Standardisation Of Reagents For Titration

Question 1

Main points to remember

Answer 1

• attention to detail (use correct solutions and equipment)
• ensure no bubble in burette and add properly dropwise (very pale pink endpoint)
• accuracy/precision calcs
• read burette volume added (starts at 0, not 50)

Question 2

COSHH Assessment - Sodium Hydroxide

1. Hazard
2. Who might be harmed
3. Measures to control risk

Answer 2

1. Corrosive
2. UG students, Staff, PG demonstrators
3. SOP, training, supervision, mop up small spills with blue roll, wear nitrile gloves, wear lab coat at all times, wear safety glasses at all times

Question 3

COSHH Assessment - Phenolphthalein

1. Hazard
2. Who might be harmed
3. Measures to control risk

Answer 3

1. Carcinogenic, reproductive toxicity, mutagenic
2. UG students, Staff, PG demonstrators
3. SOP, training, supervision, mop up small spills with blue roll, wear nitrile gloves, wear lab coat at all times, wear safety glasses at all times

Question 4

What is accuracy? What to use? Equation?

Answer 4

• How close results are to reference value (within 1%)
• Use Molarity
• [(experimental value-actual value) / actual value] x100%

Question 5

What is precision? What to use? Equation?

Answer 5

• how close results are to each other (within 1%)
• use titre volume
• (difference in titre volume/smallest titre volume) x100%

Question 6

What is a primary standard solution?

Answer 6

solution that is

- stable, 
- easily handled, 
- readily obtained in high purity (99.9%)
- free from CO2 and H2O in atmosphere
- used as a reference (known conc)

(If 1:1 stoichiometry, then 1 mole primary standard = 1000mL M solution)

Question 7

What is a secondary standard solution?

Answer 7

• solution that had its conc measured by titration with primary standard solution

Question 8

What is the purpose of this practical?

Answer 8

• to find the exact concentration of a ~0.1M NaOH solution by titration of the solution against a primary solution
• The endpoint will be defined by formation of a basic solution as assessed by colour change using Phenolphthalein as an indicator

Question 9

What are the key chemicals/ reagents you need to use?

Answer 9

• KHP: acidic primary standard, used to derive accurate NaOH concentration
• Phenolphthalein solution: indicator for titration endpoint
• ~0.1M NaOH: test substance, NOT the 0.5M NaOH used in second practical
• Distilled water: used to dissolve KHP

Question 10

What key glassware/ equipment do you need?

Answer 10

• weighing boats, accurate balance, spatula
• burette, conical flasks
• funnel, beaker or similar for transferring NaOH, distilled water wash bottle or beaker

Question 11

Which operations are critical to getting a good result?

Answer 11

• transferring an accurately weighed amount of KHP to the conical flask: weigh by difference
• set up and reading of burette, careful determination of endpoint
• amount of water/ Phenolphthalein added is not critical as long as KHP is dissolved and endpoint is clear

Question 12

What is being done to reduce error in result?

Answer 12

• weighing KHP by difference
• making sure that the burette is vertical and the funnel is removed, also ensuring there are no bubble or empty regions at the bottom of the burette
• slow addition of NaOH (especially near endpoint)
• repeating of titration to concordance