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Gooch Crucible Definition Essay

Many years ago, a friend of mine teased me about how I, as a chemist, would probably die young of some dread occupational disease. Peevishly, I replied that he, a geologist, might die romantically when a rock fell on his head; or, more prosaically, wrapping his beloved pickup round an exotic rock formation.

Even if the actuarial statistics do not bear out my friend’s hunch, there is no doubt that chemists are perceived as living dangerously. We heat things. We evacuate them. And we work with things that aren’t very good for us. Cyanide and chloral hydrate were once part of undergraduate practicals, mercury flowed in rivulets, flames were everywhere. And because of its remarkable inertness, so was asbestos - a cheap wonder-material central to, amongst other things, gravimetric analysis thanks to the work of Frank Austin Gooch.

Born in Watertown, Massachusetts, US, in 1852, his father was a lumber merchant who nurtured his son’s interests in science - helping him make electrical machines and Leyden jars to store static charge. The young Gooch found his classical schooling rather dull, so with glassware and chemicals obtained from his uncle, he started doing home experiments. These grew in scale and ambition, peaking with a cataclysmic display in the kitchen. Years later he would recall sardonically that his aunt Lucy’s ’arrival upon the scene of devastation prevented repetition of my first chemical experimentation - more successful so far as the anticipated results were concerned than many since made’.

He went to Harvard University, US, at age 16, specialising in chemistry and physics. After graduating, he became an assistant to Josiah Cooke - a brilliant undergraduate lecturer with whom he started research. Wanting to develop his interest in crystals, Gooch spent time in Vienna, Austria with the mineralogist Gustav Tschermak. He returned to Harvard to finish his PhD and then went back to Europe for an extensive tour of universities.

A prestigious fellowship took him back to Harvard, working with Wolcott Gibbs, who steered him towards analytical chemistry. It was then that he became frustrated with existing methods of gravimetric determination. Suction filtration on paper was limited by moisture retention in the paper if heated to temperatures below 100?C, or charring above. Gooch’s former boss, Josiah Cooke, had developed ’inverse filtration’, which allowed one to suck up the supernatant liquid from above a precipitate; Charles Edward Munroe, also at Harvard, had devised conical porous clay filters that were very effective and could be fired in an oven, but were slow to dry, and tended to reabsorb moisture from the air quickly; and a number of chemists, including the great Robert Bunsen, had attempted to make thermally and chemically robust filters from asbestos.

Gooch brought several of these ideas together. He took a platinum crucible and riddled its flat base with holes. A slurry of acid-washed asbestos fibres could then be poured into the crucible under suction to produce a ’felt’ filter bed. After drying to constant weight, the crucible was ready. Precipitates could be filtered directly, even from highly corrosive mother liquors, and then fired in situ. The technique required little skill and few fancy materials - highly inert platinum crucibles (exotic by today’s standards) were once fundamental kit for chemists. 

In his reminiscences, Gooch was characteristically modest about his creation, writing that his device ’proved to be of such general utility in analysis that, dubbed by the craft with my surname, it has found its way into the "Standard Dictionary", dragging me along with it.’

After adventurous stints as an analyst at the US Geological Survey and chief chemist for Yellowstone National Park, in 1886 he became professor of chemistry at Yale University, US. He rebuilt the antiquated teaching programme and taught all branches of chemistry for undergraduates himself. He was an inspiring lecturer, aiming his material at the best students, and peppering his classes with demonstrations. His reputation led to a steady expansion in student numbers, particularly to non-chemistry majors. He oversaw the construction of the Kent laboratory and its subsequent extensions. 

Postgraduate numbers also rose and Gooch supervised numerous research students, working at Yale until his retirement in 1918, after which he devoted himself mainly to family genealogy. He died in 1929 at the ripe old age of 78.

Contrary to popular belief, many of the nasties in our labs have gone. Labs are ventilated, cyanide is locked away, and asbestos - strongly associated with the aggressive form of lung cancer, mesothelioma - has vanished completely. But Gooch’s crucibles, reinvented in Pyrex with sintered glass frits, endure. As such, most of us chemists - like Gooch before us - are likely to die in our beds. Or perhaps just commuting to work.

Andrea Sella is a lecturer in inorganic chemistry at University College London, UK 

Further Reading

Proc. Amer. Acad. Arts Sci., 1878, 13, 342 
Chem. News, 1878,  37, 181   


Chapter 2

EXPERIMENTAL TECHNIQUES IN CHEMISTRY

Analytical Chemistry:

            “The science of chemical characterization is called analytical chemistry.”

OR

            “The branch of chemistry which deals with qualitative as well as quantitative analysis of a sample is called analytical chemistry”

Qualitative Analysis:

            Detection or identification of elements in a compound is called qualitative analysis.e.g. Detection of radicals of salt

Quantitative Analysis:

            The determination of the relative amounts of elements in a compound is called quantitative analysis.e.g Combustion analysis

It involves following steps

  1. Sampling
  2. Separation of desired components
  3. Measurements
  4. Calculation of results
  5. Drawing the conclusion

Experimental Techniques:

            During the laboratory work a student deals with following techniques

  1. Filtration
  2. Crystallization
  3. Sublimation
  4. Solvent Extraction
  5. Chromatography

i.        FILTRATION

            The process in which insoluble particles are separated from a liquid is called filtration. For filtration we can use a Filtration paper or Filter crucible.

Filter Paper:

            The filtration by filter paper is a slow process. In this process solvent passes through filter paper and insoluble particles are left behind on the filter paper. Many types of filter papers are available. They have different porosites (pore sizes). A suitable filter paper is chosen. Its size should be large so that it should be to full of precipitates at the end of filtration.

The funnel should be large so that its rim should extend 1-2 cm above the filter paper. The stem of funnel should be long. It should touch the side of beaker. Moreover stem of funnel should remain full of liquid during the filtration. The filter paper should be fitted properly otherwise it is time wastage.

Folding of Filter Paper:

  1. Filter paper should be folded twice.
  2. The first fold should be along the diameter.
  3. In second fold the edges should not be quite matched.
  4. The folded filter paper should be opened on the larger section.
  5. A cone is formed by three fold side and one fold side.
  6. The apex angle should be greater than 60o.
  7. Then filter paper is placed into the 60o funnel.

viii.            Finally wet the filter paper and press it firmly.

ix.            The rate of filtration can be increased by gentle suction.

  x.            The rate of filtration can also be increased by fluted filter paper.

(Fluted Filter Paper)

Fluted Filter Paper:

            A Filter Paper Raving many elevations and depressions is called fluted filter paper. A filter paper folded by fan like arrangement with alternate elevations and depressions given fluted filter paper.

Filter Crucibles:

            We use filter crucible for an easy filtration. There are two types of filter crucibles.

  1. Gooch Crucible
  2. Sintered glass Crucible

i.          Gooch Crucible:

            It is made of porcelain. Its bottom is perforated covered with paper pulp. The Gooch crucible is placed on suction apparatus. It is shown in figure. It is used to filter those precipitates which are to be heated at high temperature. Some solutions like HCl, KMnO4 react with paper. So their filtration is done by covering Gooch crucible with asbestos [CaMg3 (SiO3)4].

(Gooch Crucible Fitted on Filtring Apparatus)

ii.         Sintered Glass Crucible:

            It is made of glass. Its bottom is sealed with sintered (porous) glass disc. It needs no preparation. All solutions can be filtered by it.

(Sintered Glass Crucible)

ii.       Crystallization

            The process of crystals formation is called crystallization. When a hot saturated solution of a substance is cooled, then crystals of solid substance separate out. It is called crystallization. This process is used for purification of solid compounds. E.g. Purification of naphthalene. The process of crystallization involves the following steps.

i.          Choice of Solvent:

            A suitable solvent is chosen by hit and trial method. An ideal solvent has following properties

  1. It should dissolve maximum amount of solute.
  2. It should not react with solute.
  3. It should not dissolve the impurities.
  4. On cooling, it should form pure crystals
  5. It should be cheap (in-expensive)
  6. It should be easily removable.
  7. Its use should be safe
  8. An inflammable solvent should be heated on water bath. The most common solvents are water, rectified spirit(95% alcohol), absolute alcohol(99% alcohol), ether, chloroform, acetone, acetic acid, carbon tetra chloride.

ii.         Preparation of Saturated Solution:

            The substance is dissolved in minimum amount of solvent. To dissolve more and more solute, the solution should be heated directly or on water bath. The stirring of solution is necessary.

Precaution:There are certain solutions which are volatile and these should be heated with the help of water bath.

                                                                                    Diagram……

iii.        Filtration:

            The hot saturated solution is filtered to remove insoluble impurities. Sometimes premature crystals can form. It can be prevented by using hot water funnel.

                                                                                                                                                                                                                        Diagram…..

                                                                                                                                                                                               

iv.        Cooling:

            The hot filtered solution is cooled. The moderate cooling gives the medium sized crystals and slow cooling gives bigger crystals. Therefore slow cooling is avoided because it contains solvent as an impurity. Sometimes a few crystals of pure solid are added in solution to start the crystallization process.

v.         Collecting the :

            When the crystallization is complete, then mixture of crystals and mother liquor is filtered through Gooch crucible using a vacuum pump. The mother liquor is removed completely by full suction. Then the filter cake is pressed to remove the rest mother liquor. Finally the crystals are washed with small amount of cold solvent. This process is repeated many times till pure crystals are formed. By evaporation of the mother liquor we can get a fresh crop of crystals. The mixture through which crystals are formed called mother liquor.

                                                                                                                                                                                                                        Diagram………

vi.        Drying of :

            There are three methods for drying the crystals.

a)      Press the crystals between the folds of filter paper. Repeat this process many times but by this method crystals are crushed and fibers of filter paper can mix with them.

b)      Dry the crystals at 100oC in an oven. Here solid should not melt or decompose at 100oC. This method is also not reliable because there are many solids which have low very low m.p can not be dried.

c)      Spread the crystals over watch glass and keep in vacuum desiccators for many hours. The drying agents used in desiccators are CaCl2, P2O5, Silica gel. This is the most reliable method

                                                        Diagram…………….

vii.       Decolourization of Undesirable Colours:

            Sometimes crystals become coloured due to impurities. To prevent it, the substance is boiled with animal charcoal. Animal charcoal absorbs the coloured impurity. The pure colourless crystals are formed by cooling the filtrate.

iii.      Sublimation

            The process in which a solid, on heating directly changes into vapours without passing through liquid state is called sublimation. The hot vapours are then condensed to form the solid again. It is called sublimate. The process of sublimation is used to purify the solids. E.g. Naphthalene, NH4Cl, Iodine and benzoic acid etc. the solid substance is taken in a watch glass. A funnel is inverted on it. The solid is heated slowly over a sand bath. The funnel is cooled by wet cotton. The pure solid deposits on the inner sides of funnel and impurities are left behind in watch glass.

Limitation: Those substances can be sublimed which have high v.p than their m.p

iv.      Solvent Extraction

            We can separate a solute from a solution by shaking it with an immiscible solvent in which solute is more soluble. It is called solvent extraction. It is mainly used to separate organic compounds from water. It is done by an apparatus called.

i.          Separating Funnel:

            It is shown in figure. The example of solvent extraction is ether extraction. In this method an aqueous solution containing an organic compound is taken in separating funnel. The ether is also put into the funnel. It is stoppered, shaken well and allowed to stand. After sometime two layers are formed. The organic compound goes to ether layer and

Inorganic impurities remain in water layer. The ether layer is separated. It is evaporated on water bath to get organic compound. The repeated extraction using small portions of solvent (ether) are more efficient than a single extraction using large volume of solvent. The solvent extraction is an equilibrium process. So it is based upon Distribution law or Partition law.

This law states that a solute distributes itself between two immiscible liquids in a constant ratio of concentration irrespective of its added amount. The ratio of concentration of a solute dissolved in two immiscible solvents is a constant. It is called distribution coefficient. It is given as

                       
            For example distribution of I2 in water and Carbon tetrachloride. The I2 is not very soluble in water but readily soluble in aqueous KI. In water I2 is soluble in the form of tri Iodide ion (). When ccl4 is added to aqueous solution of ions, then equilibrium takes place.
                       
            When two solvents are gently shaken, then their area of contact increases and transfer of I2 molecules improves. Thus a good separation is achieved. The result is that brown colour of  in aqueous layer fades and purple colour of free I2 in CCl4 layer appears. When equilibrium is established, then rate at which iodine passes from CCl4 to water equals the rate at which it passes from water to CCl4 . The distribution coefficient is given as k = [I2(CCL4)]

                      [I3(water)]

                      

v.       Chromatography

            The word chromatography is derived from Greek word “Khromatos” which means colour writing. A technique by which we can separate components of a mixture due to their different distribution between stationary and mobile phase is called chromatography. The phase over which mobile phase flows is called stationary phase. It may be a solid or liquid. The phase which flows over the stationary phase is called mobile phase. It may be a liquid or gas.

Principle:

            The principle of chromatography depends upon two points.

  1. Relative affinity (solubility) of components in two phases.
  2. Distribution coefficient of components in two phases. The distribution coefficient K is given as

A component with smaller value of K remains in stationary phase.

A component with grater value of K remains in mobile phase.

Types of Chromatography

            The chromatography has two types

                        (i)         Adsorption Chromatography

                        (ii)        Partition Chromatography

i.          Adsorption Chromatography:

            The Chromatography in which solid substance is used as stationary phase is called adsorption chromatography. e.g. Column Chromatography.

ii.         Partition Chromatography:

            The Chromatography in which a liquid substance is used as stationary phase is called partition chromatography. e.g. paper chromatography.

Paper Chromatography

            It is a kind of partition chromatography. Here stationary phase is liquid (H2O) adsorbed on paper. The mobile phase is an organic liquid which passes over the paper. The solutes are distributed between two liquids. There are three kinds of paper chromatography.

  1. Ascending paper chromatography
  2. Descending paper chromatography
  3. Radial / Circular paper chromatography

Here will explain only ascending type.

Ascending Paper Chromatography

            The chromatography in which solvent front or mobile phase travels upwards by capillary action is called ascending paper chromatography.

Method:

            A solvent mixture is taken in a chromatographic tank. A 20 cm long strip of whatmann’s paper no.1 is taken and a pencil line is drawn on it at 2.5 cm from one end. A spot point of mixture solution is put on the line. Two other spots of pure components are also put on the line as standard. Now spots are dried and paper is suspended into the tank such that its lower end dips 5-6 mm into the solvent. The glass cover is put on the tank. The solvent passes the spots. The solutes begin to move upward according to their distribution coefficients.

            When the solvent has covered length of paper, then strip is removed, position of solvent is marked and paper is dried. The pattern on the is called chromatogram.
            If spots are coloured, then they can be easily identified. If spots are colourless, the chromatogram is developed by physical or chemical methods. The retardation factor (Rf) of each component is calculated as

            Each component has its specific Rf value. For example the shown chromatogram tells that mixture solution (A) has two components (B, C). Their Rf values are given as

Rf(B)= x/y   (x is distance covered by component B, Z is distance covered by component C and y is distance covered by solvent)

 Rf(C)= z/y                           Think! Why Rf value can never be greater than 1            

Uses of Chromatography:

  1. The chromatography is useful in separation of organic compounds.
  2. It is used for identification of compounds.
  3. It is useful in qualitative and quantitative analysis.
  4. It is used in checking purity of a compound.

Important note:A pure compound gives only one spot.

Think! Why Paper Chromatography is time consuming process?

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