A combination (pH-reference electrode) was obtained. A screw-like BNC connection of the electrode was inserted into the slot at the rear of the meter and turned (unsteady or sporadic readings could indicate a loose connection).
The electrode was suspended from the meter arm and after which it was kept from drying out by placing it in a beaker of water. When it was ready to make measurements, the air hole under the knurl at the top was insured to be in the open position. The meter was then calibrated. The ATC probe was inserted in order to automatically monitor the temperature. After this the electrode was rinsed with the wash bottle and blotted dry (the electrode was not rubbed as this would cause static buildup and erroneous results).
For the case of the sample, a trial run was carried out first to determine the required mass of unknown weak acid. For the trial run the sample need not be oven dry. About 0.1 g of the unknown sample was put into a 250mL beaker. The mass of sample taken for titration was adjusted, so that 30 to 40 mL of NaOH titrant was used to reach a pH of 10 or higher if no endpoint had been found. The remaining unknown acid was heated in the 600C oven for approximately 1 hour before weighing out the next sample portions.
When ready to titrate, about 100 mL of water was added to the previously weighed sample. The beaker was then placed on the magnetic stirrer and stirred vigorously in order to dissolve. The electrode was immersed in the solution taking care not to hit the fragile glass bulb with the stir bar. Then the burette was placed over the beaker. This was then titrated by adding 0.5 mL increments, recording to 2 decimal places, and reading the pH meter after each addition.
A quick preliminary titration was performed in order to determine the location of the end point(s), carrying the titration past pH = 10 until no significant change in pH occurred with additional NaOH. The endpoint was reflected by a sharp increase in the pH with respect to NaOH addition.
The mass of weak acid titrated was adjusted so that 30-40 mL of titrant was used to reach the expected endpoint. And a second more careful titration was performed with dried sample. The pH and volume of NaOH was added every 0.5 mL up to 1.5 mL of the anticipated end point was noted.
At least sixteen (16) readings were made in the vicinity of each end point, i.e. 8 readings 1 drop apart before and after each endpoint was observed. After which two more accurate titrations were done on dried sample. Other fewer readings were taken in the buffer region of the replicate titrations until the end-point region was reached. And then small additions made and many readings near the end-point(s) were made.
Data and Results
The tables below represent calculated results the lab