Acids, Bases, pH and Buffers

Acids An acid dissolves in water to donate H + In water the H+ reacts with water to make the hydronium ion H + + H 2 O  H 3 O + Acids therefore produce hydronium ions in water HCl + H 2 O  H 3 O + + Cl -

Bases A base is something that accepts an H + Examples NaOH and NH 3 NaOH + H +  H 2 O + Na + Or more accurately …. NaOH + H 3 O +  2H 2 O + Na + NH 3 + H 2 O  NH OH -

Neutralization When an acid reacts with a base an H + reacts with an OH - to make water HCl + NaOH  H 2 O + NaCl If we write the ionic substances as ions H + +Cl - +Na + +OH -  H 2 O+Na + +Cl - In a complete neutralization the amount of H+ = OH-

pH=-log[H + ] The pH of a solution tells us if it is acidic, basic or neutral

Measuring pH With a pH meter With an indicator (red cabbage here) pH is actually measuring [H + ] In pure water K w = [H + ][OH - ] = [10 -7 M][10 -7 M] = In acid or base [H + ][OH - ] = Example, if an acid [H + ] = M pH = -log[H + ] = -log[10 -4 ] = 4

Buffers Buffers maintain the pH ([H+]) of a solution Example blood must be maintained between pH = Buffers can work as both an acid or a base so they can neutralize small amounts of acid and base pH of blood is maintained by bicarbonate HCO 3 - HCO H +  H 2 CO 3 HCO OH -  H 2 O + CO 3 2- In this experiment we will study the effect of an acid and base on a buffer and compare it with a non-buffer

pH color using red cabbage indicator 13 test tubes arranged in a rack Pour 3-4mL of of each buffer into a separate test tube label the pH of each buffer on each test tube Add 2-3 mL of the red cabbage indicator to each test tube and describe the color of each Keep as a reference

Measuring pH 3-4 mL of the samples (shampoo, conditioner, mouthwash, detergents, fruit juice, aspirin etc) into a test tube Add 2-3 mL of the red cabbage solution and estimate the pH from the color Measure the pH with the pH meter

Buffers Adding Acid In four separate test tubes add 10.0mL of the following H 2 O, 0.1M NaCl, a buffer with a high pH, a buffer with a low pH Add 2-3 mL of cabbage indicator to each Add 5 drops 0.1 M HCl and stir Determine the pH Determine the change in pH if any Adding Base Repeat the above instead adding 5 drops 0.1M NaOH