an acidic solution increases but `pH` of a basic solution decreases.
`text(pH Range of Some Common Substances)`
Gastric juice | 1.0-3.0 |
soft drinks | 2.0-4.0 |
Lemon | 2.2-2.4 |
Vinegar | 2.4 - 3. 0 |
Urine (human) | 4.8 - 8.4 |
Saliva (human) | 6.5 - 7.5 |
Rain water | 6.0 |
Tears | 7.4 |
Sea water | 8.5 |
Milk of magnesia | 10.5 |
Milk (Cow) | 6.3 - 6.6 |
Blood plasma (human) | 7.36 - 7.42 |