Materials Needed and Procedures for Titration for Tartaric & Malic Acids
EQUIPMENT:
pH meter capable of 0.01 accuracy
magnetic stirrer if available but not necessary
Pipette or syringe (the pipette will be easier to control the titrations)
REAGENTS:
0.1 normal sodium hydroxide (NaOH)
6 normal hydrochloric acid (HCL)
8 normal potassium hydroxide (KOH)
(These may be diluted if reactions are too difficult to control with accuracy. I have used 10% solutions.)
BUFFERS:
pH 2 and pH 7
GRAPHS:
graphs for pH 2.70 - 3.00 and pH 4.5 - 4.80
graph for plotting tartaric and malic acid levels from the above graphs
PROCEDURE:
Obtain a juice sample which has not precipitated any tartrates and is at room temperature.
Standardize the pH meter at room temperature to pH 2 and pH 7
Put 50 ml of clear juice into a beaker and, using 6 normal HCL, lower the pH to just below pH 2.70. Using the NaOH raise the pH to exactly 2.70, record the starting point in your pipette or syringe and titrate to exactly pH 3.00. Record the volume of titrant used.
Put 50 ml of clear juice into a beaker and, using the KOH, raise the pH to just below 4.50. Using the NaOH, raise the pH to exactly 4.50, record the starting point and titrate to exactly ph 4.80. Record the volume of titrant used.
Using the volume of titrant used in step 3, locate the number on the vertical axis of the graph pH 2.70 – 3.00 and draw a horizontal line. Locate at least two points where the horizontal line intersects the curves and draw vertical line to the horizontal axis.
Using the volume of titrant used in step 4, locate the number on the vertical axis and draw a horizontal line. Locate at least two points where the horizontal line intersects the curves and draw vertical lines to the horizontal axis.
Using the graph – tartaric and malic acid, transfer the two numbers from the horizontal axis from the chart pH 2.70 – 3.00 to the vertical axes which correspond with the curves from which these numbers were derived: ie – if the lines intersected on the 10g/l and the 2.5g/l, locate those points on the corresponding vertical axes of the graph and draw a line between the point of intersection.
Repeat the above process using the numbers obtained from the ph 4.50 – 4.80 graph.
Where the two lines intersect, draw a horizontal line to the malic axis and a vertical line to the tartaric axis. They will give an estimate of the volumes of malic and tartaric acids in the juice.