Experiment 2: Determination of Total Acidity of Vinegar By Acid-Base Titration Pabilane, Alma L. & Group 6, Chem 27.1, SEJ1, Ms. Noemi S. Walican December 11, 2010 I. Abstract Titration is an analytical procedure involving a chemical reaction in which the quantity of at least one reactant is determined volumetrically. Acid-Base titration involves neutralization reaction between an acid and a base. Its basis is the equivalence point, wherein the amount of the titrant added is stoichiometrically equivalent to the amount of analyte, thus, the concentration of the unknown can be calculated. Sodium hydroxide is the titrant used in titrating vinegar to determine its total acidity. It is standardized by titrating it against primary standard grade potassium hydrogen phthalate, a type of substance with very high purity, to determine the most accurate measurement of its concentration. Three trials were made for the standardization of the NaOH and 3 more trials in titrating the vinegar with it. The concentration of the vinegar (molarity, %(w/v)) in each trial is given by: 0.48M, 2.88%; 0.46M, 2.76%; 0.42M, 2.52%; 0.44M, 2.64%, respectively. The average experimental concentration in molarity and in %(w/v) of the vinegar as determined is 0.45M, and 2.721%, respectively, with a standard deviation of 0.155, relative SD of 5.7 ppt, and CI (at 95%) of 0.152. The true value for the molarity of the vinegar is 0.4995M and for %(w/v), is 3.000%, thus, giving an experimental error of -9.91%.
II. Keywords : titration, vinegar, acid, base, titrant, indicator
Vinegar is a substance that is widely used in daily living. It is a sour-tasting liquid that is commonly used as a condiment and a preservative. It contains many organic acids. Among these acids, acetic acid (CH3COOH, a weak acid) is the one present in larger amount compared to the other. Vinegar is taken in by the human body. The concentration of the vinegar must fall in the range that is allowable for human consumption (which is usually, 4-8% by volume). Therefore, vinegar manufacturers control and monitor the acidity of vinegar. In this experiment, acid-base titration will be used to determine the concentration of vinegar, its total acidity, and not the concentration of acetic acid alone. The basic principle of titration is neutralizing the analyte (substance with unknown concentration, vinegar, in the experiment) with the titrant (substance with known concentration, usually strong acid/base, in the experiment, NaOH). When equivalent point is reached, that is, amount of analyte equals the amount of titrant, the concentration of the analyte can be calculated. But experimentally, the end point is the one determined, a point which is very close to the equivalence point. This experiment will help us know whether the vinegar is safe to be taken in by human.
dissolving it in 300 ml CO2 – free, distilled water in a 500-ml beaker, and then, transferred into 1-L PET bottle and tightly closed. For the standardization of the titrant (0.1M NaOH), in a 250-ml Erlenmeyer flask, 0.50 – 0.51 g of dried primary standard grade potassium hydrogen phthalate (KHC8H4O4) was placed. Immediately before titration, 25 ml CO2-free distilled water and 2 drops of phenolphthalein were added to the KHC8H4O4 in the Erlenmeyer flask. The solution was titrated with 0.1M NaOH, with continuous swirling while adding the titrant, until the end point was reached, indicated by the faint pink coloration of the solution. This was done for three times. The molarity of NaOH was calculated in each standardization using the formula M1V1 = M2V2. Then the average molarity (or the mean value) of NaOH was also computed. For the titration of unknown (determination of acidity of vinegar), 5 ml of the diluted vinegar (with unknown concentration) was placed into a 250-ml Erlenmeyer flask. Five ml of CO2-free water and 2 drops of phenolphthalein were added. The solution was titrated with the standardized NaOH solution, with continuous swirling while adding the titrant, until the end point was reached, indicated by faint pink coloration of the solution. The molarity of the vinegar was calculated using the formula M1V1 = M2V2.
IV. Experimental
V. Results
III. Introduction
For the experiment, 1 L of 0.10 molar of Reactions involved: sodium hydroxide (0.1M NaOH) was prepared by dissolving approximately 4.0 g of NaOH pellets and Standardization of NaOH solution: Chem 27.1. Determination of Total Acidity of Vinegar by Acid-Base Titration. Page 1 of 3
-
KHC8H4O4(s) + H2O(l) KC8H4O4 (aq) + H3O +
+ (aq)
-
KHC8H4O4(aq) + NaOH(aq) Na K C8H4O4 (aq)+ H2O -
HPhe(aq) + H2O(l) Phe (aq) + H3O
+ (aq)
Analysis of unknown: -
CH3COOH(aq)+ H2O(l) CH3COO (aq)+ H3O +
+ (aq)
-
CH3COOH(aq) + NaOH(aq) Na CH3COO (aq) + H2O(aq) -
HPhe(aq) + H2O(l) Phe (aq) + H3O
+ (aq)
Results Standardization: Trial
1 2 3
Weight of KHC8H4O4 (g) 0.5009 0.5075 0.5035
Volume of NaOH (ml) 25.05 25.13 25.05
Molarity of NaOH 0.098 0.099 0.0984
Average Molarity= 0.098 + 0.099 + 0.09844 = 0.0985 M 3 -4
Standard Deviation: 5.01 x 10 Relative Standard Deviation (in ppt): 5.08 Analysis of Unknown: Trial 1 2 3 4 5 6
V NaOH (ml) 24.35 23.75 23.45 22.15 21.35 21.65
M of vinegar 0.48 0.46 0.42 0.44
%(w/v) of vinegar 2.88 2.76 2.52 2.64
Average %(w/v) of vinegar= [(0.48mol/L/ 60.06 g/mol)x(1 L/1000ml)x100] + [(0.46mol/L/ 60.06 g/mol)x(1 L/1000ml)x100] + [(0.42mol/L/ 60.06 g/mol)x(1 L/1000ml)x100] + [(0.44mol/L/ 60.06 g/mol)x(1 L/1000ml)x100] + 4 = 2.7027% Standard Deviation: 0.155 Relative Standard Deviation: 5.7 (ppt) CI at 95% Confidence Level = (1.96x0.155)/√ 4 = 0.152
% Error = 2.7027 – 3.000 x 100 = - 9.91 % 3 VI. Discussion For human consumption, the usual concentration of acetic acid in vinegar is only about 4-8% by volume. In the experiment, acid-base titration was used to determine the acidity of a vinegar with unknown concentration. Titration is a method of analysis that is useful in determining the concentration of an unknown using the by reacting it completely with a solution of a known concentration. The substance of unknown concentration being titrated is called the analyte and the one being reacted with the analyte (the one with known concentration) is called the titrant. The titrant is delivered into the flask containing the analyte using the buret. The titrant used is usually strong acid/base so that the reaction is irreversible. In the experiment, NaOH reacts with CH 3COOH to form the salt sodium acetate and water. The equivalence point of a titration refers to that point when an amount of titrant that has been added is stoichiometrically equivalent to the amount of analyte present. For the experiment it is the point when number of moles of vinegar (or the acetic acid) equals the number of moles of NaOH, for the reaction to be complete. According to experimental results, approximately 21-25 ml NaOH is needed to react completely with the vinegar. (Note that this is quite a wide range). The end point refers to the experimental approximation of the equivalence point, since the latter is difficult to determine. Indicators, such as phenolphthalein as used in the experiment, are used to detect the end point. Phenolphthalein is a very weak acid, and is colorless in acidic solution and turns to pink in basic solution. Thus, the end point is indicated when a faint pink color appears in the solution. During titration, one will know that the equivalence point is near when the pink color disappears more and more slowly. For a more accurate titration, the titrant’s concentration must be known as near to the true value as possible. In the experiment, NaOH was prepared in an approximate concentration of 0.1 molar. Then, it was titrated against primary standard grade potassium hydrogen phthalate (KHC8H4O4). This process is called standardization which is used to determine the exact concentration (or the most accurate, as possible) of the titrant. Primary standard grade of the substance to be used in standardization is necessary because it has very high purity and possess other qualities that will be helpful to get the most accurate result, like stability
Chem 27.1. Determination of Total Acidity of Vinegar by Acid-Base Titration.
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in solid form and high equivalent weight (to minimize relative error in weighing). The results of the standardization in the experiment showed that the titrant NaOH has a concentration of approximately 0.0985M (this is the mean of the molarities of NaOH in the 3 trials which were computed by getting the number of moles of KHC8H4O4 divided by the volume of NaOH used in liters). The concentration of the vinegar as determined in the experiment is 0.45M or 2.721% (w/v), while the true value is 0.4995M or 3.00% (w/v). The experiment has an experimental error of -9.91% (indicating that the experimental result is less than the true value). VII. Conclusion and Recommendations The acid content of a vinegar is present in a very low amount (4-8%) for human consumption, and it cannot exceed because it may have bad effects to human body. Titration is a useful method in determining amount/concentration of a substance of unknown concentration. But, it greatly depends on purity of the reagents being used, especially the titrant. Use of primary standard grade substance is necessary in standardization of the titrant. It is highly recommended that experimenters make sure that the water being used is distilled and carbon dioxide-free, because, carbon dioxide may also react with water. Proper ways of measurement must also be done in measuring volumes and masses to avoid causing errors in the data. Proper lab techniques must also be observe, e.g. make sure there is no bubble at the tip of the buret, and do half-dropping when the equivalence point is near. Experimenters must always wear personal safety equipment such as lab gowns, gloves, masks and goggles and always follow the procedure in the manual to avoid unwanted accidents. Wastes must be segregated and disposed properly, in their proper waste bottles. Acids/bases must be neutralized first. Solid wastes must be put in the right trash bin and not in the sink.
http://web.lemoyne.edu/~giunta/chm151L/vi negar.html on Dec. 9, 2010. 4. Helmenstine, A. Titration Basics. Retrieved from: http://chemistry.about.com/od/acidsbases/a /aa082304a.htm on Dec. 9, 2010. 5. Titrations, Indicators, and Titration Curves. Retrieved from http://www.avogadro.co.uk/chemeqm/acidb ase/titration/phcurves.htm on Dec 9, 2010.
I hereby certify that I have given substantial contribution to this report. _______________________ MARIE GIECEL
_______________________ ALMA PABILANE
VIII. References 1. Brown T., LeMay H., Bursten B. (2002). th Chemistry the Central Science 8 ed. Pearson Education Asia Pte. Ltd. Singapore. 2. Hargis, L. (1988). Analytical Chemistry. Prentice-Hall, Inc. New Jersey. 3. Determining Molar Concentration of Vinegar by Titration. Retrieved from: Chem 27.1. Determination of Total Acidity of Vinegar by Acid-Base Titration.
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