Analysis of vinegar via titration

objectives To standardize a sodium hydroxide solution with potassium hydrogen phthalate. To determine the molar concentration and mass/mass percent concentration of acetic acid in an unknown solution To gain proficiency in the laboratory technique of titration

Safety Guidelines Wear safety glasses or goggles at all times while working in the laboratory. 2. Wear shoes at all times. 3. Eating and drinking in the laboratory are prohibited. 4. Locate the first-aid equipment and fire extinguisher. 5. Consider all chemicals to be hazardous unless instructed otherwise. 6. If chemicals come into contact with your skin or eyes, wash immediately with water and consult your laboratory instructor. 7. Never smell any vapor or gas directly. Instead waft a small sample toward your nose.

Safety Guidelines cont’d 8. Any reactions involving noxious chemicals or unpleasant odors must be performed under a fume hood. 9. While heating a substance in a test tube, never point the open end toward yourself or your neighbor – the contents may erupt and spray a hot chemical. 10. Always pour acids into water – not water into acid- because the heat released will cause the acid to spatter. 11. Clean up all broken glassware immediately. 12. Many common organic reagents are highly flammable; for example, acetone, alcohol, and ether. Do not use organic liquids near an open laboratory burner flame. 13. Do not perform unauthorized experiments. 14. Notify the instructor immediately in case of an accident.

Discussion The neutralization of an acidic solution of vinegar is investigated by the use of a basic solution of sodium hydroxide. The method of the investigation is done by titration. A titration is a process used to deliver a measured volume of solution. To continue the investigation, the acetic acid in a vinegar sample, can be titrated by delivering a measured volume of sodium hydroxide solution.

discussion Phenolphthalein is used as an indicator to signal when neutralization of the acetic acid is reached. It is colorless below pH 9 and pink above pH 9 respectively. After acetic acid is neutralized, the extra drop of NaOH increases the pH dramatically and the indicator changes from colorless to pink. The turning point is referred to as the endpoint.

discussion We will commence with 6 M NaOH for dilution with water. The new concentration can be approximated. Therefore to determine precise concentration is done by a process called standardization. Crystals of KHP is weighed for the process. The equation is KHP(aq) + NaOH(aq) --- KNaP(aq) H2O(l)

discussion Example Exercise 1: Molar Concentration of Standard NaOH A 0.905 g sample of KHP (204.23 g/mol) is dissolved in water and titrated with 19.90 mL of NaOH solution to a phenolphthalein endpoint. Find the molarity of the NaOH solution. 0.905 g KHP x 1 mol KHP/204.23 g KHP x 1 mol NaOH/1 mol KHP = 0.00443 mol NaOH 0.00443 mol NaOH/19.90 mL solution x 1000 mL/1 L = 0.223 mol NaOH/1 L solution = 0.223 M NaOh

discussion A sample of vinegar will be titrated with NaOH to a phenolphthalein endpoint. HC2H3O2(aq) +NaOH(aq) - NaC2H3O2(aq) + H2O(l)

discussion Example Exercise 2: Part (b) To calculate the m/m % concentration, we must know the density of the vinegar (1.01 g/mL) and the molar mass of acetic acid (60.06 g/mol). 0648 mol HC2H3O2/1000 mL solution x 60.06 g HC2H3O2/1 mol HC2H3O2 x 1 mL solution/1.01 g solution x 100% = 3.85% HC2H3)2

discussion Example Exercise 2: Percentage of Acetic Acid in Vinegar The titration of a 10.0-mL vinegar sample requires 29.05 mL of standard 0.223 M NaOH. Calculate the (a) molarity and (b) mass/mass percent concentration of acetic acid. 29.05 mL solution x 0.223 mol NaOH/1000 mL solution x 1 mol HC2H3O2/1 mol NaOH 0.00648 mol HC2H3O2/10.0 mL solution x 1000 mL/1 L = 0.00648 mol HC2H3O2

Equipment Graduated cylinder wash bottle with distilled water 1000-mL Florence flask w/stopper 125-mL Erlenmeyer flask (3) Buret stand ( or ring stand) Buret clamp (or utility clamp) Small, plastic funnel (optional) 50-mL buret 10-mL pipet and pipet bulb 100-mL beaker 150-mL beaker

CHEMICALS Dilute sodium hydroxide 6 M NaOH Potassium hydrogen phthalate, solid KHC8H4O4 (KHP) Phenolphthalein indicator Unknown vinegar solution, 3.00-5.00% HC2H3O2

Set Up Attach an utility clamp or one buret clamp to the ring stand 2. Carefully attach a buret to the jaws of the clamp 3. Place an Erlenmeyer flask on top of the base of the stand and adjust the height of the buret so that the bottom of the stem is just below the rim of the flask. 4. Place the funnel in the top of the buret and carefully fill the buret to the 0.00 mL mark. 5. Titrate the liquid into the Erlenmeyer flask by slowly opening the stopcock and allowing the liquid to dispense dropwise until a desired endpoint is reached.

Set Up cont’d

Procedure Preparation of Standard Sodium Hydroxide Solution Half fill a 1000-mL Florence flask with approx. 500 mL of distilled water. Measure approx. 20 mL of 6 M NaOH into a graduated cylinder and pour the NaOH into the Florence flask. Stopper the flask,and carefully swirl to mix the solution. 2. Place the buret in a buret stand. Using a small funnel, half fill the buret with the NaOH solution from the Florence flask. Allow some solution to pass through the buret tip, invert the buret, and empty the remainder into the sink. 3. Close the stopcock, and fill the buret with NaOH solution from the Florence flask.

procedure 4. Label the 125-mL Erlenmeyer flasks #1, #2. and #3. Accurately weigh out approx. 1 g of KHP into each of the flasks. Add approx. 25 mL of distilled water to each flask, and heat as necessary to dissolve the KHP crystals. 5. Perform the titration as follows: Drain some NaOH through the tip of the buret to clear any air bubbles. Position Erlenmeyer flask #1 under the buret as shown Figure 1. Record the initial buret reading (+/-0.05 mL). Add a drop of phenolphthalein indicator to the flask. Titrate the KHP sample to a permanent pink endpoint. Record the final buret reading (+/- 0.05 mL).

Procedure 6. Refill the buret with NaOH solution, record the initial buret reading, add a drop of phenolphthalein to flask #2, titrate the KHP sample, and record the final buret reading. 7. Refill the buret with NaOH solution, record the initial buret reading, add a drop of phenolphthalein to flask #3, titrate the KHP sample, and record the final buret reading. 8. Calculate the molarity of the NaOH solution for each trial. Record the average molarity of NaOH in the Data Table of Procedure B.

Procedure B. Concentration of Acetic Acid in Vinegar Obtain approx. 50 mL of vinegar solution in a dry 100-mL beaker. Record the unknown number in the Data Table. 2. Condition a pipet with unknown vinegar solution, and transfer a 10.0-mL sample into each 125-mL flask. Add approx. 25 mL of distilled water into each flask. 3. Fill the buret with NaOH solution, adjust the meniscus to zero, and record the initial buret reading as 0.00 mL. Add a drop of phenolphthalein to flask #1 and titrate the vinegar sample to a pink endpoint. Record the final buret reading.

Procedure B. Concentration of Acetic Acid in Vinegar 4. Refill the buret with NaOH solution, and adjust the meniscus to 0.00 mL. Add a drop of phenolphthalein to flask #2, and titrate the vinegar sample. 5. Refill the buret with NaOH solution and adjust the meniscus to 0.00 mL. Add a drop of phenolphthalein to flask #3, and titrate the vinegar sample. 6. Calculate the molarity of acetic acid, HC2H3O2, in the unknown vinegar solution. 7. Convert the molarity of HC2H3O2, (60.06 g/mol) to mass/mass percent concentration. Assume the density is 1.01 g/mL for the unknown vinegar solution. 8. M x MM x D x100% = m/m % concentration

Procedure fruit juices

Calculate the molarity of citric acid

Example for citric acid concentration calculation The titration of a 10.0 mL fruit juice sample requires 29.05 mL of standard 0.227 M NaOH . Calculate the (a) molarity and (b) mass/mass percent concentration of citric acid. Solution: We can calculate the moles of citric acid from the moles of NaOH solution: 29.05 mL solution X 0.227 moles NaOH /1000 mL solution X 1 mole H3C6H5O7/3 mole NaOH = 0.002198 mol H3C6H5O7 The molar concentration of H3C6H5O7 is 0.002198 mol H3C6H5O7/10.0 mL solution X1000 mL/L = 0.21981 mol H3C6H5O7/1 L solution =0.220 M H3C6H5O7

Example cont’d To calculate the m/m % concentration, we must know the density of the fruit juice (1.6650g/mL) and the molar mass of citric acid (192.13 g/mol). 0.220 mol H3C6H5O7/1000 mL solution X 192.13 g H3C6H5O7/1 mol H3C6O7 X1 mL solution /1.6650 g solution X 100% = 0.55849 % H3C6H5O7