abdellatef.weebly.comabdellatef.weebly.com/.../2/8/0/9280204/part_3_colored.docx · Web viewthe oxalate ion combines with the hydrogen ions to form a weak acid, 8-Hydroxyquinoline

الفرسان الفرسان .……… pharmaza ..………… اسرة الفرسان …… pharmaza ..……… اسرة اسرة………….. pharmaza

3. GravimetryGravimetry includes any analytical method in which the ultimate measurement is by weight.

Steps of gravimetric analysis:The individual steps required in a gravimetric analysis can be summarized as follows:

1) Preparation of the solution 5) washing2) precipitation 6) drying or igniting,3) digestion 7) weighing,4) filtration 8) calculation.

1) Preparation of the solution:

Eliminate interfering materials. Proper adjustment of the solution conditions prior to precipitation Factors that must be considered include the volume of the solution

during precipitation, the concentration range of the test substance, the presence and concentrations of other constituents, the temperature, and the pH.

The pH is important because incase of the possibility of interferences from other substances. For example:

calcium oxalate is insoluble in basic medium, while in acid medium the oxalate ion combines with the hydrogen ions to form a weak acid,

8-Hydroxyquinoline (oxine) can be used to precipitate a large number of elements, but by control of pH, elements can be precipitated selectively.

Conditions for analytical precipitation:

1. The precipitate should consist of large crystals so that they can be easily filtered.

2. This contamination should be negligible; it is minimized also by keeping the crystals large.

The precipitation occurs in a series of steps. 1. First, supersaturation occurs, that is, the solution contains more of the dissolved salt than occurs at equilibrium.. This is started by nucleation, number of particles must come together to produce microscopic nuclei of the solid phase. The higher the degree of supersaturation, the greater the rate of nucleation.

30


Following nucleation, the initial nucleus will grow by deposition of other precipitate particles to form a crystal of a certain geometric shape. Again, the greater the supersaturation, the more rapid the crystal growth rate. With increased growth rate, there is increased chance of imperfections in the crystal and trapping of impurities.

Von Wiemarn discovered that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during the precipitation process:

relative supersaturatiou = Q−SS

where Q is the concentration of the mixed reagents before precipitation occurs and is the degree of supersaturation, and S is the solubility of the precipitate at equilibrium. This ratio is also called the von Wiemarn ratio.

high relative supersaturation → many small crystals (high surface area)low relative supersaturation → fewer large crystals (low surface area)Obviously, then, we want to keep Q low and S high during precipitation. Several steps are commonly taken to maintain favorable conditions for precipitation.

1- Precipitate from dilute solution. This keeps Q low.2- Add dilute precipitating reagents slowly, with effective stirring. This also

keeps Q low. 3- Precipitate from hot solution. This increases S. 4- Precipitate at as low a pH as is possible to still maintain quantitative

precipitation.

Most of these operations also often decrease the degree of contamination. The concentration of impurities is kept lower and their solubility is increased, and the slower rate of precipitation decreases their chance of being trapped. The large crystals have a smaller surface area relative to the mass and so have less chance of adsorption of impurities.

2) Digestion of the precipitate:

When a precipitate is allowed to stand in the presence of the mother liquor (the solution from which it was precipitated), the large crystals grow at the expense of the small ones. This process is called digestion, or Ostwald ripening . The small particles tend to dissolve and precipitate on the surfaces of the larger crystals. In addition, individual particles agglomerate to effectively share a common counter-ion layer and the agglomerated particles

31


finally cement together through formation of connecting bridges. This result is an appreciable decrease in surface area.

Also, imperfections of the crystals tend to disappear, and adsorbed or trapped impurities tend to go into solution. It improves the filterability of the precipitate and its purity.Many precipitates do not give a favorable von Wiemara ratio, especially very insoluble ones. Hence, it is impossible to yield a crystalline precipitate (small number of. large particles) and the precipitate is first colloidal (large number of small particles).

Colloidal particles are very small (1-100 m) and have a very large surface which promotes surface adsorption. In AgCl, for example, there will be alternating Ag+ and Cl- ions on the surface. While there are localized + and - charges on the surface, the net surface charge is zero. The adsorption creates a primary layer that is strongly adsorbed and is an integral part of the crystal. It will attract ions of the opposite charge in a counterlayer or secondary layer and give an overall neutral particle. The primary surface charge will tend to repel like particles, creating a colloidal state. An added electrolyte will result in a closer secondary layer and will promote coagulation.

Heating tends to decrease adsorption and the effective charge in the adsorbed layers, thereby aiding coagulation. Stirring will also help.

There are two types of colloids, hydrophilic and hydrophobia Hydrophilic means "water-loving" and these colloids have a strong affinity for water. A solution of a hydrophilic colloid is therefore viscous. A hydrophobic colloid has little attraction for water. A solution of this type of colloid is called a sol. Coagulation of a hydrophobic colloid is fairly easy and results in a crudy precipitate.An example is silver chloride. Coagulation of a hydrophilic colloid, such as hydrous ferric oxide, is more difficult, and it produces a gelatinous precipitate that is difficult to filter because it tends to clog the pores of the filter. In addition, gelatinous precipitates adsorb impurities readily, because of their very large surface area. Sometimes a reprecipitation of the filtered precipitate

32


is required. To reduce concentration of impurities.

Impurities in precipitates: (contamination)Precipitates tend to carry down from the solution other constituents that are normally soluble, causing contamination of the precipitate. This process is called either, coprecipitation or postprecipitation.There are a number of ways in which a foreign material may be coprecipitated.

1. Occlusion. foreign ions are trapped inside the crystal as it is formed. For example, when BaSO4 is precipitated by the slow addition of Ba2+ (as BaCl2) to a solution of SO42-, the sulfate ion will be in excess throughout the precipitation, and the primary adsorbed layer will be sulfate ions. This is neutralized by a secondary layers of counter ions from the solution. For example, Na+ may be the counter ion (two per SO42-). As more barium chloride is added, the loosely bound Na+ is replaced by Ba2+, and the crystal continues to grow. However, if the growth takes place too rapidly, all the sodium ions may not be replaced by barium ions.

Occluded impurities are difficult to remove. Digestion may help some, but this is not completely effective. The impurities cannot be removed by washing. Purification by dissolving and reprecipitating may be helpful.

2. Surface adsorption. the surface of the precipitate will have a primary adsorbed layer of the lattice ions in excess. After the barium sulfate is completely precipitated, the lattice ion in excess will be barium, and this will form the primary layer. The counter ion will be foreign anion, say, a nitrate anion, two for each barium ion. The net effect is an adsorbed layer of barium nitrate. These adsorbed layers can often removed by washing, or they can be replaced by ions that are readily volatilized. Gelatinous precipitates are especially troublesome, though digestion reduces the surface area and the amount of adsorption.

3. Isomorphous replacement. Two compounds are said to be isomorphous if they have:

1. The same type of formula 2. Crystallize in similar geometric forms. 3. Same lattice dimensions.

one ion can replace another in a crystal, resulting in a mixed crystal. For example, in the precipitation of Mg2+ as magnesium ammonium phosphate, K+ has nearly the same ionic size as NH4+ and can replace it to form magnesium potassium phosphate. Isomorphous replacement is very serious

33


and little can be done about it.

4. Postprecipitation. Sometimes, when the precipitate is allowed to stand in contact with the mother liquor, a second substance will slowly form a precipitate with the precipitating reagent. For example, when calcium .oxalate is precipitated in the presence of magnesium ions, magnesium oxalate does not immediately precipitate. But it will come down if the solution is allowed to stand too long before being filtered.

Washing and filtering the precipitates:

Coprecipitated impurities, especially those on the surface, can be removed by washing the precipitate after filtering. The precipitate will be wet with the mother liquor, which is also removed by washing. Many precipitates cannot be washed with pure water, because peptization occurs. This is the reverse of coagulation.

The process of coagulation discussed above is reversible. As mentioned, coagulated particles have a neutral layer of adsorbed primary and counter ions. Washing with water will dilute the counter layer, developing repulsive forces between individual particles. The coagulated particles thereby peptize and form colloidal particles that pass through the filter. Prevention consists in adding an electrolyte to the wash liquid.

The electrolyte must be one that is volatile at the temperature to be used for drying or ignition, and it must not dissolve the precipitate. For example, dilute nitric acid is used as the wash solution for silver chloride. The nitric acid replaces the adsorbed layer of Ag+ /anion-, and it is volatilized when dried at 110°C. Ammonium nitrate is used as the wash electrolyte for hydrous ferric oxide. It is decomposed to NH3, HNO3, N2, and oxides of nitrogen when the precipitate is dried by ignition at a high temperature.

When a precipitate is washed, a test should be made to determine when the washing is complete. This is usually done by testing the filtrate for the presence of an ion of the precipitating reagent. After several washings with small volumes of the wash liquid, a few drops of the filtrate are collected in a test tube for testing. For example, if chloride ion is determined by precipitating with silver nitrate reagent, the filtrate is tested for silver ion by adding sodium chloride.

Drying or ignition of the precipitate:The precipitates may contain:

34


1. adsorbed water(water of humidity) 2. Occluded water (present in cavities within crystals).3. sorbet water, with substances having a large internal surface 4. essential water, as water of crystallisation [e.g. CaC204.H2O

Simply drying can usually be done by heating at 110°-120°C for one to two hours. Ignition at a much higher temperature is usually required For example, magnesium ammonium phosphate (MgNH4PO4) is decomposed to the pyrophosphate (Mg2P2O7) by heating at 900°C. In addition to the evolution of water, the ignition of precipitates often results in thermal decomposition reactions involving the dissociation of salts into acidic and basic components, e.g. decomposition of carbonates and sulphates. The temperatures at which precipitates may be dried, or ignited to the required chemical form, can be determined from a study of the thermogravimetric curves for the individual substances.

Thermogravimetry is a technique in which a change in the weight of a substance is recorded as a function of temperature or time. The results presented as a thermogravimetric (TG) curve, in which the weight change is recorded as a function of temperature or time, A typical thermogravimetric curve, for copper sulphate pentahydrate CuSO3.5H2O, is given in the next Figure.

Homogeneous precipitation: We have seen that the most favorable conditions for precipitation involve precipitating from dilute solution and adding the precipitating agent slowly with effective stirring. These operations maintain a low degree of supersaturation (low Q). local excesses of precipitating reagent are

35


unavoidable when the reagent is added to the sample solution. A process called homogeneous precipitation avoids such difficulties. In this technique, the precipitating reagent is generated by a chemical reaction which occurs uniformly throughout the solution.An example is the generation of hydroxyl ions for the precipitation of hydrous iron oxide, aluminum oxide, and so forth. The precipitating reagent is formed by hydrolysis of urea in an initially acid solution:

(NH2)2CO + 3 H2O → CO2 + 2 NH4+ + 2OH-

This hydrolysis reaction takes place slowly and occurs at temperatures just below the boiling point of water.

The precipitate formed by this technique:1. Have higher densities and much more easily filtered. 2. The degree of coprecipitation is decreased and a purer precipitate is

obtained. Examples of homogeneous precipitation reactions, Sulfate ion can be generated homogeneously by heating a solution containing sulfamic acid. The sulfamic acid hydrolyzes as follows:

HSO3NH2 + H2O → H+ + SO42-

Thus, barium sulfate or lead sulfate may be homogeneously precipitated. In the presence of nitric acid, sulfate ion is also formed homogeneously by the following reaction:

HSO3NH2 + HNO3 →2H+ + SO42- + N2O + H2O

Calculations:Calculations are usually made on a percentage basis: A%=

g AAS

x100=grams of analyte grams of sample taken for analysis

x100%

For example, if we are analyzing for the percentage of chloride in a sample by weighing it as AgCl, we can write:Cl− p⃗recipitating agent AgClWe derive one mole of AgCl from one mole of Cl-, sograms Cl−

grams of AgCl= a .w . ClF .w . AgCl

and g Cl=g AgCl x a .w .ClF .w . AgCl

If we precipitate the chloride as PbCl2,

2 Cl− p⃗recipitating agent PbCl

then grams Cl−

grams of PbCl2=2 a .w .ClF .w . pbCl2

36


and g Cl=g pbCl2 x 2 a .w .ClF .w . pbCl2

In general, the gravimetric factor is the appropriate ratio of the formula weight of the substance sought to that of the substance weighed:

Gravimetric factor = f . w (substance sought )f .w .(substance weight )

In the above example, the gravimetric factors were Cl/AgCl. and 2Cl/PbCl2.

The weight of the substance sought is obtained by multiplying the weight of the precipitate by the gravimetric factor:

We can now write a general formula for calculating the percentage of composition of the substance sought:

% sought= weight of precipitate (g) x gravimetric factor X 100 weight of sample (g)

Example. Orthophosphate (PO43-) is determined by weighting as

ammonium phosphomolybdate, (NH4)3PO4.12MoO3. Calculate the percent of P in. the sample and the percent of P2O5 if the precipitate from a 0.2711-g sample weighed 1.1682g.Solution: % p =

1. 168 x [ p/ (NH4 )3 PO4 . 12MoO 3 ]0 .2711

x100%

% p = 1. 168 x30 .97/1876 .50 .2711

x100%= 7 . 111%

% p2O5= 1 . 168 x [ p2O5 /(NH4 )3 PO4 .12MoO3 ]

0 .2711x100%

% p2O5= 1 . 168 x [141 .95/2x1876 .5 ]0 . 2711

x100%= 16 .30%

Organic precipitates:

Organic precipitating agents have the advantages of giving precipitates with:1. very low solubility in water 2. Favorable gravimetric factor. (more sensitive and precise)

Most of them are chelating agents that form slightly soluble uncharged chelates with the metal ions.

Mn+ + nHX → MXn + nH+

For increasing the selectivity the pH must adjusted

37


Dimethylglyoxime with Nickel, lead , platinum

cupferron with Iron, stannous ….

8-hydroxyquinoline (oxine)

38

الفرسان الفرسان .……… pharmaza ..………… اسرة الفرسان …… pharmaza ..……… اسرة pharmaza ..………… اسرة

1. chloride as AgCl 2. sulphate as BaSO4: 3. iron as iron(III) oxide

Precipitation by precipitation of Cl- as its silver salt after acidification with nitric acid.

Ag+ + Cl- →AgCl

based upon precipitation with Ba++ ion:Ba++ + SO4

2- →BaSO4

based upon the precipitation of ferric iron as a hydrous oxide,

2Fe+++ + 6NH3 + (x+3)H2O → Fe2O3.xH2O + 6NH4

+

ammonium hydroxide is used as precipitant. The precipitate (amorphous) forms colloidal solution. To avoid this, the precipitation must be done in hot solution in presence of a coagulating electrolyte.

Washing The precipitate washed with very dilute nitric acid, in order to prevent peptisation,

washed with water The most satisfactory wash liquid is ammonium nitrate; this voltatilizes upon ignition. Ammonium chloride is unsuitable, because iron(III) chloride which is volatile, is formed during the ignition.Fe2O3 + 6NH4Cl = 2FeCl3 + 6 NH3 + H2O

Drying and igniation

Dried at 130-150°C, and then finally weighed as AgCl.

ignited at temperature less than 600oC and weighed as BaSO4

by ignition Fe2O3.xH2O to Fe2O3.Fe2O3.xH2O → Fe2O3 + xH2O

precautions i) Precipitation is carried out in nitric acid solution in order to prevent the precipitation of other silver salts, such as phosphates and carbonates.

ii) excess of Ag+ ion is required to decrease the solubility of the precipitate.

iii) The precipitate of AgCl must be protected from light; to prevent photochemical decomposition of the precipitate occurs.

AgCl AgO (Violet color) + ½ Cl2 (

iv) I-, Br-, and SCN-, interfere.

BaSO4 ia a very small crystals, which sometimes pass through the filter pores pores. To overcome this problem:

a) The precipitant should be added slowly.b) Addition of substance that increases the

solubility of the precipitate (decreased the degree of supersaturation ). Hydrochloric acid can play this role by the formation of HSO42-.

c) The precipitation at boiling temperature.

Pure BaSO4 is not decomposed when heated in dry air until a temperature of about 1400°C is reached:

BaSO4 →BaO + SO3The precipitate is, however, easily reduced to sulphide at the temperature above 600°C by the carbon of the filter paper.

i) Cations as Al+++, Cr+++, Mn3+ , anions arsenate, phosphate, etc. must be also absent.ii) Organic hydroxy compounds e.g. citric acid, tartaric acid, sugar, etc. prevent the precipitation of iron by forming complex salts.iii) Cupferron can be used as a specific one for precipitation of ion. The precipitated complex has also to be ignited to ferric oxide (the common weighying form of iron). Iron can also be precipitated as oxinate which is filtered, washed, dried and weighed.

39

الفرسان الفرسان .……… pharmaza ..………… اسرة الفرسان …… pharmaza ..……… اسرة pharmaza ..………… اسرة

BaSO4 + 4C → BaS + 4 CO

4. aluminium as aluminium oxide:

5. magnesium:

Precipitation The same way as Fe+++, by precipitation with ammonia, and subsequent conversion of the A1(OH)3 precipitate into Al2O3 by ignition.

diammonium hydrogenphosphate, and excess of ammonia solution is added to precipitate ammonium magnesium phosphate hexahydrate, Mg(NH4)PO4.6H2O at room temperature.Mg2++ HPO4

2- + NH4+ + OH- = Mg(NH4)PO4 + H2O

The precipitate should stand for several hours before filtration (to decrease its solubility).

Washing The precipitate is washed with either ammonium chloride or ammonium nitrate solution; to prevent peptisation.

The precipitate is washed with 0.8M aqueous ammonia solution and then weighed either as the hexahydrate MgNH4PO4.6H2O or as the pyrophosphate Mg2P2O7.

For the former, the precipitate is washed with ethanol, followed by anhydrous diethyl-ether, and weighed after standing in a desiccators for about 20 minutes at room temperature. This method is of moderate accuracy and is recommended only because of the time saving achieved.

Drying and igniation

Al2O3, obtained after ignition is hygroscopic unless the temperature of ignition is raised to at least 1200°C at which a non hygroscopic form of Al2O3 is formed. A13+ may also be precipitated with 8-hydroquinoline at pH 4-10.

A13+ + 3H(C9H6NO) → A1(C9H6NO)3 + 3H+

The precipitate is ignited at a high temperature and weighed as such.

2MgNH4PO4.6H20 = Mg2P2O7 + 2NH3 + 13H2O

For complete precipitation of Al+++ the pH of the solution must be carefully controlled. To about 6.5-7.5. The addition of ammonium chloride exerts a buffering effect and also assists the coagulation of the colloidal precipitate.

40


6. Mixture containing aluminium and iron:The iron is oxidised to the ferric state, then sodium hydroxide is added in excess to

precipitate iron as ferric hydroxide and convert aluminium into soluble aluminate A1O2-. The precipitate is filtered, washed, dissolved in nitric acid and reprecipitated to obtain a

purer precipitate of ferric hydroxide and determined as before. The filtrate is acidified, and the resulting aluminium salt is determined as explained before.

7 Determination of Calcium:

The calcium is precipitated as CaC2O4.H2O by treating hot acidified solution with ammonium oxalate, and slowly neutralizing with aqueous ammonia solution.

Ca2+ + C2O42- + H2O → CaC2O4.H2O

The precipitate is washed with dilute ammonium oxalate solution and then weighed in one of the following forms:

1. As CaC2O4.H2O by drying at 100-105°C for 1-2 hours.

This method is not recommended because of the hygroscopic nature of the oxalate and the difficulty of removing the co-precipitated ammonium oxalate at this low temperature.

2. As CaCO3 by heating at about 500°C

since the CaCO3 is not hygroscopic, this is considered as the most satisfactory method:Ca2C2O4 → CaCO3 + CO

3. As CaO by ignition at 1200°C.

This method is widely used, but the resulting CaO has a comparatively small molecular weight and is hygroscopic.CaCO3 → CaO + CO2

Pyrolysis curve (relation between mass and temperature)

To obtain course crystal ammonium oxalate , followed by ammonium hydroxide to a hot acid solution of the calcium salt drop by drop

Removal of the Cl- ions by washing prevents losses due to the formation of volatile CaCl2 during ignition of the precipitate.

All metals which form slightly soluble oxalate must be absent e.g. Cu2+, Pb2+ and Zn2+.

41

Documents

abdellatef.weebly.comabdellatef.weebly.com/.../2/8/0/9280204/part_3_colored.docx · Web viewthe oxalate ion combines with the hydrogen ions to form a weak acid, 8-Hydroxyquinoline