LAB REPORT

June 13th 2017
EXPERIMENT 2 – ACID-BASE TITRATIONS, CHEM 240
Titrimetric analysis of polyprotic acids

AIM: The aim of this experiment was to study acid-base titrations using the potentiometric end point detection. First we standardized the sodium hydroxide solution necessary for titrations and determined the content of citrate in a sample of Enerlyte powder for rehydration.

PROCEDURE: The experiment is done by following the procedure given for the EXP. 2: Acid-Base Titrations, CHEM 240.

Enerlyte:
Expire: 11/16
Batch number: LoTF03L81
Label claim sodium acid citrate 0.53g
Mass Sachet = 5.1268 ± 0.0001 g

Citric acid mass = 0.60739 ± 0.0001 g
Disodium citrate mass = 2.5850 ± 0.0001 g
c(HCl) = 0.1015 mol/L

Errors:
5mL pipette ± 0.01
10mL pipette ± 0.04

RESULTS:
All titrations were performed using autotitrator Metrohm 785 Titrino system. The change of pH is detected by the Metrohm combined glass electrode.

I Strong Acid (HCl) Titration / Standardisation of NaOH titrant
The solution of NaOH is standardized using the standard solution of HCl.( c(HCl) = 0.1015 mol/L). The aliquot of 5.00 mL of HCl standard solution was transferred using a pipette into a titration vessel, diluted with approximately 65 mL of deionized water and titrated using the NaOH solution of unknown concentration.
Table 1: Titration of HCl with NaOH
# pHI pH=7 fixed mL EP mL EP pH
1 2.25 4.897 4.901 7.36
2 2.23 4.927 4.930 7.27
3 2.17 4.942 4.944 7.07
4 2.20 4.937 4.941 7.24
5 2.18 4.931 4.932 7.06

II Titration of citric acid and disodium hydrogen citrate solutions
Citric acid is accurately weighted (about 0.6 g) and dissolved

Answer the following questions: 1) What variables can an analyst...