Titration: Acid and Solution

Abstract
The aim of this experiment was to determine which of the samples of vinegar if any had been watered down, to complete this task a sample of commercial vinegar was tested twice using titration and an average was taken of the two samples to give a base line to compare the potentially tampered samples against.
Introduction
Vinegar is a solution made from the fermentation of ethanol (CH3CH2OH), which in turn was previously fermented from sugar. The fermentation of ethanol results in the production of acetic acid (CH3COOH). The typical pH of vinegar ranges anywhere from 2 to 3.5, although shop-bought vinegar usually measures 2.4 www.wisegeek.com/what-is-vinegar.htm fig 1
In this study we will determine the amount of acid in a vinegar sample by using titration, a common technique in chemistry a typical shop brought vinegar has an acid percentage of 5% see fig 1. A titration is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. Typically, the titrant (the know solution) is added from a burette to a known quantity of the analyte (the unknown solution) until the reaction is complete. Knowing the volume of titrant added allows the determination of the concentration of the unknown. Often, an indicator is used to usually signal the end of the reaction, the endpoint. chemed.chem.purdue.edu/genchem/lab/techniques/titration/what.html In this study we will use an indicator solution called phenolphthalein as an indicator. Phenolphthalein is colourless when the solution is acidic or neutral. When the solution becomes slightly basic, phenolphthalein turns pinkish, and then light purple as the solution becomes more basic. So when your vinegar solution starts to turn pink, you know that the titration is complete see fig 2. FIG 2 http://www.titrations.info/acid-base-titration-acetic-acid-in-vinegar The titration process has to do with hydrogen ions. In water a small number of molecules disassociate, some of the water molecules lose hydrogen ions and become hydroxyl ions the remaining hydrogen ions join with the water molecules and become hydronium ions. In pure water there are an equal amount of hydrogen and hydroxyl ions so the solution is neither acidic nor basic. An acid solution like vinegar (acetic acid) is a substance that donates hydrogen ions and when acetic acid is dissolved in water the balance of hydrogen and hydroxyl ions is shifted so that there are now more hydrogen ions than hydroxyl ions in the solution making it acidic. To balance (titrate) the solution you need to add a base solution, a base solution is substance that accepts hydrogen ions and when a base solution is dissolved in water the balance between hydrogen and hydroxyl ions shifts the other way leaving a solution with more hydroxyl ions than hydrogen ions making it an alkaline solution. To measure the amount of acid in vinegar you add enough hydroxyl ions to balance the hydrogen ions in the acid, this is called the equivalence point, in this experiment sodium hydroxide was used as a base solution. Sodium hydroxide is a strong base and will disassociate almost completely in water so that for every sodium hydroxide molecule that is added to the water you can expect to produce a matching hydroxyl ion.
Aim
To determine which of the samples of vinegar if any had been watered down.
Hypothesis
That the process of titration would determine whether or not the samples of vinegar had been tampered with.

Equipment 1. . Vinegar, control 2. . Vinegar Sample 3. . Small funnel 4. . 0.5% Phenolphthalein solution in alcohol (pH indicator solution) 5. . 0.1 M sodium hydroxide solution 6. . 125 mL Conical flask 7. . 50 mL burette 8. . 10 mL graduated cylinder 9. . Ring stand 10. . Burette clamp
Method 1
25ml of vinegar form an uncontaminated source was decanted into a beaker and to that 5 drops of phenolphthalein was added. A burette was placed into a burette clamp and using a funnel the burette was filled with 50ml of 0.1m sodium hydroxide solution and the beaker containing the vinegar solution was placed beneath the burette and the starting level of the sodium hydroxide solution was noted (making sure to read the level from the bottom of the meniscus (Fig 3).
Fig 3 http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p045.shtml The sodium hydroxide solution was slowly added to the vinegar solution drop by drop while the beaker was swirled to mix the solution. At this point the vinegar solution will start to turn pink but will return to clear when the solution is mixed, when the vinegar solution turns pink and remains that way even when mixed the titration is complete and the tap on the burette should be closed. Note the remaining level of the sodium hydroxide solution from the bottom of the meniscus and subtract this figure from the starting level to determine the amount of titrating solution was used, repeat the process twice to determine the average % of the purity of the sample.
Method 2
The process was repeated as per method 1 for the unknown sample and the data was recorded.

Results | Neat Vinegar | Tampered Sample | 1st Titration | 18.9 ml | 3.7 ml | 2nd Titration | 19.1 ml | 3.7 ml | Mean Results | 19.0 ml | 3.7 ml |

Calculations 19.5%
19.5%
100
100
3.7
19.0

Neat Vinegar 0.076 MOL
0.076 MOL
19.0 0.1 25
0.076 MOL = the amount of acid in sample
Tampered vinegar 0.0148 MOL
0.0148 MOL
3.7 0.1 25
0.0148 MOL= the amount of acid in sample
0.0148 X 5 = 0.074 which equates to 20%

Discussion
The purpose of this experiment was to conclude the molar concentration of a sample of acetic acid, CH3COOH, compared to a commercial vinegar sample. The first test was to smell the vinegar sample and compared to the commercial sample it had a weaker aroma this was an indication that the sample was different from the commercial vinegar. On the basis of data received after having performed this investigation, it was found how using the technique of acid base titration helps in determines the molar concentration of acetic acid present in vinegar by titrating it with standard sodium hydroxide in order to find out the concentration of acetic acid. As a result it was observed that the commercial vinegar had the strongest acidity with its molarity of 0.076mol and the tampered sample ha a molarity of 0.0148mol leading to the conclusion that the samples of vinegar had been watered down to 19.5%

Conclusion
The sample of vinegar had been watered down by 20% and after titrating the sample and calculating the percentage it was calculated that the sample had been watered down by 19.5% the discrepancy could be down to a number of things such as human error; by not taking the reading at the exact time or that the alkali/acid became contaminated. To improve the accuracy of the results the experiment could have been done more times to make sure that there were no mistakes

Bibliography http://wwwchem.csustan.edu/consumer/vinegar/analysis.htm http://www.wisegeek.com/what-is-vinegar.htm http://chemed.chem.purdue.edu/genchem/lab/techniques/titration/what.html http://www.titrations.info/acid-base-titration-indicators-preparation Fig 2 http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p045.shtml http://www.titrations.info/acid-base-titration-acetic-acid-in-vinegar Fig 3