The Determination of ph of some Common Acids & Bases

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1 The Determination of ph of some Common Acids & Bases Introduction: An acid is a substance that when dissolved in water produces hydrogen ions, H +. Inorganic acids do not contain carbon but organic acids do contain carbon. Strong acids are acids that produce lots of hydrogen ions, H + s. They almost completely ionize to form hydrogen cations, H +, and the companion anions. Once dissolved in water the very strong acid does not exist since it has ionized into its ions. Hydrochloric acid H-Cl H + + Cl Examples of Strong Acids: Strong Acid Formula Common Source Hydrochloric Acid HCl Stomach Acid Sulfuric Acid H2SO4 Battery Acid Weak acids are acids that produce only a few hydrogen ions, H + s. Only some of the molecules in solution ionize into hydrogen cations, H +, at a time. When they do ionize, the hydrogen cations, H +, and the companion anions join back together again to reform the acid. This process of the acid ionizing and then reforming over and over is called equilibrium. Because of the equilibrium there is a mixture of the original acid, the hydrogen ion, H +, and the anion all in the solution at the same time. Acetic acid H-C 2 H 3 O 2 H + + C 2 H 3 O 2 Carbonic acid CO 2 + H 2 O H 2 CO 3 H + + HCO 3

2 2 Examples of Weak Acids: Weak Acid Formula Common Source Acetic Acid HC2H3O2 Vinegar Carbonic Acid H2CO3 In Carbonated Water Citric Acid H3C6H5O7 In Lemons & Oranges Tartaric Acid H2C4H4O6 In Grapes Phosphoric Acid (A weak inorganic acid) H3PO4 In Cola Drinks A Base is a substance that when dissolved in water will produce hydroxide ions, OH, in the solution. A base that does not contain carbon is called an inorganic base. A base that contains carbon is called an organic base. Strong bases are bases that produce lots of hydroxide ions, OH. They almost completely ionize to form hydroxide anions, OH, and the companion cations. Sodium NaOH Na + + OH Examples of Strong Bases: Strong Base Formula Common Source Sodium Potassium NaOH KOH Lye, Caustic Soda, Soda Ash, Drano Potash

3 3 Weak bases are bases that produce only a few hydroxide ions, OH. Sometimes it s not obvious that hydroxide anions, OH, are even produced, but the anions that are produced can react with water to produce OH s. Weak bases are in equilibrium with their ions. Sodium Bicarbonate NaHCO 3 Na + + HCO 3 HCO 3 + H 2 O H 2 CO 3 + OH Examples of Weak Bases: Weak Base Formula Common Source Magnesium Mg(OH)2 In Milk of Magnesia Ammonium NH4OH In Glass Cleaner Sodium Bicarbonate NaHCO3 Baking Soda Calcium Carbonate CaCO3 Antacids, Sea Shells, Egg Shells, Limestone & Marble Indicators: The ph of a solution is often measured by observing how the acid or base causes the color of certain organic molecules to change. Phenolphthalein is colorless when acidic but turns pink above ph 7. Anthocyanin indicators are common in flower petals, berries, and purple cabbage. The colors change over the entire spectrum of ph making it a universal indicator. The chart below shows the relative color changes of anthocyanin pigments over a wide ph range. Colors of Anthocyanin (Red Cabbage) Indicator ph = Color Red Fuchsia Rose Purple (neutral) Blue Aqua Green Greenbrown Yellow

4 4 Preparation Procedure: 1. You will need two 50 ml beakers for each of the household products to be tested. Put into each pair of beakers one of samples of a liquid household product. Pour in enough liquid to reach a depth of about 3 cm. 2. Prepare the solids to be tested by placing a dime sized scoop of the sample into the pair of small beakers and add enough water to reach a depth of about 3 cm. Stir to dissolve. 3. Use these sample tubes for the next sections where the ph of each will be determined by various methods. Determination of ph by Paper: 1. Tear the universal indicator ph paper into small pieces and arrange them on a paper towel. 2. Dip a glass stirring rod into either beaker of the sample to be tested and touch the wet stirring rod to the ph paper. 3. Compare the color produced on the wet paper to the color on the ph paper container label. Record the results. Determination of ph by Phenolphthalein: 1. Add three drops of phenolphthalein indictor to one of your samples and observe the color change, if any. 2. Record as acidic if the color stays clear or basic if the color changes to a rose or fuchsia color. Determination of ph by Cabbage: 1. Tear a piece of purple cabbage into small pieces and place in a large glass beaker. 2. Add distilled water to the beaker to cover the cabbage and then boil the solution using a hot plate until the water is dark purple. Remove from heat. Add a half of an eye dropper of the cabbage juice to the second beaker of your samples. Stir, and note the color. 3. Compare the solution color to the chart on page 3. Record the results.

5 5 Name Lab Day Report for ph of Acids & Bases Experiment: Determination of ph by Paper, Phenolphthalein, and Cabbage: Foods: Solution ph by Paper phenolphthalein Soda Pop brand Tea Coffee Orange Juice Vinegar Baking Soda New Cabbage color produced ph by Cabbage Cleaners: Shampoo Detergent Ammonia Bleach Toilet Cleaner Others: Lye Milk of Magnesia Antacid brand Results Summary: 1. In general, foods and 2. In general, cleaning beverages are mostly: supplies are mostly: A. Acidic A. Acidic B. Basic B. Basic C. Neutral C. Neutral D. there is no general trend D. there is no general trend