IODOMETRIC TITRATION

Transcription

1 IODOMETRIC TITRATION

2 Oxidizing agents In most iodometric titrations, when an excess of iodide ion is present, the tri-iodide ion is formed: I + I - I 3 - Since iodine is readily soluble in a solution of iodide. The half cell reaction is better written: Reducing agents I 3- + ē 3I - And the standard reduction potential is: 0 E I / I V

3 Peculiarities of iodometric titration 1. Molecular iodine (I ) is only slightly soluble in water but adding iodide, I -, produces the "triiodide" ion (I 3- ) in solution. Thus, KI is almost always added when redox reactions of I are involved in quantitative analysis.. Iodine or tri-iodine ion is a much weaker oxidizing agent than potassium permanganate, and it can be used both for determination of oxidizing agents as well as for reducing agents. 3. It is recommended to perform titrations in acidic solutions. At рн 8-9 iodine decomposes. I + OH - IO - + H O 4. It is possible to analyze mixtures of oxidizing or reducing agents.

4 Iodometric titration can be of two types 1. The direct iodometric titration method (sometimes termed iodimetry) refers to titrations of reducing agents (E < 0,54) with a standard solution of iodine.. The indirect iodometric titration method (sometimes termed iodometry) deals with titration of iodine liberated in chemical reactions of potassium iodide with oxidizing agents (E > 0,54).

5 DETERMINATION OF OXIDIZING AGENTS Direct titration: Oxidizig agent KI ( exces) acid I... is not applied, because it is impossible to determine the equivalent point

6 Indirect iodometric titration method (sometimes termed iodometry) Iodometric titrations are often used to determine the concentration of oxidizing agents. The oxidizing agent is reacted with an excess of potassium iodide, producing iodine. The iodine that is produced is then titrated against sodium thiosulphate, to find how much iodine was produced by the reaction of the oxidizing agent with potassium iodide (replacement titration). Once the amount of iodine has been found, the amount of the original oxidizing agent can be calculated. Titration scheme Oxidizig agent KI ( exces) acid I Na S O I Na S O NaI n( 1. ) ( O z Ox agent n NaS 3 )

7 The important sources of errors in titrations involving iodine are: 1. Loss of iodine owning its appreciable volatility. Iodine is not particularly soluble in water 3. Reversibility and slow rate of the redox reaction 4. Decomposition of iodine at light

8 Conditions of iodometric titration (to overcome errors) Titration should be performed in acidic solution. Alkali solution should be avoided because iodine can be converted to hipoiodide. Avoid exposure to light, as iodine tends to hasten the decomposition. Before the titration solution should be kept in a glass stoppered vessel in dark cold place. Time is required for the redox reaction completeness (5-10 min) Addition of KI excess. In the presence of iodine the volatility is decreases markedly through the formation of the tri-iodide ion. At room temperature the loss of iodine by volatilization from a solution containing at last 4% of potassium iodide is negligible Low temperature. Titration should be performed in cold solutions in conical flasks not in open beakers.

9 Detection of end point A solution of iodine in aqueous iodide has an intense yellow to brown colour. One drop of 0,05 M iodine solution imparts a perceptible pale yellow colour to 100 ml of water, so that in otherwise colourless solutions iodine can serve as its own indicator. The test is made much more sensitive by the use of a solution of starch as indicator. Starch reacts with iodine in the presence of iodide to form an intensely blue-coloured complex, which is visible at very low concentrations of iodine. In titrations of iodine the starch should not be added until the iodine solution is pale yellow, as the iodine would become strongly adsorbed onto the starch making the titration less accurate.

10 The sensitivity of the colour reaction is such that a blue colour is visible when the iodine concentration is 0,00001 M. The reaction of iodine with starch depends on several factors: 1. The color sensitivity decreases with increasing temperature of the solution, thus at 50 C it is about ten times less sensitive than at 5 C.. The sensitivity decreases upon of solvents, such as ethanol: no colour is obtained in solutions containing 50% ethanol or more. 3. It cannot be used in a strongly acid medium because hydrolysis of the starch occurs.

11 Examples Determination of hydrogen peroxide H O oxidizes iodide to iodine in the presence of acid. The iodine formed is titrated with thiosulfate solution, incorporating a starch indicator. H O KI ( excess) H SO4 I KSO4 H O Na S O I Na S O NaI n( 1 H O ) n( Na O S 3 ) Determination of KMnO 4 KMnO4 10KI ( excess) 8H SO4 MnSO4 6KSO4 I 8H O Na S O I Na S O NaI n( 1 5 KMnO 4 ) n( Na O S 3 )

12 Determination of cooper sulphate An excess of KI is added to a solution of CuSO 4. This reduces Cu + to Cu + in the form of insoluble CuI and, more importantly, produces I in solution. The liberated I is then titrated with a standard sodium thiosulfate (Na S O 3 ) solution, reducing it back to I -. Starch is used as an indicator because it forms an intense blue color with I (actually, I 3- ) The end-point is the disappearance of the blue starch-iodine color. CuSO K SO I 4 4KI ( excess) CuI Na S O I Na S O NaI 4 n( CuSO4 ) n( Na SO3 )

13 Sodium thiosulphate solution with c(na S O 3 ) = 0,0 M and potassium iodide with ω(ki) = 4% are usually employed in iodometric titration.

14 Preparation of a solution of Na S O 3 with approximately the desired concentration 0,0 mol/l Sodium thiosulphate (Na S O 3 H O) is readily obtained in a state of high purity, but there is always some uncertainty as to the exact water content because of the efflorescent nature of the salt and for other reason. The substance is therefore unsuitable as a primary standard. Ordinary distilled water usually contains an excess of carbon dioxide, this may cause a slow decomposition to take place with the formation of sulphur: Na S O 3 + H O + CO NaHCO 3 + NaHSO 3 + S Moreover, decomposition may also be caused by bacteria action (e.g. Thiobacillus thioparus), particularly if the solution has been standing for some time.

15 For these reasons, the following recommendations are made for preparation: 1. Prepare the solution of sodium thiosulphate with recently boiled distilled water.. Add a few drops of chloroform to improve the keeping qualities of the solution. Bacterial activity is least when the ph lies between 9 and 10. The addition of a small amount of sodium carbonate is advantageous to ensure the correct ph. 3. Avoid exposure to light, as this tends to hasten the decomposition. 4. A freshly prepared solution should be allowed to stand for 5-7 days at room temperature.

16 The standardization of thiosulphate solution may be effected with potassium iodate KIO 3 or potassium dichromate K Cr O 7 or potassium bromate KBrO 3 as primary standards, or with potassium permanganate KMnO 4 cerium (IV) sulphate Ce(SO 4 ) as secondary standards.

17 Standardization of sodium thiosulphate solution with potassium dichromate K Cr O 7 Potassium bichromate has a purity of at least 99,9 percent. It is a standard compound and its solution can be prepared by the direct method of preparation. Primary standard K Cr O 7 is pre-treated with an excess of KI in acidic solution which again yields I in solution for subsequent titration with thiosulfate. KCrO 7 6KI ( excess) 7HSO4 Cr ( SO4 ) 3 KSO4 3I 7HO Na S O I Na S O NaI n( 1 6 K CrO7 ) n( Na O S 3 )

18 DETERMINATION OF REDUCING AGENTS Direct titration Scheme Re ducing agent I I... n( 1 Re d) n( 1 I ) z In most direct titrations with iodine (iodimetry) a solution of iodine in potassium iodide is employed as a titrant

19 Examples: Determination of Na S O 3 Na S O I Na S O NaI n( Na ) ( 1 SO3 n I ) Determination of Vitamin C C H O I C H O HI n( 1 ) ( 1 C6H 8O I 6 n )

20 Determination of arsen(iii) HAsO I H O HAsO I H E В 4 / HAsO E 0. В HAsO + NaHCO 3 рн = I I 54 / The reaction between arsenite and iodine is a reversible one and only proceeds quantitatively from left to right if the hydrogen iodide is removed from the solution as fast as it is formed. This may be done by the addition of sodium hydrogencarbonate: sodium carbonate or sodium hydroxide cannot be used, since they react with iodine, forming hypoiodide. Actually it has been shown that complete oxidation of the arsenite occurs when the ph of the solution lies between 4 and 9. HAsO I H O HAsO I H 3 4 n( 1 ) ( 1 Na3 AsO I 3 n )

21 Determination of formaldehyde Back titration is used CH O I( excess) 3NaOH HCOONa NaI H O I( leftover) Na S O I ( leftover) Na S O NaI n( 1 CH O) n( 1 I ) n( Na O S 3)

22 Preparation of a solution of I with approximately the desired concentration 0,0 mol/l It is not difficult to prepare high purity iodine through sublimation, but - due to its volatility - iodine is difficult to weight accurately, as it tends to run away. To minimize losses it should be weight in closed weighing bottle. Iodine should be kept in a closed bottles also because it is highly corrosive and it vapor can damage delicate mechanism of analytical balance. Commonly used solutions are 0.0 M. To minimalize losses it is important to transfer iodine to the solution of KI as fast as possible, or even to weight a 1% excess. Solution should be kept in dark glass bottle with grinded glass stopper and standardized every few weeks or before use.

23 m 1 (beaker + KI) = m (beaker+ KI + I ) = m - m 1 = m(i )

24 The standardization of iodine solution may be effected with arsenic (III) oxide as primary standard, or with sodium thiosulphate as secondary standard. Sodium thiosulphate solution, which has been recently standardized, can be employed. Aliquot part of the standard sodium thiosulphate solution should be transferred to a conical flask with 1 ml of starch. The iodine solution is added from a burette until the solution is a dark blue. c( 1 I ) c( Na S O3 ) V ( Na V ( I ) S O 3 )