Methodical Instruction for Students of the 1 Course Medical Faculty
LESSON № 2 (PRACTICAL – 6 HOURS)
Themes: 1. The titration method. The acid-base titration.
2. The chemical equilibrium. The Mohr’s method.
3. The oxidation-reduction titration.
Aim: To be able to prepare of hydrochloric acid solution and to determine concentration of this solution by the acid-base titration; to determine the chlorite ion concentration in investigated liquids; to determine concentration of potassium permanganate solution by oxidation-reduction titration.
Professional orientation of students:
Method of neutralization will be used in laboratory-clinical investigations (for determination of gastric juice acidity), sanitary-hygienic researches (for determination of acidity of milk, bread, et al) and analysis of drugs.
The knowledge of processes of solubility and crystallization of salts is need for understanding essence of exchange of calcium and phosphate ions in an organism. Barium sulfate BaSO4 are used as a contrast agent, at a X-ray analysis of a gastrointestinal tract. Precipitation titration have been used for chlorite ions determination in a urine and gastric juice.
Almost all oxidation-reduction processes in an organism used hydrogen ion. At a biological oxidation, the hydrogen transfer from a molecule of a substratum to oxygen implements in some steps, in which potential change since -0,42V to 0,81V. The imagination about oxidation-reduction processes can measure quantitative characteristic of phosphorylation process. Studied principles action of oxidation-reduction galvanic cells is important for understand oxidation-reduction processes in human body.
Methodology of Practical Class (900-1200).
I. The titration method. The acid-base titration.
Work 1. To make solution with normal concentration 0,1mol-equiv/l
a) To define by an areometer density of primary hydrochloric acid solution.
M a t e r i a l s: HCl solution, areometer
P r o t o c o l. In volumetric flack add some volume of HCl primary solution and put down areometer into the flask. Solution density you sound.
Calculation:
If you will be fined density of primary HCl solution, so you looked in book’s table and find mass fraction (w of %) of solution. After that calculate normality of this solution by the formula:
When you have been fined normality primary HCl solution (N), so you calculate a volume (V), which need for making same volume (V1) solution with 0.1 mol-equa/l (N1).
Where: N – normality of primary HCl solution; V – a volume of primary HCl solution; N1 – normal concentration of HCl solution equal 0.1mol-equ/l; V1 – volume of solution of HCl solution with 0.1mol-equ/l:
b) to make hydrochloric acid solution.
M a t e r i a l s: HCl solution.
P r o t o c o l. In volumetric flack add:
same volume V of hydrochloric acid solution, which you define by calculation;
distilled waters to a label. After that you mix this solution 10-12 times.
Work 2. To determine concentration of hydrochloric acid by NaOH solution
M a t e r i a l s: HCl solution, 0,1 tetraborate solution (Na2B4O7 x10 H2O), 0,05 % solution of indicator methyl orange
P r o t o c o l. Filling burette: make certain the stopcock is closed. Add 5 or 10 mL of the titrant (HCl solution), and carefully rotate the burette to wet the interior completely. Allow the liquid to drain through the tip. Repeat this procedure at least two more rimes. Then fill the burette well above the zero mark. Free the tip of air bubbles by rapidly rotating the stopcock and permitting small quantities of the titrant to pass. Finally, lower the level of the liquid just to or somewhat below the zero mark. Allow for drainage (= 1 min), and then write the initial volume reading, estimating to the nearest 0.01 ml.
In titrated flask add
10 mls of sodium tetraborate solution
2 drops solution of indicator methyl orange.
To titrate this solution by HCl solution while colour of the indicator change from yellow to orange.
Titration. Be sure the tip of the burette is well within the titration vessel (ordinarily а flask). Introduce the titrant in increments of about 1 mL. Swirl (or stir) constantly to ensure thorough mixing. Decrease the size of the increments as the titration progresses; add titrant dropwise in the immediate vicinity of the end point. When it is judged that only а few more drops are needed, rinse the walls of the container. Allow for drainage (at least 30 s) at the completion of the titration. Then record the final volume, again to the nearest 0.01 mL.
The result of titrating are written to the table under such form:
|
V (Na2B4O7) |
N2 (Na2B4O7) |
V1 (HCI) |
|
|
|
|
Calculation: the concentration of a hydrochloric acid solution, which you made defined by the formula:
II. The chemical equilibrium. The Mohr’s method.
Work 3. To determine concentration of sodium chlorate by AgNO3 solution.
M a t e r i a l s: 0.05 mol/l AgNO3 solution, 5% K2Cr2O7 solution.
P r o t o c o l. Filling burette: make certain the stopcock is closed. Add 5 or 10 mL of the titrant (AgNO3 solution), and carefully rotate the burette to wet the interior completely. Allow the liquid to drain through the tip. Repeat this procedure at least two more rimes. Then fill the burette well above the zero mark. Free the tip of air bubbles by rapidly rotating the stopcock and permitting small quantities of the titrant to pass. Finally, lower the level of the liquid just to or somewhat below the zero mark. Allow for drainage (= 1 min), and then write the initial volume reading, estimating to the nearest 0.01 ml.
In flask add: 10 mls of sodium chlorite NaCl solution
2 drops of indicator 5% K2Cr2O7.
To titrate this solution by AgNO3 solution while form а bright red solid of silver chromate.
Calculation: amount of sodium chlorate in solution by the formula:
To calculate of absolute error (Dm) and relative error by the formula:
BE CAREFUL! All titrated solutions must for pouring out in flask with argentums.
III. The oxidation-reduction titration.
Work 4. To determine concentration of solution permanganate by oxalic acid solution.
M a t e r i a l s: 0.05 mol/l oxalic solution, KMnO4 solution, solution of sulfuric acid (1:4)
P r o t o c o l. Filling burette: make certain the stopcock is closed. Add 5 or 10 mL of the titrant (KMnO4 solution), and carefully rotate the burette to wet the interior completely. Allow the liquid to drain through the tip. Repeat this procedure at least two more rimes. Then fill the burette well above the zero mark. Free the tip of air bubbles by rapidly rotating the stopcock and permitting small quantities of the titrant to pass. Finally, lower the level of the liquid just to or somewhat below the zero mark. Allow for drainage (= 1 min), and then write the initial volume reading, estimating to the nearest 0.01 ml.
In flask add: 10 mls of e oxalic acid solution.
10 mls solution of sulfuric acid (1:4)
To heat solution in flask to boiling. After that to titrate this solution by KMnO4 solution, while colour of solution in flask change from white to pink.
Calculation: The concentration of sodium chlorate solution are fined by the formula:
Individual Students Program.
I. The titration method. The acid-base titration.
1. Basis of the titrimetric (volumetric) analysis.
2. Standard solutions, primary standard. Important requirements for а primary standard.
3. Essence of the acid-base titration.
4. Ionization of water. The ion-product constant for water.
5. Indicators of a method of neutralization and mechanism of their colour change (ionic theory Osvalde).
II. The chemical equilibrium. The Mohr’s method.
1. Chemical equilibrium.
2. Solubility-product constants. Solubility of salts.
3. Essence of a precipitation method. An argentometry method: the Mohr’s method.
4. Indication of and point in argentometry method.
5. Using of argentometry at clinical-biological researches.
III. The oxidation-reduction titration.
1. Basis of oxidation-reduction titration. Oxidation-reduction reactions.
2. Maine reductants.
3. Main oxidants.
4. Indicators of an oxidation-reduction titration and mechanism of change of their colour.
Seminar discussion of theoretical issues (1230 – 1400 hour).
Test evaluation and situational tasks.
1. Which method you can determine concentration acid
A. Acid-base titration.
B. Chelatometry.
C. Precipitation.
D. Oxidation – reduction titration.
2. Which method you can determine concentration of NaCl of solution?
A. Neutralization.
B. Chelatometry.
C. Precipitation.
D. Oxidation – reduction.
3. Which method you can determine concentration of potassium permanganate?
A. Acid-base titration;
B. Chelatometry;
C. Precipitaton;
D. Oxidation– reduction titration.
4. To calculate a volume of hydrochloric acid solution with mass fraction 0,04 and density 1,018 g/ml, which necessary for making 250 mls of 0,1 mol/l HCl solution.
5. To calculate mass of Na2CO3 in solution, if is used 22,00 mls 0,1200of solution H2C2O4 for titrating its.
6. To calculate a molar concentration of solution HCl with mass fraction 0,36 (36,2 %) and density 1,18 g / sm3.
7. How much can make 0,1 mole/l H2SO4, if you have 20 mls of H2SO4 solution with mass fraction 0,90 (90 %) and density 1,82 g / sm3.
Correct answers of test evaluations and situational tasks:
1. A.
2. C.
3. D.
4. 24.79 mls
5. 0,1399 g
6. 11,70 mol / l
7. 668,5 mls
Individual student work (1415-1500 hour) are checked by solving situational tasks for each topic, answers in test evaluations and constructive questions (the instructor has tests & situational tasks).
Students must know:
– basic principal of titrimery;
– the main properties of primary standard;
– basic principal of titrimery;
– basic principles of method precipitation;
– chemical equilibrium and constant of chemical equilibrium;
– basic principles of method oxidation-reduction titration;
– oxidation-reduction reaction.
Students should be able to:
– determine concentration of acid solution by a method of neutralization;
– determine chlorate ion concentration by Mohr’s method;
– write oxidation-reduction reaction;
– prepare primary solution in oxidation-reduction titration;
– determine concentration of potassium permanganate solution.
References.
Basic:
1. Atkins P.W. Physical chemistry. – New York. – 2004. – P.299-307.
2. http://intranet.tdmu.edu.ua/Student’s facilities/Practical classes materials/Department of General Chemistry/medical chemistry/medical/1 course/English/02. The titration method. The acid-base titration.The chemical equilibrium. The Mohr’s method. The oxidation-reduction titration
3. Robert R Crichton; Frédéric Lallemand; Ioanna S M Psalti; Roberta J Ward. Biological inorganic chemistry: an introduction. Amsterdam [etc.] : Elsevier, 2010.
Additional:
1. Lawrence D. Didona. Analytical chemistry. – 2002: New York. – P. 97– 118.
2. John B.Russell. General chemistry. New York.2002. – P. 382-386
The methodical instruction has been worked out by: Associate Prof. Dmukhulska Ye. B.
Methodical instruction was discussed and adopted at the Department sitting
25.06.2013. Minutes N 11
Methodical instruction was adopted and reviewed at the Department sitting
27.08.2013. Minutes N 1
