Unit IV Gravimetric Analysis Gravimetric Analysis · Gravimetric analysis is the process of...

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Unit IV Gravimetric Analysis . 1 Gravimetric Analysis Gravimetric methods – are quantitative methods in which the mass of the analyte or some compound that is chemically related to the analyte is determined. PRINCIPLE : Gravimetric analysis is the process of isolating and weighing an element or a definite compound of the element in as pure a form as possible. The element or compound of the element is separated from a weighed portion of the substance being examined. A large portion of the determinations in gravimetric analysis is concerned with the transformation of the element or radical to be determined into a pure stable compound which can be readily converted into a form suitable for weighing. The weight of the element or radical may then be readily calculated from a knowledge of the formula of the compound and the relative atomic masses of the constituent elements. The separation of the element or of the compound containing it may be effected in number of ways, the most important of which are 1. Precipitation Methods 2. Volatilisation or evolution methods 3. Electro analytical methods - Electro Gravimetry 1. Precipitation Methods The most important is the precipitation method in gravimetric analysis, which is explained in detail below. A weighed sample of the substance to be analysed is brought into solution by a suitable method, and the element to be determined is then precipitated as an insoluble compound. The precipitate is filtered off, washed throughly, ignited (or dried) and weighed accurately. The content of the element is calculated from the weight of the precipitate and its formula and expressed as a percentage of the sample taken. The precipitate may be collected on a filter paper or in a Gooch crucible. 2. Volatilisation or evolution methods In addition to precipitation, the other methods are also used. This Volatilisation or Evolution method depends essentially upon the removal of volatile constituents. This may be effected in several ways. (i) By simple ignition in air or in a current of an indifferent gas. (ii) By treatment with some chemical reagent whereby the desired constituent is rendered volatile and (iii) By treatment with a chemical reagent whereby the desired constituent is rendered non-volatile. The volatilised substance may be absorbed in a weighed quantity of a suitable medium when the estimation is a direct one, or the weight of the residue remaining after volatilisation of a component is determined, and the proportion of the constituent calculated from the loss in weight. 3. Electro analytical methods - Electro Gravimetry In electro-gravimetric analysis, the element to be determined is deposited electrolytically upon a suitable electrode. Filtration is not required, and provided the experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided. In each of these analytical methods, the amount of a given component is found from the results of weighing . These methods can therefore be regarded as different varieties of gravimetric analysis.

Transcript of Unit IV Gravimetric Analysis Gravimetric Analysis · Gravimetric analysis is the process of...

Unit IV Gravimetric Analysis � .1

Gravimetric Analysis Gravimetric methods – are quantitative methods in which the mass of the analyte or some compound that is chemically related to the analyte is determined.

PRINCIPLE :

Gravimetric analysis is the process of isolating and weighing an element or a definite compound of the element in as pure a form as possible. The element or compound of the element is separated from a weighed portion of the substance being examined. A large portion of the determinations in gravimetric analysis is concerned with the transformation of the element or radical to be determined into a pure stable compound which can be readily converted into a form suitable for weighing. The weight of the element or radical may then be readily calculated from a knowledge of the formula of the compound and the relative atomic masses of the constituent elements.

The separation of the element or of the compound containing it may be effected in number of ways, the most important of which are

1. Precipitation Methods 2. Volatilisation or evolution methods 3. Electro analytical methods - Electro Gravimetry

1. Precipitation Methods

The most important is the precipitation method in gravimetric analysis, which is explained in detail below.

A weighed sample of the substance to be analysed is brought into solution by a suitable method, and the element to be determined is then precipitated as an insoluble compound. The precipitate is filtered off, washed throughly, ignited (or dried) and weighed accurately. The content of the element is calculated from the weight of the precipitate and its formula and expressed as a percentage of the sample taken. The precipitate may be collected on a filter paper or in a Gooch crucible.

2. Volatilisation or evolution methods

In addition to precipitation, the other methods are also used. This Volatilisation or Evolution method depends essentially upon the removal of volatile constituents. This may be effected in several ways. (i) By simple ignition in air or in a current of an indifferent gas. (ii) By treatment with some chemical reagent whereby the desired constituent is rendered volatile and (iii) By treatment with a chemical reagent whereby the desired constituent is rendered non-volatile.

The volatilised substance may be absorbed in a weighed quantity of a suitable medium when the estimation is a direct one, or the weight of the residue remaining after volatilisation of a component is determined, and the proportion of the constituent calculated from the loss in weight.

3. Electro analytical methods - Electro Gravimetry

In electro-gravimetric analysis, the element to be determined is deposited electrolytically upon a suitable electrode. Filtration is not required, and provided the experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided.

In each of these analytical methods, the amount of a given component is found from the results of weighing . These methods can therefore be regarded as different varieties of gravimetric analysis.

Unit IV Gravimetric Analysis � .2

Steps Involved in Gravimetric Analysis

1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing of the Precipitate 6. Drying or igniting 7. Weighing 8. Calculation

1. PREPARATION OF THE SOLUTION

This may involve several steps including adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties, removing interferences, adjusting the volume of the sample to suit the amount of precipitating agent to be added.

2. PRECIPITATION or NUCLEATION

Gravimetric precipitation are usually made in beakers. Except during the actual precipitation, the beaker is covered with the clock-glass. A thin stirring rod, rounded at each end, is also required.

NUCLEATION

In practice, supersaturation influences one of the mechanisms of precipitation, namely nucleation and crystal growth. Nucleation is the first stage of precipitation and consists in the formation of stable microcrystals that can grow spontaneously, i.e. crystallization nuclei. Further precipitation leads to competition between new and existing nuclei (particle growth). If nucleation predominates, then a crystal will form that is easy to filter. Low supersaturation encourages the growth of existing microcrystals rather than the formation of new nuclei. Higher temperatures also favour the formation of crystalline precipitates, because they increase the solubility of the precipitate (S), and other factors that encourage crystal growth are diluting the solution (Q), and adding the precipitation agent slowly while shaking energetically.

PRECIPITATION

The solution of the precipitating reagents is usually added slowly from a teat pipette or burette. with efficient stirring, to a suitably diluted solution of the sample. Efficient stirring. is necessary to avoid the possible high local concentrations of precipitating reagent which would tend to give contamination of precipitate due to co-precipitation. The reagent is introduced down the side of the beaker to avoid splashing. Precipitation is usually made from hot dilute solutions, a procedure which tends to give an easily

Unit IV Gravimetric Analysis � .3filterable precipitate and reduces the possibility of co-precipitation. The formation of coarse particles is favoured by slow addition of the precipitant. vigorous stirring of the solution during precipitation and by carrying out the precipitation from hot solutions. Only a moderate excess of precipitating reagent is required. When the precipitate has settled somewhat. a few more drops of precipitant should be added to test for the completeness of precipitation; if further precipitation occurs. then the process of addition of precipitant, stirring and allowing the precipitation settle and testing again must be carried out until there is no doubt about completeness of precipitation. The stirring rod should not come into contact with the sides or bottom of the beaker during stirring in order to avoid scratching particles of glass from the surfaces. Also, a scratched surface is difficult to wash clean. Reagents should be examined for the clarity before use and filtered if necessary.

Before filtration, precipitates may need to be digested by allowing them to stand overnight, or by heating the precipitate and its supernatant liquid nearly to boiling point for some time. Digestion in this manner increases the degree of coarseness of the precipitate: A further check for completeness of precipitation should be carried out when the supernatant liquid becomes clear during the period of digestion. No further precipitate should be produced when a few drops of the must be added as described above.

If the precipitate is much more soluble in hot water than in cold, the solution is allowed to cool to room temperature, before filtration. Otherwise, solutions are filtered hot to speed up the filtration.

(i) CO-PRECIPITATION

When a precipitate separates from a solution, it is not always perfectly pure: it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation.

Hence the term co-precipitation is a phenomenon in which the soluble compounds are removed from solution during precipitate formation. It is important to understand that the solution is not saturated with the co-precipitated species. Moreover, contamination of a precipitate by a second substance whose solubility product has been exceed does not constitute co-precipitation.

Types of Co-precipitation There are four types of co-precipitation: (i) Surface adsorption (ii) Mixed crystal formation (iii) Occlusion (iv) Mechanical entrapment.

Surface adsorption and mixed crystal formation are equilibrium processes., where as occlusion and mechanical entrapment arise from the kinetics of crystal growth.

(i) Surface adsorption: Adsorption is a common source of co-precipitation that is likely to cause significant contamination of precipitates with large specific surface areas i.e., coagulated colloids.

Note the extensive internal surface area exposed to solvent

Although adsorption does occur in crystalline solids, its effect on purity are usually undetectable because of the relatively small,

Unit IV Gravimetric Analysis � .4specific surface area of these solids. Coagulation of a colloid does not significantly decrease the amount of adsorption because the coagulated solid still contains large internal surface areas that remain exposed to the solvent which can be seen in the figure.

The co-precipitated contaminant on the coagulated colloid consists of the lattice ion originally adsorbed on the surface before coagulation and the counter ion of opposite charge held in the film of solution immediately adjacent to the particle. The net effect of surface adsorption is therefore the carrying down of an otherwise soluble compound as a surface contaminant.

For example

The coagulated silver chloride formed in the gravimetric determination of chloride ion is contaminated with primarily adsorbed silver ions along with nitrate or other anions in the counter-ion layer. As a consequence, silver nitrate, a normally soluble compound, is co-precipitated with the silver chloride.

(ii) Mixed - Crystal formation

Mixed-crystal formation is a type of co-precipitation in which a contaminant ion replaces an ion in the lattice of crystal.

In mixed-crystal formation, one of the ions in the crystal lattice of a solid is replaced by an anion of another element. For this exchange to occur, it is necessary that the two ions have the same charge and that their sizes differ by no more than about 5% furthermore, the two salts must belong to the same crystal class.

EXAMPLE

Barium sulphate formed by adding barium chloride to a solution containing sulphate, Lead and acetate ions is found to be severely contaminated by Lead sulphate even though acetate ions normally prevent precipitation of Lead sulphate by complexing the lead. Here. Lead ions replace some of the barium ions in the barium sulphate crystals other examples of co-precipitation by mixed-crystal formation include, MgKPO4 in MgNH4PO4 and MnS in Cds.

When mixed-crystal formation occurs, separation of the interfering ion may have to be carried out before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used.

(iii) Occlusion and Mechanical entrapment

When a crystal is growing rapidly during precipitate formation, foreign ions in the counter- ion layer may become trapped or occluded, Within the growing crystal. Because supersaturation, and thus growth rate decrease as a precipitation progresses, the amount of occluded material is greatest in that part of a crystal that forms first:

Mechanical entrapment occurs when crystals lie close during growth. Here, several crystals grow together and in so doing trap a portion of the solution in a tiny pocket.

Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low-that is, under conditions of low super saturation. In addition, digestion is often remarkably helpful in reducing these types of co-precipitation. Undoubtedly, the rapid solution and re precipitation that goes on at the elevated temperature of digestion opens up the pockets and allows the impurities to escape into the solution.

(ii) POST PRECIPITATION

Post-precipitation is the precipitation which occurs on the surface of the first precipitate after its formation. It occurs with sparingly soluble substances which form supersaturated solutions-, they usually have an ion in common with the primary precipitate. Thus in the precipitation of calcium as oxalate in the presence of magnesium. magnesium oxalate separates out gradually upon the calcium oxalate; the longer the precipitate is allowed to stand in contact with the solution, the greater is the error or mercury (II) sulphide in 0.3 M hydrochloric acid in the presence of zinc ions; zinc sulphide is slowly post-precipitated.

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Post - precipitation differs from Co-precipitation in the following

(a) The contamination increases with the time that the precipitate is left in contact with the mother Liquor in post-precipitation, but usually decreases in co-precipitation.

(b) With post-precipitation, contamination increases the faster the solution is agitated by either mechanical or thermal means. The reverse is usually true with co-precipitation.

(c) The magnitude of contamination by post-precipitation maybe much greater than in co- precipitation.

It is convenient to consider now the influence or digestion. This is usually carried out by allowing the precipitate to stand for 12-24 hours at room temperature, or sometimes by warming the precipitate for some time in contact with the liquid from which it was formed; the object is, of course, to obtain complete precipitation in a form, which can be readily filtered. During the process of digestion or of the ageing of precipitates, at least two changes Occur. The very small particles, which have a greater solubility than the larger ones, will, after precipitation has occurred, tend to pass into,solution, and will ultimately re-deposit upon the larger particles; co-precipitation on the minute particles is thus eliminated and the total co-precipitation the ultimate precipitate reduced. The rapidly formed crystals are probably of irregular shape and possess a comparatively large surface; upon digestion these tend to become more regular in character and also more dense, thus resulting, in a decrease in the area of the surface and a consequent reduction of adsorption. The net result of digestion is usually to reduce the extent of co-precipitation and to increase the size of the particles rendering filtration easier.

3. DIGESTION

The particle size of the coagulated colloid can be further increased by a process called digestion. In this context, digestion refers to letting the precipitate stand in its “mother liquor” (the solution in which it is formed), usually at elevated temperatures. The smaller precipitate particles tend to dissolve faster than large ones and re precipitate onto larger particles. This relatively slow dissolution/re-precipitation process also tends to remove foreign ions that have been trapped inside rapidly growing precipitate particles. Letting the AgCl precipitate stand at room temperature between lab periods will generally result in a complete settling of the AgCl and a clearing of the supernatant (the liquid above the precipitate).

4. FILTRATION

The separation of the precipitate from the solution by filtration is carried out either by filter paper or sintered crucible or Gooch crucible.

1. Filter Papers

Different grades of filter-paper have to be used for different precipitates. Ash-less filter- papers are used routinely. The size of the filter paper is selected, according to the bulk of the precipitate. The filter paper must be selected in such a way, that the pores of the filter paper must be smaller than the size of the particles of the precipitate.

The precipitate is separated by filtration either through crucibles or Whatman's No. 40, 41, or 42 filter papers. The diameter of filter paper may vary according to the bulkiness of precipitate. Bulky precipitate like aluminium hydroxide needs a large filter paper than dense precipitate, i.e. barium sulphate. The folded filter paper is kept in a funnel moistened with water and pressed to expel the air. Then , precipitate is transferred in a usual way and gets separated.

Apparatus used in filtration

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Various types of filtering, apparatus are used in gravimetric analysis.

(a) Gooch crucible of either porcelain or silica in which ready-made, disposable glass filter mats may only due used for precipitates at temperatures below 200°C.

(b) Sintered glass or sintered silica ( vitreosil) crucibles,

Gooch or sintered crucibles have now almost completely superseded the use. of filter paper, largely because filtration and washing of the precipitates. can be carried out so much more rapidly and efficiently with their aid. The ignition of precipitates in crucibles also avoids the necessity of first drying and then burning off the filter paper. Filtration through paper. may, however. occasionally be advantageous with gelatinous precipitates.

2. Gooch Crucibles

A Gooch crucible should be supported by a soft rubber collar in a glass adapter which passes through a rubber bung fitting into a Buchner flask.

The rubber collar should not project below the bottom or above the top of the crucible. The tip of the adapter should project below the side arm of the Buchner flask to avoid the risk of the filtrate being sucked out of the flask. A trap is required to prevent any back flow from the pump contaminating the filtrate. Furthermore, it reduces the effects of changes in water pressure which may cause the filter pod to lift from the base of the crucible.

Disposable glass fibre filter discs

Standard glass filter discuss are available. (whatman; 2.1 cm diameter) for use as filtration beds in Gooch crucibles. These filter discs are cheap, easy to handle, stable upto temperatures of atleast 180° and, like the asbestos filter beds formerly used, are readily removed for cleansing and disposal at the end of the analysis.

They very coarse grades (0 and 1) are used for such purposes as dispensing as gases in wash-bottles or gas absorption apparatus, and for sintered glass funnels for collecting solid material in preparative work. The very fine grade 5 is designed for the collection of precipitates in gravimetric analysis, but for fine precipitates (such as barium sulphate or silver chloride) grade 4 should be used. The rate of filtration is naturally slower the finer the porosity.

Sintered glass crucibles are easiest and more convenient to use than Gooch crucibles which require some skill and experience in preparation. They can be safely heated at temperature upto 400°, and are resistant to all reagents except hydrofluoric acid and strong alkalis.

One disadvantage is that they are much more difficult to clean than Gooch crucibles. The following is the recommended procedure;

1. Brush out as much of the dry material as possible with a hard brush. 2. Wash the crucible out by water drawn through it backwards (this is why sintered glass crucibles have a glass Rim or skirt below the sintered plate). 3. Soak the crucible for several hours in a solvent appropriate to the material to be removed (but not in solutions of caustic alkalis). 4. Wash the crucible well with water, and dry it.

Unit IV Gravimetric Analysis � .7Preparation of the Gooch crucible for filtration

Place the crucible in the suction filtration apparatus, sit a glass fibre filter disc gently on top of the porous base of the crucible. Gently wash the crucible and filter disc with water to remove any loose fibers, and apply gentle suction to drain off excess water. Such the pad as dry as possible, covering the mouth of the crucible during the suction to prevent dust particles being drawn in Dry (or ignite) the prepared Gooch crucible under the conditions specified for the drying (or ignition) of the precipitate, cool in a desiccator and weigh. Reheat, cool, and weigh; repeat until constant weight is attained.

3. Sintered glass or Sintered Silica Crucibles:

Sintered glass crucibles are made of resistant glass and have a porous bottom of sintered .ground glass which is fused in so as to be an integral part of the crucible. The filtering beds are fused-in sintered discs of the same material as the crucible. The sintered glass type of crucible is most frequently used, they are available in various degrees of porosity numbered 1, 2, 3, 4 and 5 indicating decreasing pore diameter with increasing number. A number 3 crucible is suitable for precipitates of medium particle size, such as silver chloride, while a number 4 is necessary for fine precipitates, such as barium sulphate. Sintered glass crucibles should not be heated at temperatures above 400°.If the precipitate has to be ignited, or requires drying at temperatures above 400°, Sintered silica Crucible (vitreosil) should be used.

Sintered Crucibles are prepared for use in filtration by washing and passing water through under suction and then drying (or igniting) to constant weight under the conditions which are specified for the drying (or ignition) of the filtered precipitate.

5. WASHING THE PRECIPITATE: USING A GOOCH CRUCIBLE

Care must be taken to avoid dislodging the filter disc. Suction must, therefore, be applied before pouring liquid into the crucible and should be adjusted, before filtration is commenced, to a low level just sufficient to draw liquid through the filter at a conveniently steady rate.

First, moisten the pad with a few drops of water from a teat Pipette. Apply suction, add more water and then pour the liquid gently down a stirring rod on to the central portion of the filter mat. Pour the supernatant liquid above the precipitate down the glass rod into the filter without-disturbing the precipitate more than is necessary. Keep the lower end of the glass rod close to, but not touching the filter pad. Do not allow the liquid to rise above a level of 1 cm from the top edge of the crucible. Keep the beaker inclined and a stream of liquid passing into the filter as long as the liquid filters freely. Wash the precipitate in the beaker by decantation as follows. Add about 10 mL of the wash liquid to the precipitate, stir the mixture, allow the precipitate to settle and pour off the supernatant liquid down the glass rod onto the filter paper. Repeat this procedure three or four times, then Stir the precipitate with about 20 ml or wash liquid transfer as much of the solid as possible to the filter. Wash the traces of precipitate which adhere to sides and bottom of the

Unit IV Gravimetric Analysis � .8beaker into the filter with a jet of water from a wash-bottle. Hold the wash-bottle in the right hand and the beaker in the left hand, with the stirring rod pressed firmly against the lip of the beaker with the left thumb. Incline the beaker and direct a stream of wash liquid against the precipitate to wash it down the stirring rod into the filter. Small amounts of precipitate which adhere tenaciously to the walls of the beaker and the rod should be dislodged by rubbing with a 'policeman' (a glass rod covered at one end with a small piece of rubber or plastic tubing). When all the precipitate has been dislodged, rinse the 'policeman' with wash liquid and transfer the trace of precipitate down the rod to the filter. Finally, hold the beaker and stirring rod up to the light and examine carefully for traces of precipitate; repeat the treatment if necessary. (Note; a 'policeman' should not be used during precipitation or the early stages of filtration and should be removed from the beaker immediately after use).

When the precipitate has been transferred quantitatively to the filter. it should be washed immediately with was liquid from a wash-bottle. Unwashed precipitate should not be allowed to stand for any length of time. because it will dry out and the mass will crack and. in this form. it cannot be washed properly. The jet of liquid should be directed onto the side of the crucible. The filter should be allowed to drain completely between each washing. From time to time. test for the completeness of washing by collecting small samples of the washings in a test-tube as the pass through the funnel, applying appropriate qualitative tests. When negative test are obtained. the precipitate and crucible are ready for drying.

In some determination, it is advantageous to wash the precipitate finally with ethanol to remove most of the water and thus reduce the time require: for subsequent drying of the precipitate.

Using of sintered glass or sintered silica crucibles. The technique is similar to that described for Gooch crucibles except that a filter mat is not used.

Some general theoretical consideration in washing the precipitate

Most precipitates are produced in the presence of one or more soluble compounds, and it is t0he object of the washing process to remove these as completely as possible. It is evident that only surface impurities will depend upon the solubility and the chemical properties of the precipitate upon its tendency to undergo peptisation, the impurities to be removed, and the influence of traces of the wash liquid upon the subsequent treatment of the precipitate before weighing. Pure water cannot, in general, be employed owing to the possibility of producing partial peptisation of the precipitate and, in slight solubility of the precipitate; a solution of some electrolyte is employed. This should posses a common ion with the precipitate in order to reduce solubility errors, and should easily be volatized in the preparation of the precipitate for weighing. For these reasons, ammonium salts, ammonia solution, and dilute acids are commonly employed. If the filtrate is required in a subsequent determination, the selection is limited to substances which will not interfere in the sequel. Also, hydrolysable substances will necessitate the use of solutions containing an electrolyte which will depress the hydrolysis whether the wash liquid is employed hot or at some other temperature will depend primarily upon the solubility of the precipitate: if permissible, hot solutions are to be preferred because of the greater solubility of the foreign substances and the increased speed of filtration.

It is convenient to divide wash solutions into three classes:

1. Solutions which prevent the precipitate from becoming colloidal and passing through the filter. This tendency is frequently observed with gelatinous or flocculated precipitates but rarely with well-defined crystalline precipitates. The wash solution should contain an electrolyte. The nature of electrolyte is immaterial, provided it is without action upon the precipitate either during washing or ignition. Ammonium salts are therefore widely used. Thus dilute ammonium nitrate solution of employed for washing iron (III) hydroxide [hydrated iron (III) oxide], and 1% nitric acid for Washing silver chloride.

2. Solutions which reduce the solubility of the precipitate. The wash solution may Contain a moderate concentration of a compound with one ion in common with the precipitate use being made of the fact that substances tend to be less soluble in the presence of a slight excess of a common ion. Most salts are insoluble in ethanol and similar solvents, so that organic solvents can sometimes be used for washing precipitates. Sometimes a mixture of an organic solvent eg.,ethanol) and water or a dilute electrolyte is effective in reducing the solubility to negligible proportions. Thus 100 mL of water at 25°C will dissolve 0.7 mg calcium oxalate, but the same volume of dilute ammonium oxalate solution dissolves only a negligible weight of the salt. Also 100 ml of water a room temperature will dissolve 4.2 mg of lead sulphate, but dilute sulphuric acid or 50 per cent aqueous ethanol has practically no solvent action on the compound.

Unit IV Gravimetric Analysis � .9

3. Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base; the wash liquid must therefore be basic. Thus (Mg (NH4)PO4 may hydrolyse appreciably to give the hydrogen phosphate ion HPO42- and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron (III), chromium (III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid.

The addition of an acid to the wash solution will prevent the hydrolysis of iron (III) or similar salts: thus dilute hydrochloric acid will serve to remove iron (III) and aluminium salts from precipitates that are insoluble in this acid.

Solubility losses are reduced by employing the minimum quantity of wash solution consistent with the removal of impurities. It can be readily shown that washing is more efficiently carried out by the use of many small portions of liquid than with a few large portions, the total volume being the same in both instances.

6. DRYING AND IGNITION OF THE PRECIPITATE

When the precipitate has been washed free from excess of the precipitant and other soluble impurities, it is convered to a substance of constant compositon before it is weighed. The details of the actual temperature of drying or the temperature of ignition are given in the description of each determination. General techniques are outlined below.

The funnel containing the washed precipitate is covered with a piece of filter paper (not ashless), moistened with distilled water. The paper projecting beyond the rim of the funnel is pressed firmly against the outside of the funnel and turn off. This forms a close-fitting lid which protects the precipitate against dust, currents of air etc. The funnel with precipitate is then put for 20 - 30 minutes into a drying oven at about 90 — 105°C.

Electric drying ovens are generally used in laboratories. The lower part of the oven contains several heating coils covered with a perforated metal sheet. The outside of the oven is provided with switches for the

SAMPLE

Unit IV Gravimetric Analysis � .10separate coils so that the temperature can be regulated. An oven fitted with a thermoregulator, which maintains the required temperature automatically, is especially convenient. The oven has shelves with holes

into which the funnels containing the precipitates are inserted.

Of course, if the precipitate is not ignited on the same day, there is no need to put it in the drying oven, as it will dry at room temperature.

Precipitates are ignited in porcelain or platinum crucibles. Before the ignition the weight of the empty crucible must be known, and it is necessary to be sure that this weight will not alter during the ignition. The crucible is therefore first heated to constant weight,i.e until its weight ceases to change. The crucible is heated under the same conditions as in the subsequent ignition of the precipitate. This is done in good time during the preceding stages of analysis. A clean and absolutely dry crucible is put in a porcelain triangle resting on a tripe and heated in the burner flame so that the blue cone of the flame is a few millimeters below the bottom of the crucible.

It will be remembered that when gas burners are used the air supply must be carefully adjusted. With excess air the flame may `strike back' or go out, while with a deficiency of air a smokey flame at a low temperature is obtained. After 15 - 20 minutes, the burner is turned out, the hot crucible is allowed to cool in air for 1 -2 minutes and then put into a Desiccator for final cooling so that the crucible does not absorb moisture from the air and so increase in Weight.

7. WEIGHING

The ignited samples in the crucibles are cooled for a few minutes and then they are kept in a desiccator, so that the moisture in atmosphere does not get adsorbed by the residue. Then it is accurately weighed on a chemical balance.

8.CALCULATION