Redox titration.

networksection III redox titration method 1, overview redox titration method is based on the redox reaction titration analysis method. Redox reaction is more complex, the general reaction speed is slower, the side reaction is more, so not all redox reaction can be used for titration reaction, should meet the general requirements of titration analysis, that is, the reaction is complete, the reaction speed is fast; Appropriate conditions for response must therefore be created on a case-by-case basis.(1) determines the degree of reaction based on the size of the equilibrium constant. Generally K≥ 106 or when the reaction is complete.(2) reacts quickly. There are generally several ways to increase the speed of the reaction.(1) plus catalyst. For example, with MnO-4 Oxidizing Fe2 plus, a little Mn2 plus is added as a catalyst to allow the reaction to be carried out quickly.(2) raise the temperature. For example, with MnO-4 oxide C2O2O2-4, the reaction is slow at room temperature and can be carried out quickly when the temperature rises to 80 degrees C.(3) no side effects. If the redox reaction used for titration analysis is accompanied by a side reaction, efforts must be made to eliminate it. If there is no method to suppress the side reaction, the reaction cannot be used for titring.according to the different oxidants used in redox titration, the redox method is divided into potassium permanganate method, iodine quantity method, potassium heavy chromate method and so on. This section mainly discusses potassium permanganate and iodine.II, potassium permanganate(i) Basic principle in microacic, neutral or weak alkaline solution, MnO
-4
reduced For brown insoluble MnO
2
: because MnO2 can cloud the solution, hindering the observation of doubt, so potassium permanganate method is usually carried out in a strong acidic solution. Ticking uses H2SO4 to control acidity and avoid HNO3 (oxidizing) and HCL (reduced). . The reaction to KMnO4 reverting to Mn2 plus is slower at room temperature. Therefore, when titration of substances that are difficult to oxidize, it is
to
or catalysts. For example, when C2O2-4 is titration with KMnO4, the reaction does not begin quickly, even when heated in a strongly acidic solution. Only after the purple fade of the 1-2 drops of KMnO4 solution that was initially added, Mn2 plus was in the solution, and the reaction was carried out more quickly. This catalytic action caused by the reaction product itself is called auto-catalysis.the potassium permanganate method is KMnO4 itself, in 100 ml of water as long as 1 drop of 0.1mol L-1KMnO4 solution can show obvious fuchsia. Therefore, potassium permanganate method does not need to be other indicators.. KMnO4 oxidation is strong, in a strong acidic solution can directly titrate some reductive substances, such as Fe2 plus, AsO3-3, NO-2, Sb3 plus, H2O2, C2O2-4, formaldehyde, glucose and saliotic acid, etc.; Some oxidizing substances, such as MnO2, PbO2, CrO-3, CLO-3, etc., can also be determined with oxidants or reducing agents, but not oxidizing or reductive substances, such as Ca2, Ba2, Zn2 and Cd2. For example, Ca2 plus can form precipitation with C2O2-4 in H2SO4, and then use KmnO4 solution titration to produce H2C2O4, so as to measure the content of Ca2 plus.disadvantages of potassium permanganate method are poor selectivity and insufficient stability of standard solutions.(ii) example of the application of potassium permanganate - the determination of H2O2 content in commercially available hydrogen peroxidein an acidic solution, H
2
O
2
oxidized by KmnO
4
2MnO
- 4

2
O
2


→2Mn
2
The 2
8H
2
o redox titration also has to go through three processes of preparation, calibration and determination of the content of the substance under test: 1. KMnO4 standard solution preparationsaid to take about 0.86g KMnO4 plus distilled water into 500ml 0.01mol L-1 standard solution. Heat to boil and keep slightly boiling about 1h. Cool and, after about a week in the dark, use
glass
or glass
to
. Move the filter to a dark place inside the brown bottle to store.2. Calibration of KMnO4 solution concentrationthe concentration of KMnO4 solution by the reference substance Na2CO3, reactively as follows: 3. Determination of hydrogen peroxide content1.00ml of commercially available hydrogen peroxide with pipelet pipe, placed in 250ml
capacity bottle
, diluted 25.00ml in conical bottle, plus 6ml3mol L-1H2SO4, KMnO 4 standard solution titration solution is pink, the density of H2O2 after dilution is 1, H2O2 content can be calculated according to the press: 3, iodine method(i) basic principles iodine quantity method is the use of I2 oxidation and I-reduction of titration analysis method.. I
2 -2I
--0.535V From the
value, I2 is a weaker oxidant, while I-is a medium-strength reducing agent. Reductive substances below
electric pairs such as S2-, SO2-3, AsO3-3, SbO3-3,
vitamin
C, etc., can be titration directly with I2 standard solution, this method is called direct iodine or iodine titration. Oxidizing substances higher than
pairs such as Cu2 plus, Cr2O2-7, CrO2-4, MnO2-4, NO-2, CL2, H2O2, bleach, etc., can oxidize I-oxidize into I2 and then titrate the resulting I2 with Na2S2O3 standard solution. This titration method is called indirect iodine or titration iodine. 1. Direct iodine measurement methodWhen using direct iodine measurement method to determine reductive substances, it should generally be carried out in a weak alkaline, neutral or weak acidic solution, such as as AsO3-3 in a weak alkaline NaHCO3 solution.reaction is carried out in a strong acidic solution, the balance moves to the left and I-is susceptible to O2 oxidation in the air: 4I
-
-O
2
-4H→2I
2I
2 O If the alkalinity of the solution is too strong, I
2
will have a dissambiguation reaction: the I
2
standard solution can be directly rationed with pure iodine from sublimation. However

I
2 is volatile and corrosive and should not be weighed on
tension
, so it is usually first matched with a solution of approximate concentration, and then calibration. Because iodine is very soluble in water, excess KI is usually added to the preparation of I
2
solution to increase its solubility and reduce the volatility of I
2
. The direct iodine method can use iodine's own yellow or added starch as an indicator, I
2
the starch is blue. 2. Indirect iodinewhen determining oxidizing substances by indirect iodine amount method, it shall be carried out in a neutral or weakly acidic solution. For example, the reaction to
K2
2
O
7
content is as follows: Cr
2
O
7 2-
6I
-
s 14H

→ 2Cr
3
s 3I
2
s 7H
2 O
I
2
2S
2
O
2 2-
→2I
-
s
4
O
2-6
If the solution is alkaline, there is a reaction of I
2
plus S br2. O
2 2-
-10OH
-




.