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LECTURE NOTES: Reduction- Oxidation Titration
(Expt. 12- 19)
1. Neutralization- acid and base2. Redox- reducing agent, oxidizing agent
gpure NaNO2= [(NxmL) Total KMnO4 - (N x 25mL) H2C2O4] MW
hx1000
g pure NaNO2= [(N x mL) Total KMnO4 -(N x 25mL) H2C2O4] x MW
h x 1000
% NaNO2(w/w) = g pure NaNO2 _______X 100
0.4 to 0.5g = X (aliqout)
100 mL 10 mL
EXPERIMENT 14: LECTURE NOTES
3. Computation: Molarity
mn NaNO2 = mn KMnO4 that reacts with NaNO2 x rr
gNaNO2= [(MxmL) Total KMnO4 - (M x 25mL) H2C2O4 X KMnO4 ] X HNO2 X NaNO2 X MW
H2C2O4 KMnO4 HNO2 1000
gNaNO2= [(M x mL) Total KMnO4 -(M x 25mL) H2C2O4 X 2 KMnO4 ] x 5HNO2 X 2NaNO2 x MW
5 H2C2O4 2KMnO4 2HNO2 1000
% NaNO2(w/w) = g pure NaNO2 X 100
0.4 to 0.5g = X (aliqout)
100 mL 10 mL
\
EXPERIMENT 15:Preparation and Standardization of
Sodium thiosulfate Solution
EXPERIMENT 15:LECTURE NOTES
Experiment 15-18:
Type of Titration: Redox
Method: Iodimetry/ Iodometry1. Iodimetry- is the process wherein a standard solution of iodine is the
titrating agent and it acts as an oxidizing agent.
Pharmaceutical solutions assayed Iodimetrically:
2. Arsenites 6. ascorbic acid
3. Sb+3 compds 7. methenamine
4. Thiosulfates
5. Sulfites
6. Mercurous compds
EXPERIMENT 15:LECTURE NOTES
2. Iodometry- is a process where in the sample of an oxidizing agent is made to liberate an equivalent amount of iodine from KI which is titrated with Na2S2O3 solution.
Pharmaceutical solutions assayed Iodometrically:
1. Iron 6. arsenous
2. Copper 7. chlorine
3. Manganese 8. Bromine
4. Chromium 9. Iodine
5. Cobalt
EXPERIMENT 15:A. Preparation of Starch Solution
1. Triturate 1g of arrowroot starch with 10 mL of distilled water. (starch paste)
2. Boil 200 mL of distilled water
3. Add the starch paste to it with constant stirring
4. Boil the mixture gently until it forms a thin, translucent liquid.
EXPERIMENT 15:LECTURE NOTES
Starch test solution- serves a indicator
Starch grains contain:
1. α- amylose (water insoluble)- with Iodine forms a violet color
2. ß-amylose (water soluble)- with Iodine forms blue color
EXPERIMENT 15:LECTURE NOTES
1. Larger grains- arrow root and potatoes( will give more ß- amylose)
2. Smaller grains- rice and corn ( will give lesser ß- amylose)
EXPERIMENT 15:B. Preparation of Sodium Thiosulfate Solution/
Class
1. Weigh roughly 26 g of sodium thiosulfate and 0.4 g of sodium carbonate (preservative to prevent acid catalyzed hydrolysis)
2. Dissolve this in 1000 mL of recently boiled and cooled distilled water (for bacterial sterilization and to expel CO2)
3. Store in an amber colored bottle and label.
EXPERIMENT 15:C. Standardization of Sodium Thiosulfate
Solution
1. Weight accurately 1.1g of primary standard KIO3 into a 500-mL volumetric flask;
2. Dissolve in about 200 mL of distilled water. Dilute to the mark and mix thoroughly.- (one per class)
3. Pipet 50.0 mL aliquot of standard KIO3 solution into a 250 mL conical flask ( one per paired groups)
4. Introduce 2g of KI (iodate free) and swirl the flask to speed up solution.
EXPERIMENT 15:C. Standardization of Sodium Thiosulfate
Solution
5. Add 2 mL of 6M HCl (do not add if not Na2S2O3 is not yet ready)- catalyze the liberation of I2
6. Immediately titrate with sodium thiosulfate until solution is pale yellow (reduction of the amount of Iodine)
7. Introduce 5 mL of the starch solution
(There will be a formation of big lumps of the blue iodo starch complex if the indicator is added before the titration with Na2S2O3 to the pale yellow endpoint. This will result to difficulty of the blue color to disappear.)
EXPERIMENT 15:C. Standardization of Sodium Thiosulfate
Solution
8. Continue the titration with standard sodium thiosulfate to the disappearance of the blue color.
9. One determination per pair
10. Compute the Mean N and M of Na2S2O3 as a class
EXPERIMENT 15:LECTURE NOTES
Standardization: primary standardization
Type of Titration: Redox
Method of Titration: Iodometry
1. KIO3- primary standard (oxidizing agent)
2. Na2S2O3 – titrant; reducing agent
EXPERIMENT 15:LECTURE NOTES
3. Equations:Molecular:
+5 gained=5eX2= 10e 0
• 2KIO3+ 10 KI + 12HCl 6I2 + 12KCl + 6H2O
-1 lost= 1 x2=2eX 5= 5e 0
reduced to lowest term:
+5 gained = 5e x2= 10e 0
• KIO3 + 5KI + 6HCl → 3I2 + 6KCl + 3H2O
-1 0
lost= 1 x 2=2e X5= 10e
EXPERIMENT 15:LECTURE NOTES
3. Equations:Molecular:
+5 gained=5eX2= 10e 0
• KIO3+ 5 KI + 6HCl 3I2 + 6KCl + 3H2O
-1 lost= 1 x2=2eX 5= 5e 0
0 gained = 1x2=2e X 1= 2e -1
• I2 + 2Na2S2O3 → 2NaI +Na2S4O6
+2 +2.5
lost= 0.5 x 2=1e X2= 2e
EXPERIMENT 15:LECTURE NOTES
3. Equations:Half- ionic equation:
• IO3- → I2
I- → I2
• S2O3-2 → S4O6
-2
I2 → I-
EXPERIMENT 15:LECTURE NOTES
3. Equations:Half- ionic equation:
• IO3- + 5 I-→ 3I2+ 3H2O
• 2S2O3-2 + I2
→ S4O6-2 + 2I-
EXPERIMENT 15:LECTURE NOTES
4. Computation: Normalitymeq Na2S2O3 = meq KIO3
(NxmL) Na2S2O3 = (g /MEW) KIO3
N Na2S2O3 = g KIO3
MW
h X 1000
mL Na2S2O3
Note: g= 1.1g = x
500 mL 50mL h=6
EXPERIMENT 15:LECTURE NOTES
4. Computation: Molarity
mn Na2S2O3 = mn KIO3 x rr
(MxmL) Na2S2O3 = mg KIO3 x 3 I2 x 2Na2S2O3
MW 1 KIO3 1 I2
M Na2S2O3 = mg KIO3 X 3 I2______ x 2Na2S2O3
MW___ 1 KIO3 1 I2
mL Na2S2O3
EXPERIMENT 16:Preparation and Standardization of
Iodine Solution
EXPERIMENT 15:A. Preparation of Iodine Solution/Class
1. Weigh roughly 14.0g of Iodine crystals
2. Dissolve it in a solution of 36 g KI in 400 mL of distilled water. (KI is a solubilizing agent to increase the solubility of Iodine crystals)
3. Add 12 drops of 6N HCl (neutralized any alkali present in KI)
4. Add enough distilled water to complete the volume to 1000 mL
5. Store in an amber colored bottle.
EXPERIMENT 16:B. Standardization of Iodine Solution
1. Run down 30 mL of sodium thiosulfate (Mohr: IR, FR)
2. Dilute with 100 mL distilled water
3. Add 5 mL of starch solution
4. Titrate with standard iodine solution to a blue color endpoint. (Geissler: IR and FR)
1. Weigh 0.4 to 0.5g tartar emetic accurately (tared flask)
2. Dissolve it in 30 mL distilled water
3. Add 25 mL of saturated solution of sodium bicarbonate (neutralize the HI formed thus preventing a reversible reaction and allowing the reaction to proceed to completion)
4. Add 5 mL of starch T.S.
5. Titrate with standard iodine solution to a blue color end point. (Geissler: IR and FR)
6. One determination per pair
7. Compute for the mean % purity of tartar emetic as a class.
4. Add 3 g of KI (Note the color change after adding: reddish brown color indicates plenty of I2 is liberated)
5. Titrate the liberated iodine with sodium thiosulfate until a golden yellow color is obtained. ( Mohr: IR) (indicates the reduction of the amount of I2)
6. Add 3 mL of starch T.S. (There will be a formation of big lumps of the blue iodo starch complex if the indicator is added before the titration with Na2S2O3 to the golden yellow endpoint. This will result to difficulty of the blue color to disappear.)
EXPERIMENT 18:Assay of Cupric Sulfate
7. Continue the titration with std. sodium thiosulfate solution until the disappearance of blue color. (FR)
8. One determination per pair
9. Compute for the % purity of cupric sulfate as a class.
10. Look for USP/NF specs
11. Disposition
EXPERIMENT 18:LECTURE NOTES
Assay of Cupric sulfate
1. CuSO4.5H2O (oxidizing agent)
• blue vitriol
2. KI (reducing agent)
3. Type of Titration: Redox
4. Method of Titration: Iodometry
EXPERIMENT 18:LECTURE NOTES
5. Equations:Molecular
+2 gained= 1e X2 =2e +1
2CuSO4.5H2O +4KI 2CuI + I2 + 2K2SO4 +10 H2O
lost= 1e x2=2e X1= 2e
0 gained= 1ex2=2e x1= 2e -1
I2 + 2Na2S2O3 2NaI + Na2S4O6
+2 lost= 0.5 X2= 1e X2=2e +2.5
EXPERIMENT 18:LECTURE NOTES
5. Equation:Half ionic
Cu +2 Cu +1
I2 I-1
EXPERIMENT 18:LECTURE NOTES
4. Computation: Normality
meq CuSO4O6. 5 H2O = meq Na2S2O3
(g /MEW) CuSO4. 5H2O = (NxmL) Na2S2O3
g pure CuSO4 5 H2O = (NxmL) Na2S2O3 X MW CuSO4 5 H2O
h X1000
% CuSO4. 5H2O= g pure CuSO4. 5H2O x 100
g impure or g sample
EXPERIMENT 18:LECTURE NOTES
4. Computation: Molarity
mn CuSO4. 5H2O = mn Na2S2O3 x rr
g pure CuSO4. 5H2O = (MxmL) Na2S2O3 x 1 I2 __________ X 2 CuSO4. 5H2O
MW 2 Na2S2O3 1 I2
1000
gpure CuSO4. 5H2O = (MxmL)Na2S2O3 x 1 I2 __________ X 2 CuSO4. 5H2O X MW
2 Na2S2O3 1 I2 1000
% CuSO4. 5 H2O (w/w) = g pure CuSO4. 5H2O X 100
g impure or g sample
EXPERIMENT 19:ASSAY OF ASCORBIC ACID
EXPERIMENT 19:A. Preparation of Standard Potassium
Bromate/Class- Group 8
1. Weigh approximately 1.6 g into a 1000 mL volumetric flask.
2. Dissolve the KBrO3 in about 400-mL of distilled water.
3. Dilute to the mark, mix thoroughly.
4. Keep in an amber bottle.
EXPERIMENT 19:Assay of Ascorbic Acid
1. Weigh accurately 3 to 5 vitamins
2. Pulverize them thoroughly in a mortar, and transfer the powder to a dry weighing bottle.
3. Weigh accurately 0.4 to 0.5 g sample into a dry 250 mL conical flask (with cover)
4. Dissolve the sample in 50mL of 1.5 M H2SO4 (freshly
prepared; converts BrO3 to Br2); then add about 5g of KBr. ( will produce excess Br2)
5. Titrate immediately with standard KBrO3 to the first faint yellow due to excess Br2 (Geissler: IR, FR)
EXPERIMENT 19:Assay of Ascorbic Acid
6. Record the volume of KBrO3 used.
7. Add 3g of KI and 5mL of starch indicator; back titrate with standard Na2S2O3 to the disappearance of blue color. (titration should be done without delay to prevent the air oxidation of ascorbic acid)
8. Calculate the ave. mass (in mg) of ascorbic acid tablet.
9. Two determination /pair
10. Compute Mean % purity per pair
11. Look for USP specifications for ascorbic chewable tablets
12. Disposition
EXPERIMENT 19:LECTURE NOTES:
Assay of Ascorbic Acid
1.Type of Titration: Redox
2. Method: Bromination
Applicable for chewable vitamin C (not coated tablets)
The binder in most Vitamin C tablets remains in the suspension through out the analysis. If the binder is starch, the characteristic color of the complex with I2 appears upon the addition of KI
The volume of Na2S2O3 needed for the back titration seldom exceeds a few millimeters.