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NaOH Determination of Grape Juice Acidity: An Acid/Base TitrationIntroductionThe addition of a basic solution to an acid solution causes the hydroxide ions from the base to react with the hydrogen ions produced from the acid. Water is formed as a result. Since pH is a measure of the concentration of the hydrogen ions, the pH will increase as more hydroxide ions are added. The point at which the number of moles of hydroxide ions added is equal to the number of moles of hydrogen ions present at the beginning of the lab is called the equivalence point. See the figure on the left.There are typically three types of acid found inside grapes: tartaric, malic, and citric acid. These acids have a large impact on the taste and character of the wine produced from the grapes. Each of these acids are monoprotic (releasing one H+ ion) and thus can be considered to have equivalent acidity (called titratable acidity by vintners). In this experiment you will titrate grape juice from your vineyard trip with standardized NaOH solution to determine the acidity content of the grapes.The reaction that will take place is: H++ OH-H2OUsing the concentration and volume of the NaOH solution needed to neutralize the acid (determined at the equivalence point), will allow you to determine the acid concentration of the grapes. This lab procedure can also be used to titrate fruit juices, wine samples or any other acid containing substance. PurposeTitration is an essential process in chemistry. In this lab, you will use a Chemistry Sensor with a pH electrode and a drop counter to titrate a grape juice solution of unknown acid concentration with a standardized sodium hydroxide solution. You will use your data and the stoichiometry of the reaction to determine moles of titratable acid present in your grapes. You will then extrapolate from this number to determine the probable percentage of titratable acid in a bottle of wine.MaterialsPasco pH electrode and drop counterRing standand clampsMortarand pestleWash bottle with distilled water3beakers (2 x 250, 1 x 50mL)100 mL graduated cylinderPipette(graduated) with bulbProtective gear10 GrapesSodium hydroxide (0.01M, 100 mL)Procedure1.Ensure the equipment at your station is set up as shown in the diagram on the right.2.Check to make sure that the buret or syringe valve is filled with the NaOH standard. Record the exact concentration of the standard in your lab data.
3.Determine the mass of your 10 grapes, and record this value in your data table.4.Crush your grapes one or two at a time in the morter and pestle, ensuring that no juice is lost. 5.Pour all the juice into a clean 50 mL beaker.6.Use a graduated pipette to measure the volume of the juice produced. 7.Transfer the juice to a clean 250 mL titration beaker. Add 100 mL of water to dilutethe juice. Add the stir bar.8.Make sure that the lab laptops are turned on and the program Data Studio is running. 9.Ensure the pH electrode and the drop counter are connected to two USB ports. The equipment has already been calibrated for you.10.Start the magnetic stirer.11.Click the Start button in Data Studio to prepare the drop counter to read drops.12.Turn the bottom valve of the burettesyringe to the full “on” position to releasethe titrant. 13.Watch the pH change. After the equivalence point is reached, allow about 5 mL of titrant to drop until the pH curve flattens. Then click Stop to end the data collection.14.Follow your teacher’s instructions regarding cleaning up your workspace.Sample Data TableDataValueType of Grape UsedMass of 10 grapes (g)Volume of grape juice added for titration (mL)Concentration of NaOH standard (M)Volume of NaOH used to reach equivalence point (mL)Data Analysis1.Determine the number of moles of base needed to reach the equivalence point.2.Use the stoichiometry of the net ionic equation to determine the moles of titratable acid in your grapes.3.In Canada and the U.S., it is assumed that all grape acidity comes from tartaric acid (it makes the calculations easier). Calculate the grams of tartaric acid found in your grapes by multiplying the moles of acid (question 2) x the molar mass of tartaric acid (C4H6O6).4.Divide your result in question 3 by the volume of grape juice used in the titration (in mL), giving a concentration of acid in g/mL. 5.To convert the value in question 4 to a weight per volume percent, multiply the result x 100, giving you the weight per volume percent of acid in your grapes. Most wines have acidities of 0.5 –1.0% percent. Sweeterwines typically have a slightly higher acidity.6.Summarize your findings in a results table.DiscussionDiscuss whether the grapes you picked from the vineyard were ready to produce wine. You will need to take into account the answers to the worksheet fromthe field trip (ex. the sugar content) in addition to the percent acidity results from this lab. You may want to research the acceptable acidity range for the grape variety you were working with.