QCEChemistry

Mandatory practical - Determining the concentration of CH3COOH in white vinegar

Important: Look through practical sheets, and answer all questions prior to exam

Errors in experimental titrations

Systematic errors

A systematic error produces a constant bias in a measurement that cannot be eliminated by repeating the measurement. Systematic errors that affect an acid-base titration could include:

• a faulty balance
• a 20.0 mL pipette that delivers 20.2 mL
• an unsuitable indicator being used.

Whatever the cause, the resulting error is in the same direction for every measurement and the average will be either higher or lower than the true value.

Systematic errors are eliminated or minimised through calibration of apparatus and the careful design of a procedure. If the error cannot be eliminated, an effort should be made to determine its size so that the error can be taken into account in calculations.

Random errors

Random errors follow no regular pattern. The measurement is sometimes too large and sometimes too small. Random errors in volumetric analysis could include:

• inherent uncertainty in the last value after the decimal place in the measurement of the mass of a primary standard on an analytical balance
• difficulty in judging where the meniscus sits on the line when measuring a volume using a pipette
• difficulty in judging the fraction between two 0.1 mL scale markings on a burette.

The effects of random errors can be reduced by taking multiple measurements of the same quantity, then calculating an average. In volumetric analysis, the average of three concordant titres is used to reduce random error.

Rinsing equipment

Step 1: Rising with water

- Rinse both burette and pipette with deionised water

Burette

- Close the burette tap and pour deionised water using a wash bottle into the top of the burette to rinse the inside

- Open the tap over the sink to rinse the bottom of the burette

- Pour the rest of the water out from the top of the burette

Pipette

- Use a wash bottle to add deionised water to the top of the pipette

- Slowly rotate the pipette at the same time to make sure the sides are cleaned

Conical flask

- Rinse with deionised water

- A little bit of leftover water doesn't impact the number of moles of solution in the conical flask, so won’t affect calculations

Step 2: Rising with solution

- Rinse both burette and pipette with solution. If burette is not rinsed after adding deionised water, there may be some deionised water left in the pipette, meaning it would dilute the analyte and underestimate the concentration.

Burette

- Use a funnel to pour some titrant into the top of the burette (ensure tap is closed)
- Open the tap to rinse the tip of the burette and pour the rest of the titrant out of the top of the burette, rotating it to ensure all of the inside surfaces are rinsed

Pipette

- Ensure tip of pipette is clean using paper towel

- Use pipette filler to fill about a third of the pipette, then remove the filler over the sink

- Slowly rotate the pipette as the analyte runs out to clean all of the sides