UNIT 9 FILTRATION Structure 9.1 Introduction Objectives 9.2 Choosing Techniques and Filter Media 9.3 Special Techniques for Gravimetric Analysis 9.4 Filter Paper Filter Speed and Porosity Paper Quality Paper Size Paper Surface 9.5 Folding Circular Filter Paper 9.6 A Simple Filtration Selecting Apparatus Preparing Apparatus The Filtration 9.7 Washing the Residue 9.8 Reduced Pressure Filtration Selecting Apparatus The Filtrat~on 9.9 Small Scale Methods 9.10 Safety 9.1 1 Summary 9.12 Terminal Questions 9.13 Answers 9.1 INTRODUCTION There are a number of areas of chemistry where it is essential to be able to separate the liquid and the solid components of a mixture quickly and efficiently. In analytical chemistry, a technique known as gravimetric analysis, requires the determination (usually to an accuracy of four decimal places) of the mass of precipitate produced by a particular chemical reaction. This is a highly skilled operation and the key step involves transferring the contents of a beaker, containing liquid and solid, to a filtering device which traps the solid and allows the liquid to pass through. No loss of solid should accompany this process. The purification of an organic substance often involves a filtration step. Thus, when the desired product is first obtained, it is often contaminated with numerous impurities. The process of recrystallisation (covered in Unit 11) enables most of these impurities to be removed. Again it is necessary by the process of filtrtion to remove the crystals from the liquid which contains dissolved impurities. You should note that this liquid, which consists of a saturated solution of the solid together with dissolved impurities is known as the 'mother liquor'. Perhaps the most basic method of filtration is that of pouring a mixture through a plug of fibres such as cotton wool or glass wool. But you would find it difficult to remove the residue or pecipitate from the fibres. This is therefore a crude method. The most commonly used filter material, or medium, is paper. Filter paper Filtration circles are probably quite a familiar sight in your lab. Different grades of paper are produced for a variety of applications and there are different techniques for folding filter raper circles to suit various applications. Papers vary from the small types through which liquids are poured in a science lab, to large sheets through which liquids or gases are forced in industrial applications. Other materials are also often used as filters. Porous sintered glass is one that you will probably encounter. The main advantage of these kinds of filters is that they are re-usable and strong, although they are expensive. Pressure and suction are used to enhance the speed of filtration, although you are more likely to find pressure techniques used in an industrial environment rather than a school or college lab. Objectives After studying this Unit, you should be able to filter a suspension using a normal folded filter paper and a funnel. fold and use a fluted filter paper, wash the residue collected on a filter paper until free of contaminant, e.g. chloride, explain the need for a range of filter papers, state the meanings of the terms porosity, ashless, hardened, acid-washed, etc. as applied to filter papers, describe the arrangement of apparatus necessary for Buchner funnel tiltration. including those needed to recover both the residue and the filtrate, carry out a filtration using a Buchner funnel and flask to collect both residue and filtrate, explain the need for small Buchner type funnels, use Hirsch funnel and/or Willstatter filter nail correctly, state the dangers associated with, and the necessary precautions in the use of partly evacuated glassware, and discuss the factors which influence the choice of filtration techniques in a given set of circumstances. 9.2 CHOOSING TECHNIQUES AND FILTER MEDIA If you intended to carry out a filtration, you will be interested in the particulate matter (the residue or precipitate), or in the filtrate or both. For qualitative purpbies, you will not usually be concerned that some of the filtrate will be absorbed by the filter medium and thus contaminate the particulate matter. However, if you intend to cany out quantitative filtration (gravimetric analysis), the filtration process is always followed by washing of the filter (precipitate) to remove traces of filtrate. In gravimetric analysis, you often have to be very careful to ensure no loss of filtrate due to spalshing. Let us now study about the factors which govern the choice of correct filtration technique and correct filter medium. (1) PARTICLE SIZE Theapproximate size of the particles to be filtered determines the porosity of the filter medium. If the medium has large pores, a fine precipitate would pass Laboratory Techniques 11 through. On the other hand if you are filtering a coarse precipitate using a fine filter, the process would be unnecessarily slow. (2) VOLATILITY OF THE LIQUID The nature of the liquid carrying the particles influences the choice of technique. In many filtrations involving inorganic compounds, such as barium sulphate or silver chloride, the liquid is aqueous, Since water is relatively involatile and not hazardous, you could very well choose a simple technique involving a filter funnel and paper, especially where time is not an important consideration. If you were tackling a filtration involving an organic compound, you might find that the liquid is non-aqueous, such as an alcohol, ether or petroleum spirit. There the problem arises that the liquid is volatile and the vapour could be flammable or toxic or both. (3) TYPE OF ANALYSIS In qualitative analysis, you may be only interested in removing a precipatate from a mixture in order to test the remaining liquid. There a cheap filter paper would suffice. However, if you were performing a gravimetric analysis, you would be very concerned that the paper should not contaminate the precipitate or filtrate. In such a case, you would chose a paper which has been processed to remove contaminating mineral matter. These are the single acid washed or double acid washed papers, which are more expensive - they are known as 'ashless papers7. Analysts often prefer to dispense with paper altogether by using, e.g. sintered-glass or porous-alumina filter crucibles. (4) VOLUME TO BE FILTERED Some apparatus is quite suitable for handling large volumes of mixtures to be filtered, e.g. the suction filtration method employing a Buchner funnel. Other apparatus is more suitable if you were filtering a few cubic centimetres of liquid, such as the Willstatter filtration nail (apparatus for filtration will be discussed in more detail in Secs. 9.6 & 9.9). You should note that the size of a filter paper is determined by the volume of the particles not the volume of the mixture to be filtered- a small amount of solid in a large volume of mixture would be 'lost' on a large paper. The volume of the mixture determines the technique to be used. (5) COST Cost is a very important factor. A school technician having to provide filtration equipment to 30 pupils would find glass filter funnels and cheap qualitative papers the least costly option. On the other hand, a science teacher pushed for time may want you to set up a suction filtration apparatus, which is more expensive, if hetshe wished to demonstrate a filtration. In industry, it may be necessary to strive for the greatest yield attainable together with high purity, and consequently the cost of the filtering process is secondary to the cost of recovering the product. If this is so, considerable sophistication is applied in refining the filtering process. Now try the following SAQ. Filtration Before you start a filtration exercise, you will have to consider a number of factors in order to choose the correct technique and correct filter medium. List these factors. (i) ..: ............................................................................ (ii) ................................................................................ (iii) ............................................................................... (iv) .............................................................................. (v) ................................................................................ (vi) .............................................................................. Check your answer with ours at the end of the unit before continuing. 9.3 SPECIAL TECHNIQUES FOR GRAVIMETRIC ANALYSIS In gravimetric analysis, you must not lose even a single grain of a precipitate and you will need to know how to use a pouring rod, a 'policeman' and a plastic wash bottle. The technique of washing the precipitate would need to be mastered to avoid splashing and you should be aware that it would be difficult to avoid spalshing if a fluted paper (see Sec.9.5) was used. Very often the filtrate from a gravimetric separation is required for a further precipitation. In this case you would need to ensure that no loss of filtrate occurs due to splashing or 'sucking back'. Both of these problems will be dealt with in Sec. 9.8. There are a number of parameters regarding the filter paper to be used in a filtration. Let us now study them one by one. 9.4.1 Filter Speed and Porosity The aim of filtration is to produce a clear filtrate as fast as possible. This aim is directly dependent on the type of filter paper you use, and the type of filter , paper you use will be determined by the particle size of the precipitate in the mixture you are to filter. The fact is that fast paper retains coarse particles and slow paper retains fine particles; so the finer the precipitate, the slower the filtration. The size of the gaps (pores) between the fibres determines whether a paper is fast or slow. A fast paper has large gaps (pores) between its fibres and allows a lot of liquid (and fine particles ) to pass through it and a slow paper has small pores and retains fine particles allowing only small quantities of liquid to pass through it.