Methods Used To Calculate The Total Acidity Within The Wine
There are many contributing factors that affect the tastes and characteristics of wine, where acids and ethanol concentration are just two of the four fundamental traits in wine. These factors are all individual preferences, but there a few crucial factors which is easily scientifically researched known to have a big impact of the flavor of wine, being the acidity and ethanol content within it. The acidity contribution is through the concentration of Hydrogen ions in the wine. In Figure 1, the overall pH of wine can be seen to be between approximately 2.5 to 4.5 pH relative to distilled water having a pH of 7.00.
There are several different types of acids found within wine, all contributing to the overall finishing acidic taste, for instance tartaric acid, malic acid and citric acid can all be found. These acids can also be classed due to different properties traditionally called the total acidity. This is usually grouped into volatile acids and non-volatile acids (or fixed acids). The total acidity is the relationship that allows us to calculate accurate values for each of the concentrations of acidity in the wine. Volatile acids typically are organic acids which are mainly come in the forms of acetic, citric etc. Wines of high volatile acidity are typically prone to a quicker spoilage rate due to the tendency to vaporize and leave the wine. Thus, non-volatile acids are formed from a longer fermentation process, meaning the acids formed from this process are weaker. The main non-volatile acid found are malic which converts to lactic acid during this process, therefore the longer the fermentation proceeds the more malic acid which is converted, thus the more basic the wine.
The ethanol content being the other contributing factor is controlled by the amount of sugar present in the grape juice which is then ferments to produce the ethanol. As grapes contain about 80% water, and 20% sugar the fermentation process breaks this into water, ethanol and flavor. Legally, to follow government regulations only between 10.0-14.0% (V/V) of ethanol is permitted to be contained within the wine to be sold.
Experiment
In these experiments the non-volatile and volatile acidities of wine samples were measured using acid-base titration and a pH metre. Following that the ethanol content was found by distillation.
Method
Determination of Non-volatile Acidity: In this experiment, the non-volatile acidity of wine samples was determined by pipetting 5 mL of wine into 100 mL beakers, along with 20 mL hot distilled water. This was evaporated until there was approximately 5 mL remaining. This process of reconstitution and evaporation was repeated three times more, each time adding 25 mL distilled water again. Once cool, the substance remaining was transferred into clean conical flasks and diluted to 50 mL, with 2-3 drops of Phenolphthalein indicator. This was titrated with 0.01930 mol/L of NaOH solution. This process was used to calculate the concentration of non-volatile acidity in the wine.
Determination of Total Acidity Using Phenolphthalein Indicator: In this experiment, the total acidity of wine samples was found by filling a burette with 0.01930 mol/L of NaOH. To get an average result of the total concentration, this was done using three 250 mL conical flasks containing 5 mL aliquot of wine, diluted to 50 mL with distilled water. The phenolphthalein indicator was used in two of the samples, and the solution was titrated with NaOH. An alternative indicator was used in the third solution and also titrated with the NaOH solution. The results from this process was used to calculate the concentration of the wine. Determination of Total Acidity Using the pH metre: Before starting the experiment, the calibration of the electrode had to occur to ensure the accuracy when trying to find the pH of the wine, which was done over a large range of pH values. By filling a burette with the standardized 0.09667 mol/L of NaOH solution. 25.0 mL of wine was then pipetted into a 150 mL beaker, where the solution was then diluted with a further 25.0 mL of water. After adding phenolphthalein indicator, the initial pH of the solution was taken. Whilst continuing to stir, 2.0 mL of the NaOH solution was added and the pH value was recorded, where this process was repeated until 16 mL of the solution was added to the wine. The 2 mL was reduced to 0.3 mL and this was added to until it was 5 mL past the equilibrium point reached in the previous two experiments. Then the increments were increased back to 2 mL and continue to be added for a further 10 mL, so approximately 40 mL of NaOH was added to the wine.
Determination of Ethanol Content in Wine Samples by Distillation: In this experiment the ethanol content of the wine samples was determined by the distillation process. First 50 mL of the wine sample was placed into a 100 mL round bottom flask, where the solution was made basic by adding approximately 2 mol/L NaOH. After each addition, the red litmus paper was used to check whether the solution was basic, where this process was continued until the desired affect was achieved. The sample of wine was distilled until the temperature of near 980C was reached, so the solution didn’t boil over. The distillate of wine was diluted to 50 mL using distilled water, this was then used to find the density. Separately, five 50 mL volumetric flasks were weighed, and their weights recorded, ensuring that they were all labeled. Five separate solutions were prepared, each with a percentage of ethanol added to them (0, 5, 10, 15 and 20%) using 100% ethanol and distilled water to get the percentages.
Results
The results found when testing the non-volatile acidity were: Test Indicator Titre (mL) using 0.0193 molL-1 NaOH 1 Phenolphthalein 18.0 2 Phenolphthalein 16.0 3 Phenolphthalein 19.4 Average Titre 17.8 mL of 0.0193 molL-1 NaOH.
To analyze these results a number of formula were used to calculate the concentrations of the certain types of acids that can be determined by the two phenolphthalein indicator tests. Non-volatile acidity (phenolphthalein indicator):The concentration of the non-volatile acid in the sample of wine was found to be approximately 0.069 molL-1, as the number of moles of the NaOH solution required to neutralize the acid in the wine solution is equal to the number of moles of acid in the wine.
The results found when testing the total acidity using phenolphthalein and bromophenol blue indicator were: Test Indicator Titre (mL) @ 0.0193 molL-1 NaOH 1 Phenolphthalein 21.0 2 Phenolphthalein 21.0 3 Bromophenol Blue 11.0 Average Titre 17.7 mL of 0.0193 molL-1 NaOH.
The total acidity (phenolphthalein indicator): The concentration of the total acid in the wine, being 0.068 molL-1 was calculated in the same manner as above for the non-volatile acid, as again there is an equivalence between the moles of acid and the moles of base in any titration problem.
As the bromophenol result is an anomaly, the calculation with disregarding this result gives a concentration value of 0.081 mol/L. Therefore, using the values from the non-volatile acidity and total acidity the concentration of the volatile acid could be calculated.
The results obtained for the pH of wine, were gathered by measuring the pH levels each time an increment of NaOH was added. This was then graphed which can be seen below in Graph 1. The independent variable being the amount of NaOH (in mL) added through the method of titration which controlled the dependent variable the pH.
Therefore, using this density we can predict the value of the ethanol concentration in the wine by using the graph. It was found that the percentage of ethanol in wine was 13.6%Discussion Although, the methods involved in calculating the concentration of acid within the wine samples were considerately different in process, the results obtained where evidentially close, and showed little variation. By calculating the non-volatile acidity and then the total acidity, this enabled the volatile acidity level to be calculated. By using the method of reconstitution, this removed the volatile acidity from the solution, leaving the remaining non-volatile acidity. The use of the phenolphthalein indicator gave the indication of the end-point where the non-volatile acidity was found to be approximately 0.069 mol/L.
By using the phenolphthalein indicator again, the titration method was used to find the total acidity of the wine, which was found to be 0.068 mol/L, which was a slight decrease from the non-volatile concentration. An alternate indicator being bromophenol blue was chosen also which gave an indication much earlier than the phenolphthalein indicator did. This is because the bromophenol registers the pH when it reaches between 6.0-7.6 pH range rather than the 8.2-10 that the phenolphthalein indicator does, therefore the earlier indication shown by the bromophenol indicator was expected (Compound Interest, 2014).The titration for the total acidity was 17.7 mL which was lower than that of the non-volatile acidity titration value of 17.8 mL. As this isn’t consistent with the rest of the data trend, this result was chosen to be disregarded as the change in indicator didn’t have the desired effect. With changing the indicator, it then gives an overall concentration for the value of volatile acidity to be negative where a negative concentration cannot occur. Therefore, by removing this result, it gives a total acidity concentration of 0.081 mol/L which is a much more reasonable result. This higher number suggests a greater fermentation process as the longer the process the more concentrated the whine meaning more of the acids were converted to the weaker acids. A slightly larger value was found using the second method of measuring pH and finding the endpoint of titration from the graph. Using titration, the endpoint was expected to be approximately 7.0 pH, which was where 20 mL of NaOH was added. This was used to calculate the total acidity of the wine sample and was 0.004 mol/L greater than that of the acidity found using the phenolphthalein indicator method. It was found that the titration method was more accurate due to the multiple tests conducted in comparison to the pH method when the inconsistent pH indicator was removed, as it was more of a guestimate from the graph at where the end point was. Another contributing factor to this error is the fact that the pH probe has to be calibrated to and stored in distilled water when not in use, which was not done properly throughout this experiment, and is likely the main cause of the discrepancy between the two values obtained for the acidity concentration of the wine sample. The difference between the pH method and titration method was approximately 4.9% which is a reasonable error to have between the two methods.
Therefore, the results achieved can be concluded to be within a reasonable range. This then gives the final volatile acidity to be approximately 0.012 mol/L from the two titration methods. To compare this to a volatile acid which would be within the white wine sample, it was found that up to 400 mg/L of acetic acid could be found in wine. Using this it was calculated that approximately 0.006 mol/L of the wine could possibly be acetic acid. This shows that approximately half of the wines volatile acid could be primarily made up of acetic acid. As this is an unreasonable amount it can be assumed that less than 400 mg/L of this was acetic acid as, but it puts the value attained from calculations in perspective and makes it more reasonable.
Using the density found from the wine, the ethanol content within the wine was predicted. It was found that the wine contained approximately 13.6% within it. This can be assumed reasonable as majority of wines have an ethanol content between 12-15%, where white wines have a larger range being between 5-15%. The reasoning for this value being on the larger end of the scale was probably because when the distillation of the wine occurred the wine boiled slightly over which contaminated the solution. Therefore, there was an impurity within the distilled solution.
Conclusion
Although, the two methods used to calculate the total acidity were two completely different process they gave the total concentration of acid within the wine of a 4.9% range of each other. This can be considered reasonable as this can be assumed to be due to human error as it was pointed out that the calibration of the pH meter might not have been correctly maintained throughout the experiment. It is assumed the more accurate value of the two would be the titration method due to the multiple times the experiment was conducted in comparison to the pH meter method. In conclusion the total acidity concentration of the wine was found to be approximately 0.081 mol/L with an ethanol concentration of approximately 13.6% which is reasonable as white wines ethanol content range is typically between 5-15%.
