Acids and Bases: Determination of pH

Arrhenius acid solution contains excess of

H+ ions

Arrhenius base solutions contains excess of

OH- ions

Bronsted Lowry acid is a

proton (H+) donor

Bronsted Lowry base is a

proton (H+) acceptor

Lewis acid is an

electron pair acceptor

Lewis base is an

electron pair donor

Conjugate base

Acid loses a hydrogen ion

HA+H2O→

(H3O+)+(A-)

Bronsted Lowry acid will donate a

hydrogen ion to any base whose conjugate acid is weaker than the donor acid

Sulfuric acid, nitric acid, and hydrochloric acid will donate

protons to conjugate base of any weak acid

Metal hydroxides, will accept

hydrogen ions from weak acids

Detecting acids and bases using compounds called acid-base indicators is easy because of the

Transfer of hydrogen ions

Acid-base indicator is a weak Bronsted-Lowry acid that changes color when it loses it’s

hydrogen ion

Phenolpthalein (HPhth) Acid form is colorless

Base form without hydrogen ion is bright pink

I an neutral solution, the two forms are in equilibrium and the colorless form

predominates

If Phenolpthatlein is put into solution of strong acid

Strong acid will donated hydrogen ions→phenolpthatlein equilibrium further in colorless form. (Le Chatelier’s Principle)

LeChatelier’s Princple

if stress is applied to a system in equilibrium, the system will respond in such a way as to relieve that stress and restore equilibrium under a new set of conditions.

If phenolphthalein is added to a solution of a strong base,

the strong base will accept ions from phenolpthalein and will shift towards its pink form.

phenolphthalein is pink in x and colorless in y

x = basic y = acid

Methyl Red

color in acid red, color in base yellow, 4.8-6.0

Litmus

color in acid red, color in base blue, 5.2-7.5

Bromthymol Blue

color in acid yellow, color in base blue, 6.0-7.6

Phenolphthalein

color in acid colorless, color in base pink, 8.2-10

Universal Indicator

Color in acid red orange yellow, color in base, blue violet, 0-14

Objectives

1. To observe the characteristic reactions of several acid-base indicators

2. To use acid-base indicators to estimate the pH of solutions

Equipment

Safety goggles and aprons

Test tube rack

stirring rod

plastic wash bottle

dropping pipettes

spot plate

reagent bottles

white paper

Materials

0.1M hydrochloric acid, HCL

0.1M acetic acid, CH3COOH

0.1M ammonium hydroxide NH4OH (releases NH3)

0.1M sodium hydroxide, NaOH

Mortar and pestle

dropping pipettes

methyl red solution

litmus solution

bromthymol blue solution

phenolphthalein

universal indicator

distilled water

household chemicals

lemon juice

vinegar

bleach baking soda

carbonated beverage

shampoo

cold tea

aspirin

milk

antacid liquid

Safety

1. Wear goggles and aprons at all times

2. Hydrochloric acid and sodium hydroxide are irritants at the concentrations used in this experiment. If you pill either of these solutions on yourself immediately flush the affected area with water for 2-3 minutes and notify the teacher. If either solution gets into your eyes, begin flushing your eyes with running water immediately and continue doing so for at least 20 minutes or use eye wash fountain. Notify teacher immediately.

3. Ammonia is an irritant. Do not inhale ammonia fumes directly and avoid skin contact with this chemical