Acids & Bases pH Scale

What is pH? pH is a measure of how acidic or basic a solution is. “p” stands for potential and “H” stands for hydrogen; hence, the potential of a substance to attract hydrogen ions

pH Scale The pH scale ranges from 0 to 14. Acidic solutions have pH values below 7 A solution with a pH of 0 is very acidic. A solution with a pH of 7 is neutral. A solution with a pH above 7 is basic

How Can You Actually Determine the pH of a Solution? Use a pH meter—read the number. Use pH paper (color patterns indicate pH). Titrate the solution with precise amounts of base or acid in conjunction with a soluble dye, like phenolphthalein, whose color changes when a specific pH is reached. 8

pH Scale A change of 1 pH unit represents a tenfold change in the acidity of the solution. For example: Solution A has a pH of 1 and Solution B has a pH of 2 Solution A is not twice as acidic as Solution B—it is ten times more acidic.

Why is pH important in biology? pH affects solubility of many substances. pH affects structure and function of most proteins - including enzymes. Many cells and organisms (esp. plants and aquatic animals) can only survive in a specific pH environment. Important point - pH is dependent upon temperature

Buffers Definition: a solution that resists change in pH Typically a mixture of the acid and base form of a chemical Can be adjusted to a particular pH value

Learning Check Identify each solution as A) acidic, B) basic, or N) neutral ___ 1) HCl with a pH = 1.5 ___ 2) Sprite soft drink, pH = 3.0 ___ 3) pH = 7.5

Solution Identify each solution as A) acidic, B) basic, or N) neutral A 1) HCl with a pH = 1.5 A 2) Sprite soft drink pH = 3.0 B 3) pH = 7.5

Calculating pH

Calculating pH Mathematically, pH is the negative log of the hydronium ion concentration pH = −log [H3O+] For a solution with [H3O+] = 1 x 10−4, pH = −log [1 x 10−4 ] pH = [4.0] pH = 4.0

How do we calculate pH? STEP 1 Press negative key and then log key STEP 2 Enter the [H3O+] value and press equal key STEP 3 Make sure you have the correct number of significant figures (the number of digits for the pH equals the number of digits of the [H3O+] value )

Example of Calculating pH Find the pH of a solution with a [H3O+] of 1.0 x 10−3 STEP 1 Press negative key and then log key: log ( STEP 2 Enter the [H3O+] value and press equal key: log (1 x 10−3) = 3 STEP 3 Make sure you have the correct number of significant figures (the number of digits for the pH equals the number of digits of the [H3O+] value ) : [H3O+] = 1.0 x 10−3 pH is 3.00

Learning Check What is the pH of coffee if the [H3O+] is 1 x 10−5 M? A) pH = 9.0 B) pH = 7.0 C) pH = 5.0

Calculate the pH of a solution with a [H3O+] of 7.0 x 10 -9 M Answer: pH = 8.2 Calculate the pH of a solution with a [H3O+] of 7.0 x 10 -9 M