Learning Target

By the end of this station, the student should be able to:

1. Perform titration of urine to estimate titratable acidity.
2. Identify the correct endpoint and interpret the result in relation to urinary acid excretion.

Required Material

• Fresh urine sample
• Measuring pipette
• Conical flask
• Burette
• Standard sodium hydroxide solution, usually 0.1 N NaOH
• Phenolphthalein indicator
• Potassium oxalate powder, if provided
• Distilled water
• White tile/background
• Gloves and waste container

Student Task / Procedure

1. Wear gloves and check the labeled urine sample.
2. Mix the urine sample gently.
3. Take the required measured volume of urine in a conical flask.
4. Add potassium oxalate, if provided.
5. Add 2–3 drops of phenolphthalein indicator.
6. Fill the burette with standard NaOH solution.
7. Note the initial burette reading.
8. Titrate the urine with NaOH slowly while shaking the flask.
9. Stop when a faint permanent pink color appears.
10. Note the final burette reading.
11. Calculate the volume of NaOH used.
12. Record the result as titratable acidity of urine.

Observation / Identification Points

The student should demonstrate:

• Proper handling and labeling of urine sample.
• Correct use of pipette, conical flask, and burette.
• Addition of phenolphthalein indicator before titration.
• Slow titration near the endpoint.
• Correct endpoint identification: faint permanent pink color.
• Accurate burette reading at eye level.
• Correct interpretation of titratable acidity.

Result / Interpretation

Principle:
Titratable acidity of urine is estimated by titrating acidic urine with standard sodium hydroxide until the phenolphthalein endpoint is reached.

Endpoint:
Faint permanent pink color.

Main urinary buffers contributing to titratable acidity:

• Phosphate buffer
• Organic acids

Important point:
Titratable acidity does not directly measure ammonium ion excretion.

Simple calculation concept:

Volume of NaOH used = Final burette reading − Initial burette reading

The result is usually expressed as the amount of standard alkali required to neutralize urinary acids.

Clinical significance:
Increased titratable acidity may occur when the body needs to excrete more acid, such as in acidosis, fasting, or high protein intake. Reduced titratable acidity may occur in alkaline urine.

Viva Questions

1. What is titratable acidity of urine?

Ideal Answer:
It is the amount of standard alkali required to titrate urine to the phenolphthalein endpoint.

2. Which indicator is used in this practical?

Ideal Answer:
Phenolphthalein indicator.

3. What is the endpoint of titration?

Ideal Answer:
Appearance of a faint permanent pink color.

4. Which urinary buffer mainly contributes to titratable acidity?

Ideal Answer:
Phosphate buffer is the major contributor.

5. Does titratable acidity include ammonium ion excretion?

Ideal Answer:
No. Ammonium excretion is measured separately.

Marking Scheme

Total Marks: 5

Checklist Breakdown

Common Student Mistakes

• Over-titrating beyond faint pink color.
• Reading the burette above or below eye level.
• Confusing titratable acidity with urinary pH.

AIM Feedback

Remember that urinary pH only tells how acidic the urine is at one moment, while titratable acidity estimates the amount of alkali needed to neutralize urinary acids. In this station, the most important OSPE point is correct endpoint identification: faint permanent pink color with phenolphthalein.