ACIDS AND BASES: PH AND BUFFERS PURPOSE: To determine the ph of common acids and bases using a ph meter, ph paper, and red cabbage indicator. To test the effect of adding an acid or base to a buffer solution. SAFETY CONCERNS: Always wear safety goggles. Wash with soap and water if skin contacts acids or bases. ACIDS: An Acid is a substance that when dissolved in water will produce hydrogen ions, H +, in the solution. An acid that does not contain carbon is called an inorganic acid. An acid that contains carbon is called an organic acid. 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 any more since it has ionized into its ions. Ionization of Hydrochloric acid H-Cl H + + Cl Examples of Strong Acids: Strong Acid Formula Common Source Hydrochloric Acid HCl Stomach Acid Sulfuric Acid H 2 SO 4 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 1+, and the anion all in the solution at the same time. Ionization of Acetic acid H-C 2 H 3 O 2 H + + C 2 H 3 O 2 Ionization of Carbonic acid CO 2 + H 2 O H 2 CO 3 H + + HCO 3
2 Examples of Weak Acids: Weak Acid Formula Common Source Acetic Acid HC 2 H 3 O 2 Vinegar Carbonic Acid H 2 CO 3 In Carbonated Water Citric Acid H 3 C 6 H 5 O 7 In Lemons & Oranges Tartaric Acid H 2 C 4 H 4 O 6 In Grapes Phosphoric Acid (A weak inorganic acid) H 3 PO 4 In Cola Drinks BASES: 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. Ionization of Sodium Hydroxide NaOH Na + + OH Examples of Strong Bases: Strong Base Formula Common Source Sodium Hydroxide NaOH Lye, Caustic Soda, Soda Ash, Drano Potassium Hydroxide KOH Potash 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. Ionization of Sodium Bicarbonate NaHCO 3 Na + + HCO 3 HCO 3 + H 2 O H 2 CO 3 + OH
3 Examples of Weak Bases: Weak Base Formula Common Source Magnesium Hydroxide Mg(OH) 2 In Milk of Magnesia Ammonium Hydroxide NH 4 OH In Glass Cleaner Sodium Bicarbonate NaHCO 3 Baking Soda Calcium Carbonate CaCO 3 Antacids, Sea Shells, Egg Shells, Limestone & Marble PH: The ph of a solution is an indicator of the number of hydrogen ions, H + s, present in a solution. We measure the concentration of hydrogen ions, H + s, in moles per liter, M, which we symbolize with square brackets, [H + ]. Pure water contains a small amount of both hydrogen ions, and hydroxide ions. In pure water the concentrations of hydrogen ion and hydroxide ions are the same. [H + ] = [OH ] = 0.000,000,1 M. It is awkward to work with numbers as small as 0.000,000,1 M. It helps to report the hydrogen ion concentration, [H + ], in scientific notation. To simplify reporting the concentration of H + s even more, we use the positive value of the exponent only and call it the ph. (in scientific notation) [H + ] [H + ] = 1 10 -ph (log means opposite of exponent on base 10) ph = log [H + ] 0.000,1 M 1 10-4 M 4 0.000,000,1 M 1 10-7 M 7 0.000,000,000,1 M 1 10-10 M 10 If the [H + ] = [OH ] then the solution is considered to be neutral and the ph = 7 If the [H + ] > [OH ] then the solution is considered to be acidic and the ph < 7 If the [H + ] < [OH ] then the solution is considered to be basic and the ph > 7 [H + ]= 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 ph = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Increasingly Acidic [H + ] > [OH ] Neutral [H + ] = [OH ] Increasingly Basic [H + ] < [OH ]
The [H + ] will not always be a simple 1 10 whole number value. In such cases we can estimate the ph range or we can calculate the ph exactly using the ph equation: ph = log [H + ]. Examples: [H + ] = 1 10 -ph ph = -log [H + ] Given [H + ] If [H + ] = 0.000032M = 3.2 x 10-5 M Calculated [H + ] Then [H + ] = 1 10-8.6 = 2.5 10-9 M We can estimate ph to be between 4 & 5 We can estimate [H + ] to be between 10-8 & 10-9 Calculated ph Then ph = -log(3.2x10-5 ) = 4.5 Given ph If ph = 8.6 4 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. Litmus, a chemical found in lichen, is one of many acid-base indicators. Litmus turns from blue to red in acidic solutions and from red to blue in basic solutions. Phenolphthalein, a laxative, is colorless when acidic but turns brilliant 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 = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Color Red Fushia Rose Purple (neutral) Blue Aqua Green Greenbrown Yellow NEUTRALIZATIONS: A Neutralization reaction is a reaction between a strong acid and a strong base in which the product formed is neither acidic nor basic. For example, if hydrochloric acid (an acid) is mixed with sodium hydroxide (a base), the products will be sodium chloride (table salt) and water. Neutralization: Strong Acid + Strong Base Salt + Water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O
BUFFERS: A Buffer is a solution that will not drastically change in ph even when strong acids or bases are added to it. A buffer is made by combining a weak acid and a salt in a water solution. By having both an acid and a base in the solution together there will always be an acid to react with any base added, or there will be a base to react with any acid added. For example, a simple buffer could be made by combining equal amounts of the weak acid, Acetic Acid, and the weak base, Sodium Acetate. Buffer = Weak Acid + Weak Base 5 HC 2 H 3 O 2(aq) + NaC 2 H 3 O 2(aq) (weak acid) (salt) + NaOH If a strong base gets added to the buffer, it reacts with the weak acid present (rather than giving the solution more OH s) and gets turned into NaC 2 H 3 O 2(aq) a weak base that does not drastically change the ph of the solution. + HCl If a strong acid gets added to the buffer, it reacts with the weak base present (rather than giving the solution more H + s) and gets turned into HC 2 H 3 O 2(aq) a weak acid that does not drastically change the ph of the solution. As long as the number of OH s and H + s does not drastically change then the ph does not drastically change. Buffer solutions are important in keeping the ph of biological solutions in a narrow range.
6 PROCEDURES: ACTIONS: I. PREPARATIONS: 1. Arrange your small 50 ml beakers and into each one put a sample of a liquid 1 household product to be tested. Pour in enough liquid so that the probe of your ph meter can just be immersed into the liquid. 2. Prepare the solids to be tested by placing a dime sized scoop of the sample into the small beakers and add enough water 2 to reach a depth of about 3 cm. Stir or stopper and shake to dissolve. 3. Use these sample tubes for the next sections where the ph of each will be determined by various methods. II. DETERMINATION OF PH BY METER: 1. Clean the ph electrodes with deionized water between each sample test. 3 Be gentle with the electrodes as they are delicate and can easily break or scratch. 2. Carefully submerge the cleaned electrode of a ph meter into a sample. Read the ph and record it on your report sheet. 3. Calculate the [H + ] from your ph meter reading and record it on your report sheet. III. DETERMINATION OF PH BY PAPER: 1. Tear the universal indicator ph paper into 1 cm square pieces and arrange them on a watch glass or paper towel. 4 2. Dip a glass stirring rod into the sample to be tested and touch the wet stirring rod to the ph paper. 3. Compare the color produced on the wet 5 paper to the color on the ph paper container label. Record the results. IV. DETERMINATION OF PH BY CABBAGE: 1. Tear a piece of purple cabbage 6 into small pieces and place in a glass beaker. 7 2. Add deionized water to barely cover the cabbage and then boil it using a hot plate, burner, or microwave oven until the water is dark purple. Remove from heat. 3. Into the beaker of the sample to be tested, add a few mls of purple cabbage juice and observe the color change. Record the ph range. 8 NOTES: 1 If the liquid is highly viscous, (thick like shampoo) treat it as a solid. 2 Use deionized water here. 3 We must have clean electrodes for each test. If you do not rinse off the previous sample then the new sample gets contaminated and the ph can be altered. 4 Bring the sample to be tested to the ph paper rather than inserting the ph paper into the solution. Putting the ph paper into the solution washes pigment from the paper into the sample. 5 Compare the color when the paper is wet. The color will change as the paper dries and not compare accurately. 6 Roses, hydrangeas, and blueberries also contain anthocyanin pigments and will work as indicators. We use red cabbage because it is so inexpensive and readily available. 7 If working at home use a coffee mug or glass container. Metal pans cause the purple pigments to turn blue. 8 Compare the colors of your substance-cabbage mix with the colors on the chart in the discussion section. This is not exact but just allows you to predict a general ph range.
7 V. PROPERTIES OF WATER SOLUTIONS: A. Water with Carbon Dioxide, CO 2 : 1. Place about 10 mls of deionized water into a small beaker and determine the ph using a ph meter. 9 2. Keeping the ph electrode in the tube, place a straw in the water and blow bubbles into it. 10 Watch the ph meter to observe changes. 3. Continue to blow bubbles into the water for a couple of minutes. Record any ph changes on the report sheet. B. Water with Acid: 1. Place about 10 mls of deionized water into a small beaker. Measure and record the ph of the water using a meter. 9 9 The deionized water may not be at a ph of 7 depending on the amount of CO 2 gases already dissolved in it. Record the ph as you find it and use this as your starting point. 10 As you blow into the tube you are adding Carbon Dioxide, CO 2 to the water. CO 2 + H 2 O forms H 2 CO 3. Review the disassociation of Carbonic acid, H 2 CO 3, in the laboratory discussion of weak acids. 2. Add 2 drops of 1.0 M HCl, Hydrochloric Acid, and stir the sample to mix. Record the ph. 3. Add 2 more drops of HCl, and stir the sample to mix. Record the ph.. Discard the solution in the sink. 4. Analyze the results and report your conclusions on the report sheet. C. Water with Base: 5. Place about 10 mls of deionized water into a small beaker. Measure and record the ph of the water using a meter. 9 6. Add 2 drops of 1.0 M NaOH, and stir the sample to mix. Record the ph.. Discard the solution in the sink. 7. Add two more drops of NaOH and stir the sample to mix. Record the ph.. Discard the solution in the sink. 8. Analyze the results and report your conclusions on the report sheet.
8 D. Buffer with Acid: 1. Into a small beaker combine 5 mls of 0.1 M Sodium Acetate, (NaC 2 H 3 O 2 ) with 5 mls of 0.1 M Acetic acid, (HC 2 H 3 O 2 ). Mix well. 11 Not all buffers are neutral. The ph depends on the weak acid and the weak base used. Each buffer will have its own unique ph. Record the ph of yours and use this as your starting value. 2. Measure and record the ph of the buffer solution you have just made using a meter. 11 3. Add 2 drops of 1.0 M HCl, Hydrochloric Acid, and stir the sample to mix. Record the ph. 4. Add 2 more drops of HCl, and stir the sample to mix. Record the ph.. Discard the solution in the sink. 5. Analyze the results and report your conclusions on the report sheet. E. Buffer with Base: 6. Into a small beaker combine 5 mls of 0.1 M Sodium Acetate, (NaC 2 H 3 O 2 ) with 5 mls of 0.1 M Acetic acid, (HC 2 H 3 O 2 ). Mix well. 7. Measure and record the ph of the buffer solution you have just made using a meter. 11 8. Add 2 drops of 1.0 M NaOH, Sodium Hydroxide, and stir the sample to mix. Record the ph. 9. Add two more drops of NaOH and stir the sample to mix. Record the ph.. Discard the solution in the sink. 10. Analyze the results and report your conclusions on the report sheet.
9 ACIDS AND BASES: REPORT: NAME DATE I-IV: Determination of ph by Meter, Paper, and Cabbage: Foods: Solution A. ph by Meter [H + ] = 10 -ph B. ph by Paper C. New Cabbage color produced ph by Cabbage Soda Pop brand Tea Coffee Orange Juice Vinegar Baking Soda Cleaners: Shampoo Detergent Ammonia Bleach Toilet Cleaner Lye Others: Milk of Magnesia Antacid brand V: Properties of Water Solutions: A. ph of Carbon Dioxide, CO 2, in water: ph of Deionized Water = ph of Breath Carbonated Water = Explanation of Results: (Why did the CO 2 from your breath have the effect that it did?)
10 V. B-C Water with Acid or Base V. D-E. Buffer with Acid or Base Solution: ph Solution: ph B. Water with Acid D. Buffer with Acid Deionized Water After 2 drops 1.0M HCl After 4 drops 1.0M HCl NaC 2 H 3 O 2 :HC 2 H 3 O 2 Buffer After 2 drops 1.0M HCl After 4 drops 1.0M HCl C. Water with Base E. Buffer with Base Deionized Water After 2 drops 1.0M NaOH After 4 drops 1.0M NaOH NaC 2 H 3 O 2 :HC 2 H 3 O 2 Buffer After 2 drops 1.0M NaOH After 4 drops 1.0M NaOH Results Summary: 1. In general, foods and beverages 2. In general, cleaning supplies are mostly: 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 3. In general, adding a strong acid or a strong base to water: A. neutralizes the solution making a solution that is neither acidic nor basic. B. drastically changes the ph to be either strongly acidic if strong acid is added, or strongly basic if strong base is added. C. causes only a slight change in ph making the solution a only a little more acidic than it already was if strong acid is added, or only a little more basic than it already was if strong base is added. 4. In general, adding a strong acid or a strong base to a buffer solution: A. neutralizes the solution making a solution that is neither acidic nor basic. B. drastically changes the ph to be either strongly acidic if strong acid is added, or strongly basic if strong base is added. C. causes only a slight change in ph making the solution a only a little more acidic than it already was if strong acid is added, or only a little more basic than it already was if strong base is added.