Titration Analysis of Weak Acid Solutions: Potassium Hydrogen Phthalate and Citric Acid in Fruit Juice

Introduction

The purpose of this lab was to determine the citric acid concentration in commercial fruit juice. This was achieved by titration of a sample of fruit juice with standardized sodium hydroxide. Sodium hydroxide is hygroscopic and had to be standardized, which was achieved using KHP (hydrogen phthalate, KHC₈H₄O₄)(eq 1).

KHC₈H₄O₄(aq) + NaOH(aq) KNaC₈H₄O₄(aq) + H₂O(l) (1)

In the second part of this lab the standardized base was used to determine the concentration of citric acid in lemon juice (eq 2). 

H₃C₆H₅O₇(aq) + 3NaOH(aq) Na₃C₆H₅O₇(aq) + 3H₂O(l) (2)

The neutralization point of the base and acid were found with titration methods.

Procedure

A standard solution of sodium hydroxide was created by dissolving 1.05 g of sodium hydroxide pellets into approximately 500 mL water. Four samples of approximately 0.4 g KHP were weighed and each was put into separate Erlenmeyer flasks. Approximately 50 mL of distilled water and three drops phenolphthalein were added to each sample. The buret filled with the sodium hydroxide solution and it was titrated into each sample until the indicator turned pink. The molarity of each trial was calculated. The average and standard deviation of the values that agreed within 

0.0005 M were calculated.

Distilled water, indicator, and lemon juice were put into Erlenmeyer flasks to create three samples. Each sample was titrated and the amount of sodium hydroxide required to turn the indicator pink was recorded. The molarity of citric acid in the juice sample, the weight/weight percent and the weight/volume percent of citric acid were calculated for each of the trials, along with the averages and standard deviations. A pipet was used to measure 2.00 mL of lemon juice, which was weighed and it’s density calculated.

Detailed procedures can be found in reference 1.

Results 

Table 1 shows the mass of KHP, the initial buret volume, the final buret volumes, and the concentration of sodium hydroxide for each trial done during the standardization of sodium hydroxide.

Table 1. Data collected during standardization of NaOH.

Sample	Mass KHP (g)	Initial Buret Volume (mL)	Final Buret Volume (mL)	[NaOH]
1	0.4178	0.51	44.11	0.04692
2	0.4051	0.41	42.01	0.04768
3	0.4072	0.39	42.38	0.04749
4	0.4097	0.62	42.99	0.04735

The average [NaOH] of samples 2, 3, and 4 was 0.04751 ± 0.00017 M.

Table 2 shows the volumes of juice, the initial buret volumes, and final buret volumes  from the titration of lemon juice with standardized sodium hydroxide.

Table 2.  Data collected during the titration of lemon juice with standardized NaOH.

Sample	Juice Volume (mL)	Initial Buret Volume (mL)	Final Buret Volume (mL)
1	2.00	0.65	32.69
2	2.00	0.97	32.21
3	2.00	0.32	32.28

Table 3 shows the citric acid concentration, the weight/weight percent, and the weight/volume percent of citric acid in the final solution. 

Table 3. Data calculated from titration of lemon juice with standardized NaOH.

Sample	Citric Acid Concentration (mol/L)	Wt/Wt % Citric Acid	Wt/Vol % Citric Acid
1	0.2537	3.891	4.874
2	0.2474	3.794	4.752
3	0.2531	3.881	4.862
Average	0.2514	3.855	4.830
Standard Deviation	0.0034	0.053	0.067

References

1. General Chemistry Experiments: A Manual for Chemistry 204, 205, and 206, Department of Chemistry, Southern Oregon University: Ashland, OR, 20010