Inorganic Chemistry Practical by Dr Deepak Pant (best large ereader TXT) 📕

refer to same analytical method and they have resulted from the use of EDTA (Ethylene diamine tetra acetic acid) and other chilons. These chilons react with metal ions to form a special type of complex known as chelate.
Titration Selectivity, Masking and Demasking Agents

EDTA is a very unselective reagent because it complexes with numerous doubly, triply and quadruply charged cations. When a solution containing two cations which complex with EDTA is titrated without the addition of a complex-forming indicator, and if a titration error of 0.1% is permissible, then the ratio of the stability constants of the EDTA complexes of the two metals M and N must be such that KM/KN ≥ 106 if N is not to interfere with the titration of M. strictly, of course, the constants KM and KN considered in the above expression should be the apparent stability constants of the complexes. If the complex-forming indicators are used, then for a similar titration error KM/KN ≥ 108.

Use of masking and demasking agents:

Masking agents act either by precipitation or by formation of complexes more stable than the interfering ion-EDTA complex.
a) Masking by Precipitation: Many heavy metals e.g.- Co, Cu and Pb, can be separated either in the form of insoluble sulphides using Sodium sulphide, or as insoluble complexes using thioacetamide. These are filtered, decomposed and titrated with disodium EDTA. Other common precipitating agents are sulphate for Pb and Ba, oxalate for Ca and Pb, fluoride for Ca, Mg and Pb, ferrocyanide for Zn and Cu, and 8-hydroxy quinoline for many heavy metals. Thioglycerol (CH2SH.CHOH.CH2OH) is used to mask Cu by precipitation in the assay of lotions containing Cu and Zn.
b) Masking by Complex formation: Masking agents form more stable complexes with the interfering metal ions. The most important aspect is that the masking agent must not form complexes with the metal ion under analysis. The different masking agents used are enlisted below:
Ammonium fluoride will mask aluminium, iron and titanium by complex formation.
Ascorbic acid is a convenient reducing agent for iron(III) which is then masked by complexing as the very stable hexacyanoferrate(II) complex. This latter is more stable and less intensely coloured than the hexacyanoferrate(III) complex.
Dimercaprol (2,3-Dimercaptopropanol); (CH2SH.CHSH.CH2OH). Cations of mercury, cadmium, zinc, arsenic, tin, lead and bismuth react with dimercaprol in weakly acidic solution to form precipitates which are soluble in alkaline solution.

All these complexes are stronger than the corresponding edetate complexes and are almost colourless. Cobalt, copper and nickel form intense yellowish-green complexes with the reagent under the above conditions. Cobalt and copper, but not nickel, are displaced from their edetate complexes by dimercaprol.

Potassium cyanide reacts with silver, copper, mercury, iron, zinc, cadmium, cobalt and nickel ions to form complexes in alkaline solution which are more stable than the corresponding edetate complexes, so that other ions, such as lead, magnesium, manganese and the alkaline earth metals can be determined in their presence. Of the metals in the first group mentioned, zinc and cadmium can be demasked from their cyanide complexes by aldehydes, such as formaldehyde or chloral hydrate (du
selectively titrated.

Potassium iodide is used to mask the mercury(II) ion as (HgI4) and is specific for mercury. It can be used in the assay of mercury(II) chloride

Tiron (disodium catechol-3,5-disulphonate) will mask aluminium and titanium as colourless complexes. Iron forms highly coloured complexes and is best masked as its hexacyanoferrate(II) complex.

Triethanolamine [N (CH2.CH2.OH)3] forms a colourless complex with aluminium, a yellow complex with iron(III), the colour of which is almost discharged by adding sodium hydroxide solution, and a green manganese(III) complex which oxidizes mordant black II. For these reasons, if murexide is used in the presence of iron and manganese.
Demasking
It is the process in which the masked substance regains its ability to enter into a particular reaction. This enables to determine a series of metal ions in one solution containing many cations.
Example of using masking and demasking agents in complexometry is the analysis of 3 metals, Cu, Cd and Ca. the following method of analysis is followed:

1. Direct titration of the mixture with the EDTA gives the sum of the 3 metals.
2. Cu and Cd may be masked with the addition of cyanide to the solution, leaving only Ca ion.
3. When formaldehyde or chloral hydrate is added to the cyanide containing mixture, only Cd is demasked and the EDTA titrates the sum of Ca and Cd. In this manner, the concentration of three ions is determined by 3 individual titrations.


Indicators used in complexometric titrations
Name of the Indicator Colour change pH range Metals detected
Mordant black II Eriochrome blackT Red to Blue 6-7 Ca, Ba, Mg, Zn, Cd, Mn, Pb, Hg
Murexide
or
Ammonium purpurate Violet to Blue 12 Ca, Cu, Co
Catechol-violet Violet to Red 8-10 Mn, Mg, Fe, Co, Pb
Methyl Blue Blue to Yellow 4-5 Pb, Zn, Cd, Hg
Thymol Blue Blue to Grey 10-12 Pb, Zn, Cd, Hg
Alizarin Red to Yellow 4.3 Pb, Zn, Co, Mg, Cu
Sodium Alizarin sulphonate Blue to Red 4 Al, Thorium
Xylenol range Lemon to Yellow 1-3 Bi, Thorium
4-5 Pb, Zn
5-6 Cd, Hg


Type Exercise:
Experiment 1. Masking and demasking for the stepwise complexometric determination of aluminium, lead and zinc from the same solution.
A complexometric method based on selective masking and de-masking has been developed for the rapid determination of aluminium, lead and zinc from the same solution in glass and glass frit samples. The determination is carried out using potassium cyanide to mask zinc, and excess disodium salt of EDTA to mask lead and aluminium. The excess EDTA was titrated with standard Mn(II)SO4 solution using Erichrome Black-T as the indicator. Subsequently selective de-masking agents – triethanolamine, 2,3-dimercaptopropanol and a formaldehyde/acetone mixture – were used to determine quantities of aluminium, lead and zinc in a stepwise and selective manner.
Apparatus
Calibrated pipettes and volumetric flasks supplied by Borosil Glass Works Ltd India were used. The digestion process was carried out on a Laminar flow bench equipped with an appropriate ventilation system.
Reagents
Hydrofluoric acid 40% (m/m), nitric acid (16 N), ammonia solution (NH4OH), triethanolamine 30% (v/v), potassium cyanide solution 20% (m/v), 2,3-dimercaptopropanol, formaldehyde solution, acetic acid, ascorbic acid 98% of AR/GR grade and all other standard chemicals supplied by E. Merck (Germany) were used. De-ionized water (18 mega ohm resistivity) prepared from the Millipore milli-Q water purification system, USA, was used throughout.
Standard solutions
Standard zinc solution, 0.01 M
0.656 g of zinc metal (99.99% purity) was dissolved in hydrochloric acid and diluted to 1 L with distilled water.
Standard EDTA solution, 0.01 M
3.744 g of the disodium salt of EDTA were dissolved in deionized (DI) water and diluted to 1 L. The stock solution was standardised according to the conventional method [6] with standard 0.01 M zinc solution using EBT as the indicator.

Mn(II)SO4 solution 0.01 M and 0.005 M
For the preparation 0.01 M and 0.005 M Mn(II)SO4 solutions, respectively, 1.7 g and 0.85 g of 1-hydrate were dissolved in water and diluted to 1 L with water. The stock solution of 0.01 M Mn(II)SO4 was standardised against standard aluminium according to the conventional method using EBT indicator. Similarly 0.005 M Mn(II)SO4 solution was standardised against standard lead solution according to the conventional method [6] using EBT indicator.


Lead nitrate Solution, 0.01 M
For the 0.01 M stock solution, 3.312 g of lead nitrate were dissolved in water and acidified with a few drops of HNO3, before being diluted to 1 L with DI water.

Standard aluminium solution, 0.025 M
0.6745 g of polished aluminum foil were cleaned with absolute alcohol and then dissolved in 25 mL of hydrochloric acid and 150 mL of DI water, before being further diluted to 500 mL. Erichrome Black-T
This was prepared by dissolving 0.12 g of EBT and 1.2 g hydroxylamine hydrochloride in ethanol.
Preparation of the sample solution
0.5 g of a well ground sample obtained after loss on ignition (100 ± 5°C) was put into a cleaned teflon basin, moistened with a few drops of water, before the addition of 2 mL of HNO3 and 10 mL of 40% HF acid. The Teflon basin containing whole components was evaporated to dryness on a hot plate and the process repeated several times to ensure the total evaporation of silica as SiF4. The residual mass was then dried several times with 10 mL of DI water to remove the acids. Finally the residue was dissolved with 5 mL of HNO3 and diluted to 250 mL with DI water. Subsequently, 25 mL of stock solution were taken in a conical flask and diluted to 100 mL to which was added a known excess amount of 0.01 M EDTA solution and 0.1 g of ascorbic acid, before being boiled on a hot plate for 1 minute. To this solution 25 mL of 20% potassium cyanide solution were added and immediately 10 mL of concentrated ammonium hydroxide were added to prevent the formation of HCN. During work with KCN gloves and masks must be used as a precaution. The total solution was cooled to 10°C in an ice bath, and excess EDTA was titrated against standard 0.01 M manganese solution, with a few drops of a 0.1% alcoholic solution of Erichrome Black T as indicator. At the end point the blue colouration was seen to change to red. The total volume of the EDTA consumed in the reaction corresponded to the sum of all the constituents which complexed with EDTA.
Procedure
Determination of Al
To determine aluminium, 20 mL of triethanolamine and 5 mL of hydroxyl amine 10% (w/v) were added to the solution remaining after the above titration, before being boiled for 1 minute. The liberated EDTA was then titrated with standard 0.01 Mn(II)SO4 solution using Erichrome Black-T as the indicator. At the end point a blue colouration was seen to change to red. The consumed manganese solution was equivalent to the aluminium content of the sample.
Determination of lead
After the titration of aluminium, 5 mL of 20% alcoholic solution of 2,3-dimercaptopropanol were added with slow swirling. The red colouration was seen to change to blue. Again liberated EDTA was titrated with 0.005 M manganese using Erichrome Black-T. At the end point the blue colouration was seen to change to a sharp red wine colour. The consumed manganese solution corresponded to the lead content of the sample.
Determination of zinc
After the titration of lead, zinc was quantitatively de-masked from its cyanide complex by the addition of (3:1) 20 g of 4% formaldehyde:acetone solution. Then the liberated zinc was titrated with a 0.01 M EDTA solution using EBT as the indicator. The end point was indicated by a sharp colour change from red to blue. After titration addition of a formaldehyde:acetone mixture was repeated followed by a second titration, until the solution no longer turned red. The total amount of EDTA consumed corresponded to the zinc content of the sample.

Experiment 2. Determination of Calcium and magnesium ions using Complexometric titration
Complexometric titration is based on the formation of a complex ion. Ethlyenediaminetetraacetic acid (EDTA or H4Y, where Y = C10H12N2O8) is a complexing agent designed to bind metal ions quantitatively, forming stable, water soluble complexes with a 1:1 stoichiometry for most metal ions . EDTA binds to both calcium and magnesium, but binds more tightly to calcium, thus:
Ca2+ + Y4– ----CaY2–
Mg2+ + Y4– -----MgY2–
Ca2+ + MgY2–----CaY2– + Mg2+

PROCEDURE
The EDTA Solution.
You will be using the disodium salt of EDTA (M.W. = 372.24 g/mole). It has been dried for 1 week at 80°C to drive off any superficial moisture. It is in the TA desiccator. Be sure to return it to the desiccator