Volume NaOH ph ph/ Vol (ml)

Transcription

1 Determining Acidity of Foods I. Purpose/Objective: The purpose is to identify the normality of a prepared sodium hydroxide solution by titrating samples of KAP. With the known normality of the base solution, samples of cheese and juice were titrated to measure their titratable acidity and ph values. Also, samples of three types of juices, two types of cheeses, and four solutions of vinegar and varying sugar concentrations were tasted to compare their perceived acidity to sweetness ratios. II. III. Introduction: Many foods contain weak organic acids, which help to promote taste, color and longevity of the foods. These acids stabilize foods and act as buffers, in addition to being antioxidants, ph adjusters, and nutrients. In order to evaluate the acids present in foods, the food industry uses a standard base to titrate into the acidic solution to determine the amount of acid present in the sample. In the base and acid reaction, the base accepts the hydrogen ions that the acid is donating. When the ph reaches neutral, there is an equal amount of base and acid present in the solution. This point, the equivalence point, determines the amount of acid originally present in the sample. In this experiment, these concepts relating to food acids are used in titrating different juice and cheese samples. Procedure: This laboratory procedure is found in the Principles of Food Composition, Laboratory Manual, FST 101A (Fall 01) Lab A, Preparing a Standard Base Solution, pages and Lab B, Total Acidity and ph of Liquid and Solid Foods, pages Modifications for part A of this experiment include titrating ml additions of NaOH until ph 11.5 is reached and determining two vials of KAP that are different in weight. Modifications for part B of this experiment include no titration of vinegar; the volume of cheese solution to be titrated is 50 ml; titrating the cheese solution at 0.1 ml NaOH per addition until ph 9, then at 1.0 ml NaOH per addition until ph 1; a solution of juice and water to be titrated is 5 ml juice and 5 ml bottled water; no filter is to be used in the titration of juice; and titrating the juice and water solution with 0.5 ml NaOH per addition until ph 6.0, then 0.1 ml NaOH per addition until ph 9.0, and 1.0 ml NaOH per addition until ph

2 Data/Result: Table 1: Group 10 s starting and ending volumes of NaOH and ending ph for the titrations of the first 3 known KAP samples and the unknown KAP samples. Titration Weight of Starting NaOH Ending NaOH Ending KAP (g) Volume Volume ph 1 (vial 53) (vial 310) (vial 306) unknown 1 (vial 541) See Table unknown (vial 414) See Table Table : Group 10 s ph vs. volume of NaOH for the 4 th known KAP sample titration Table 3: Group 10 s NaOH concentration and unknown KAP calculated and true weight. Unknown KAP Calculated KAP True KAP [NaOH] (N) Sample Weight (g) Weight (g) Vial Vial

3 Table 4: Sensory tastings of juices, vinegar, and cheeses. Rank In Order of Acidity Rank In Order of Sweetness Preference Juice Orange>Apple>Grape Grape>Apple>Orange Apple Vinegar D>S>C>B B>C>S>D B Cheese Cheddar>Mozzarella Mozzarella>Cheddar Cheddar Table 5: Group 10 s ph vs. volume NaOH for apple juice titration

4 Table 6: Group 10 s ph vs. volume of NaOH for cheddar cheese titration

5 Table 7: Section 5 Group 7 s ph vs. volume of NaOH for mozzarella cheese titration Table 8: Data for each juice s end point volume, equivalent ph, Brix, and concentration of NaOH. Group Juice N NaOH ph Equivalent Brix V eq -V initial ( o Bx) Orange Grape Apple

6 Table 9: Data for each cheese s initial ph, equivalent ph, Brix, end point volume, weight, and the concentration of NaOH. Group Cheese ph initial ph Equivalent N NaOH Brix V eq -V i Weight of ( o Bx) Cheese (g) Mozzarella Cheddar Table 10: Data for percent acidity, equivalent ph, Brix, Brix/titratable acidity, and taste ranking for each juice and cheese using average data in each group. Orange Juice Grape Juice Apple Juice Mozzarella Cheese Group % acidity (g/100ml juice) (g/weight g of cheese) ph Brix ( o Bx) Brix/ Titratable Acidity average average average average Taste Ranking (1=most preferred) 3 1 6

7 Cheddar Cheese average ph or First Derivative Equivalence Point Titration First Derivative Figure 1: Titration curve and the first derivative for the fourth KAP titration ph or First Derivative Equivalence Point Apple Juice Titration Apple Juice First Derivative Volume of NaOH Figure : Titration curve and first derivative for the apple juice titration. 7

8 1 10 ph or First Derivative Equivalence Point Cheddar Cheese Titration Cheddar Cheese First Derivative Volume of NaOH Figure 3: Titration curve and first derivative for the cheddar cheese titration ph or First Derivative Equivalence Point Mozzarella Cheese Titration Mozzarella Cheese First Derivative Volume of NaOH Figure 4: Titration curve and first derivative for the mozzarella cheese titration. IV. Calculations: weight KAP (g) 04. g *1000 meq mol [NaOH] = mol added 8

9 Example: For Group 10, KAP Titration 1 Data (see Table 1) g 04. g *1000 meq mol [NaOH] = mol 14. ml NaOH [NaOH] = mol*1000meq mol 14. ml NaOH meq [NaOH] = 14. ml NaOH [NaOH] = N weight KAP (g) = N * V NaOH NaOH 1000 meq * 04. g mol mol Example: For Group 10, unknown KAP 1 (see Tables 1 and 3).1078 N *14. ml weight KAP = 1000 meq * 04. g mol mol 1.53 meq weight KAP = 1000 meq *04. g mol mol weight KAP = 31.6 meq*g mol 1000 meq mol weight KAP = g pka = ph - log [A_ ] [HA] Example: For Group 10, pka of lactic acid in cheddar cheese. (see Tables 6 and 9) pka = ph - log [A_ ] [HA] pka = ph midpoint when [HA] = [A _ ] ph equivalent = 7.56 at 3.5 ml NaOH ph midpoint =, ph = 6.49 at 1.7 ml NaOH and ph = 6.56 at ph 1.8 ml ph midpoint = 6.53 pka = 6.53 % acidity = N acid * EqWt * 0.1L 100mL 9

10 Example: For Group 10, % acidity of apple juice. (see Table 8 and 10) % acidity = N acid *EqWt * 0.1 L 100 ml % acidity = V * N NaOH NaOH *EqWt * 0.1L V acid 100 ml % acidity = L * eq L.05 L % acidity = eq 0.05 L *67.05 g eq * 0.1 L 100 ml % acidity = eq L * g eq * 0.1 L 100 ml % acidity = g 100 ml o Bx o titratable acidity = Bx % acidity V. Discussion: *67.05 g eq * 0.1 L 100 ml Example: For Group 10, o Bx/titratable acidity of apple juice (see Table 10) o Bx titratable acidity = 11.7 o Bx o Bx titratable acidity = 37.8 For the unknown weights of the two KAP samples, the calculated weight for vial 541 was g and for vial These values were close to their actual weights of g and 0.37 g, respectively. The differences in values could be caused by not adding enough NaOH to the solution, thus undershooting the equivalence point, or could be due to mass of KAP lost in transfer in making the solution. In comparing the total acidity and Brix, the values differed for each juice. The average percent acidity of orange juice was 0.799% and the average Brix was 1. o Bx. For grape juice, the average percent acidity was % and the average Brix was 1.8 o Bx. Apple juice had an average percent acidity of 0.300% and an average Brix of This data did not correspond to the sensory test. In sampling each juice, grape juice was found to be the least acidic (most sweet), as found in the data. However, in tasting orange and apple juice, orange juice was found to be the most acidic, though the data suggests apple juice to be more acidic. In comparing data to the sensory tastings, the rank in order of acidity differs because people s taste buds perceive acidity and sweetness at different concentrations. 10

11 The titration curve for the fourth KAP titration looks like an expected titration and first derivative curve. However, the curves for the juice and two cheeses greatly differed. The equivalence points are more difficult to discern when looking at the graphs. Also, the titration curves for the two cheeses did not look like the expected titration curve. The differences in the curves could be because the KAP was a total weak acid solution. The apple juice and the two cheese solutions were a mixture of weak acids and other foodstuffs, which inhibited the NaOH from fully reacting with only the acid. The results of the titrations of the juices were overall fairly consistent. For the orange juice, the highest ph value was found at the lowest volume of NaOH added. For grape juice, the ph value as compared to the added volume of NaOH was fairly consistant. For apple juice, the lowest ph value was found at a relatively average volume of NaOH added. Also, the highest ph value was found at the lowest volume of NaOH added. An average ph value was found at the highest volume of NaOH added. For the mozzarella cheese, each of the ph values compared to the volumes of NaOh added were consistent. For the cheddar cheese titrations, the lowest ph value, which was not very low when compared to other ph values, the volume of NaOH added was very low when compared to the other volumes of NaOH added. These discrepancies in the data could be due to errors in setting up the titration and when calculating the ph. For the cheeses, there were different cheese weights used in the groups, and each prepared the cheese solution slightly differently. Also, errors in calculating the ph could be found if the ph was written down before the ph electrode was absolutely ready. VI. Conclusions: In this two-part laboratory solution, a standardized base solution was prepared and used to titrate a weak acid solution and used to titrate acids in foods. Many foods contain acid, and the amount of acid for foods can be found through titrations with a standard base solution. In titrating the food samples, the amount of acid present is found when the concentration of hydroxyl ions equals the concentration of hydrogen ions. In the experiment, three types of juices (each diluted with water) and two types of cheeses (blended then strained to create a solution) were titrated with the standard base solution, with the ph being tracked. Both members of group 10 both participated in preparing the juice solution and the cheese solution. Both also titrated one of the solutions with the base solution while the other recorded the ph. 11

12 VII. Questions: Part A: 1. When a standard base solution of NaOH reacts with CO, it forms sodium carbonate in the solution. This causes the solution to have an imprecise concentration, making it very difficult to accurately titrate an acidic sample.. When the indicator changes color, the ph of the sample is at it s equivalence point. This ph provides a good estimate for the end-point of a titration of a weak acid by a strong base because the concentration of hydroxyl ions equals the concentration of hydrogen ions. 3. ph or First Derivative Acetic Acid Data Volume of NaOH Acetic Acid Titration First Derivative equivalence point = ph 9.4 Phosphoric Acid Titration ph or First Derivative Volume of NaOH Phosphoric Acid Titration First Derivative equivalence point 1 = ph equivalence point = ph

13 Part B: 1. The average percent acidity and average Brix for orange juice and for apple juice fell between the acceptable ranges. However, the average percent acidity and average Brix for grape juice were below and higher than their respective acceptable ranges, if only considering tartaric acid as the primary acid in grape juice.. ph and titratable acidity are related because as the titratable acidity increases, so does ph. This is because the greater the amount of acid, the more base is required to neutralize it, making the ph higher. 13