Titration of acids of gastric contents.

Laboratory work 12. Digestion of proteins. Determination of acids of gastric contents

Normal gastric juice has a strongly acidic reaction because of the presence of hydrochloric acid. Gastric contents extracted by the gastric tube after a certain time after the test breakfast, is used for analysis. It normally has pH 1.5-2.5, and the concentration of HC1 is 0.1-0.2%. The following types of acidity are distinguished in a clinical analysis of gastric juice:

1) "free hydrochloric acid", which is in the free state and dissociates into ions;

2) "bound hydrochloric acid" in the form of salt-like compounds with proteins and products of their digestion;

3) "total hydrochloric acid" which is a combination of free and bound hydrochloric acids;

4) "total acidity", which is the sum of all acidic substances (bound and free hydrochloric acid, hydrophosphates; and in pathologies - lactic acid and volatile fatty acids).

Qualitative determination of free hydrochloric acid in gastric juice using indicator congo red.

Congo red has blue color in the acidic medium, and red color in slightly acidic, neutral and alkaline media. The transition pH range of congo red is between pH 3.0 and 5.2. Such pH value in gastric juice is a result of only free hydrochloric acid presence.

Pour 1 ml of hydrochloric acid in the test tube and add 2 drops of indicator congo red. The same experiment is done with a gastric juice samples (samples 1 and 2). Register the change of indicator color, and make a conclusion about the presence or absence of hydrochloric acid in the studied juices.

Observations:

Qualitation of free hydrochloric acid in gastric juice with tropeolin solution.

The transition pH range of tropeolin indicator is between pH 1.3 and 3.0. When pH is above 3.0 the color of indicator is orange-yellow. Only in the presence of mineral acids (when pH is 1.3 and below), color is crimson-red.

Pour 1 ml gastric juice 1 into the first test tube, 1 ml of gastric juice 2 – into the second test tube, and and 2) and 1 ml of hydrochloric acid into the third one. Add 2 drops of tropeolin in all test tubes. Compare the color of the indicator in all the samples.

Titration of acids of gastric contents.

Acids of gastric contents are titrated with 0.,1 n. sodium hydroxide solution. A variety of indicators with different transition pH range: phenolphthalein – 8.2-10.0; congo-red – 3.0-5.2; alizarin red – 4.3-6.3 is used to determine equivalent point for hydrochloric acid and other acidic compounds.

Pour 5 ml of gastric contents (sample 1) into a conical flask. Add 10 drops of phenolphthalein solution, 2 drops of congo red solution and titrate with 0.1 n. sodium hydroxide solution till the transition of the original blue color to red. The amount of alkali, used for the titration, corresponds to the amount of free hydrochloric acid and identifies by indicator congo red. Not adding alkali into the burette until zero, continue titrating until transition of the color in the crimson. The total amount of alkali corresponds to the total acidity and is detected by phenolphthalein indicator.

Measure 5 ml of gastric contents in another flask, add 2 drops of alizarin red solution and titrate to the transition of the original yellow color to the purple one. The amount of alkali used for titration, corresponds to the sum of all the substances that give an acid reaction, except bound hydrochloric acid, and is detected by alizarin red indicator. Results are entered in the table.

Results of titration are expressed in ml of 0,1 n. sodium hydroxide solution, used on neutralizing of free hydrochloric acid and other acidic compounds in 100 ml of gastric contents (conventional units). One conventional unit corresponds to the concentration of hydrochloric acid of 1 mmol/l.

Compare the received data with the norm. Make a conclusion about the gastric acid secretion (hyper-, norm-, hypo- or achlorhydria).

Table. Results of the determination of gastric acidity

Volume of gastric juice, ml	Volume of 0.1 n. sodium hydroxide solution consumed in titration, ml	Number of conventional units, mmol/l
to red	to crimson	to purple	free hydro-chloric acid	total acidity	bound hydro-chloric acid

5. Qualitative reaction to lactic acid (Ufelmann’s test).

The reaction is based on the interaction of lactic acid with iron phenolate, which is colored in purple. Iron lactate of a greenish-yellow color is formed in this reaction. Iron phenolate is formed in the reaction of ferric chloride (III) with phenol:

Pour 3 ml of 2% phenol solution in the test tube; add 2-3 drops of ferric chloride (III) 1% solution. Dark-violet color is observed. Diluted the reagent with water to weak staining and pour into three test tubes. Add 1% solution of lactic acid in the first tube dropwise until a greenish-yellow color. Add gastric juice dropwise in the second and third tubes (samples 3 and 4). Greenish-yellow color appears only if hydrochloric acid is absent, or present in a very small quantity in the gastric juice. Strong hydrochloric acid completely destroys the iron complex with phenol, as well as displaces the weaker lactic acid from its salt.

Make a conclusion about presence of the studied acids in gastric juice.

Дата добавления: 2015-10-26