42. Titration of gastric juice

Question 1

What does human gastric juice contain?

Answer 1

hydrochloric acid and other compounds consuming NaOH (lactic acid, proteins etc).

Question 2

If gastric juice is titrated by NaOH, what is the pH that HCl is completely neutralized?

Answer 2

HCl is practically neutralized completely at pH 4

Question 3

If gastric juice is titrated by NaOH, what is the pH that other acids (except HCl) are neutralized?

Answer 3

the other acids (lactic acids, protein) are not titrated

→ their complete neutralization can be observed at pH 8 or above

Question 4

What are the pH indicators used for gastric juice titration?

Answer 4

methyl red and phenolphtalein

Question 5

What is methyl red?

Answer 5

pH indicate that indicates the neutralization of free HCl

Question 6

What does phenolphtalein indicate?

Answer 6

the neutralization of weak acids

Question 7

The acid content of gastric juice is characterized clinically by two data
What are they?

Answer 7

• total acidity is the number of used milliliters of 0.1 N NaOH
  
  • for the neutralization of 100 ml gastric juice using phenolphtalein indicator
• free HCl content is the number of used mililiters of 0.1 N NaOH
  
  • for the neutralization of the HCl content of 100 ml gastric juice using methyl red indicator

Question 8

How can we model gastric juice in lab?

Answer 8

with a solution containing HCl and CH3COOH

Question 9

How can we titrate gastric juice in lab? What is the principle behind this? (a solution containing HCl and CH3COOH)

Answer 9

• We titrate the strong acid first and then the weak acid afterward.
• H+ ions from the complete dissociation of the strong acid suppress the deprotonation of the weak acid until it is present in the solution
  
  • since the dissociation equilibrium of the weak acid is shifted to the left according to the Le Chatelier principle.

Question 10

After titration of model gastric juice with the strong acid (HCl) the equivalence point of the solution will be in __ range

Answer 10

alkaline

Question 11

Materials needed for titration of gastric juice

(2 solutions + 2 indicators)

Answer 11

• 0.1 N NaOH
• HCl-CH3COOH solution mimicking filtered gastric juice
• methyl red and phenolphtalein solution

Question 12

Describe the produce of gastric juice titration

Answer 12

• 20 ml gastric juice has to be measured into titrating flasks and 3 drops methyl red are added.
• During vigourous shaking the solution is titrated by 0.1 N NaOH until it becomes reddish-yellow.
  
  • Then read the consumption; this is the free HCl content.
• Add 3 drops of pheolphtalein and continue the titration until the occurence of the pink color of pheolphtalein.

Question 13

Difference between methyl red and methyl orange

Answer 13

1. Methyl orange has pH range of 3.1 - 4.4; Methyl red has pH range 4.4-6.2
2. Methyl red has a carboxylic group; Methyl orange has a sulfonic acid group