Introduction
The purpose of this lab is to determine citric acid concentration in fruit juice. The purpose was achieved by using titration methods. The first step was standardizing a strong base and sodium hydroxide. NaOH is hydroscopic and had to be standardized, which was achieved using KHP. The second part of this lab was using the standardized base to determine the concentration of citric acid in grape juice. Methods were done similarly to the ones done in part one, finding the neutralization point of the base and acid using titration. Many calculations were done as the last step in the lab. To cary out this lab an analytical balance, a buret, Erlenmeyer flasks, a bottle, and a graduated cylinder were used.
Procedure
To begin, a standard solution of sodium hydroxide was created by dissolving NaOH pellets and mixing the resulting solution with the contents of a 500 mL bottle full of dissolved water.
Four samples of KHP were weighed and their masses recorded. Each of these samples were put into separate Erlenmeyer flasks. Distilled water and and phenolphthalein were added to each then they were mixed and covered. The buret was cleaned. It was then filled to the top with the sodium hydroxide solution and drained until it reached 0.00 mL. Each of the KHP samples were titrated-NaOH was released into the flask until the indicator turned pink. The molaritys were calculated for each trial, and the average and standard deviation found.
A pipet was used to accurately measure 10 mL of white grape juice, which was then weighed and it’s mass recorded. Three Erlenmeyer flasks had the appropriate amount of distilled water, indicator, and juice in them. They were then titrated and the mL need for each were recorded. The molaritys of citric acid in the juice sample, and the weight/weight and the weight/volume percents of citric acid were calculated for each of the trials, along with the averages and standard deviations.
For detailed procedure refer to reference 1.
Results
Table 1 shows the mass of KHP used, the initial buret volume, the final buret volumes, and the concentration of sodium hydroxide for each trial.
Table 1. Data collected during standardization of NaOH with KHP.
Sample Mass KHP (g) Initial Buret Volume (mL) Final Buret Volume (mL) [NaOH]
1 0.4005 0.00 43.30 0.04529
2 0.4003 0.00 41.20 0.04757
3 0.3991 0.00 43.30 0.04513
4 0.3998 0.00 45.80 0.04470
Average [NaOH] of the closest three was 0.04970 ± 0.0003051.
Table 2 shows the volume of juice, the initial buret volume, and final buret volume for each of the trials that were using the white grape juice.
Table 2. Data collected during the titration of white grape juice with standardized NaOH.
Sample Volume Juice (mL) Initial Buret Volume (mL) Final Buret Volume (mL)
1 10.00 0.00 18.80
2 10.00 0.00 18.35
3 10.00 0.00 18.05
Table three shows the citric acid concentration, the weight/weight percent, and the weight to volume percent of citric acid in the final solution. The calculations done to get these results can be found in the attached pages. The averages and standard deviations were calculated using Excel.
Table 3. Citric acid concentration (mol/L), wt/wt %, and wt/vol % citric acid in white grape juice.
Sample Citric acid concentration (mol/L) Wt/wt % citric acid Wt/vol % citric acid
1 0.7986 0.514 0.542
2 0.8182 0.502 0.529
3 0.8318 0.494 0.526
Average 0.8162 0.503 0.532
Standard deviation 0.01669 0.0101 0.00850
References
1. General Chemistry Experiments: A Manual for Chemistry 204, 205, and 206, Department of Chemistry, Southern Oregon University: Ashland, OR, 2009
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