Quantitative Determination of Acidity of Softdrinks

Alvarez, Ma. Lemie G.

Quantitative Determination of Acidity of Softdrinks

I.Introduction

Acids are substances that release hydronium (H3O+) ions whereas bases release hydroxide (OH-) ions in an aqueous solution. A solution’s acidity or basicity can be measured using the process called Acid – Base Titration (Chang, 2009, p. 429).

Titration is the gradual addition of one solution to another until the solution in the first solution has reacted completely with the solute in the second solution. it is a way of measuring the concentration of the solution in a given quantity of the solution.

An indicator was added to the solution being titrated in order to determine when the reaction between the acid and the base was complete. An indicator                                                                                                                                                was a compound that exhibit different colors depending on the pH of the solution. Upon completion, the reaction between the acid and the base was called Neutralization Reaction (Chang, 2009, p. 429).

Acid + Base -----→salt + water

Sodium hydroxide, NaOH, was a known base in general chemistry I laboratory class. It can be used as a titrant or an indicator.

Titrant was a substance that was gradually added into the other solution to be titrated. Since a titrant was used to determine the amount of an acid or a base in a sample solution that was titrated, the concentration of a titrant should be accurately known. This was done through the process of standardization.

NaOH solution was used as a titrant in determining the concentration of citric acid, C6H8O7, which was commonly found in commercial fruit juices. The objective of this experiment was

(a) Prepare and standardize NaOH solution,

(b) Determine the acidity of fruit juices using the standardized NaOH solution,

(c) Calculate the weight of citric acid present in the fruit juice of citric acid    

    Present in the softdrinks.

II. Materials

1. Reagents

0.50 g NaOH pellets

0.150 g potassium acid phthalate

300 ml Distilled water

2-3 drops of phenolphthalein

2 trials of 75 ml soft drink sample (Sprite)

2 trials of 75 ml soft drink sample (7-up)

1. Apparatus

Four 250 ml beaker                                     Top loading balance

50 ml buret                                                       White background

250 ml Erlenmeyer flask                              Stirring rod

100 ml volumetric flask                                Buret holder

Iron stand                                                    Bunsen burner

Iron ring                                                       10 ml pipet

III. Procedure

1. Preparation of NaOH

          0.50 g of 0.05 M NaOH pellets was obtained and placed in a preweighed beaker. For two to three minutes, it was left to stand then it was weighed again. The new mass was recorded.

         250 mL of distilled water was introduced to the beaker. The beaker was swirled until NaOH pellets were completely dissolved. The NaOH solution was transferred to a 250 mL volumetric flask. The solution was diluted with distilled water until the 250 mL mark was reached. The volumetric flask was covered then shook.

1. Standardization of NaOH

          0.5 g of Potassium acid phthalate was obtained and placed in a 250 mL Erlenmeyer flask. 50 mL of distilled water was put in the flask then three drops of phenolphthalein was introduced to the flask. The prepared NaOH solution earlier was put in a 50mL buret that was placed on the iron stand.

         The potassium acid phthalate solution on the Erlenmeyer flask was titrated until the color of the solution changed to faint pink. The sides of the flask were washed with distilled water to assure that all the NaOH solution was put in the potassium acid phthalate solution. After the solution’s color changed into faint pink, the volume of NaOH used was recorded. The concentration of NaOH in molars was calculated. The same procedure was done for the second trial.

1. Determination of the acidity of softdrinks

          75 mL of softdrink sample was obtained and placed in a 250 mL Erlenmeyer flask. The two samples were heated for five minutes and were shaken until there were no visible bubbles formations. Two trials were obtained each with 2-3 drops of phenolphthalein. The two samples were titrated until the color of the solution changed to faint pink. After the solution’s color changed into faint pink, the volume of NaOH used was recorded. The concentration of citric acid was calculated. The same procedure was done for the second trial and for the other sample of softdrinks and its second trial.

IV. Data and observations

Table1. Preparation of the NaOH solution

Table 2a and 2b. Standardization of NaOH solution    

Average molarity of NaOH solution: 0.06 M