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Separation Techniques

Some of the common methods of separating substances or mixtures are:

1. Sieving

Sieving is done to separate mixtures that contain substances mostly of different sizes. The mixture is passed through the pores of the sieve. All the smaller substances pass through easily while the bigger components of the mixture are retained.

2. Evaporation

Evaporation is a technique that is used in separating a mixture usually a solution of a solvent and a soluble solid. In this method, the solution is heated until the organic solvent evaporates where it turns into a gas and mostly leaves behind the solid residue.

3. Sedimentation

Sedimentation is a process by which heavier impurities present in liquid normally water settle down at the bottom of the container containing the mixture. The process takes some amount of time.

4. Magnetic Separation  When one substance in the mixture has some magnetic properties then this method is quite useful. Strong magnets are commonly used to separate magnetic elements
5. Filtration

Filtration is a process used to separate solids from liquids or gases using a filter medium that allows the fluid to pass through but not the solid. The term “filtration” applies whether the filter is mechanical, biological, or physical. The fluid that passes through the filter is called the filtrate. The filter medium may be a surface filter, which is a solid that traps solid particles, or a depth filter, which is a bed of material that traps the solid.

Filtration is typically an imperfect process. Some fluid remains on the feed side of the filter or embedded in the filter media and some small solid particulates find their way through the filter.

Filtration Techniques

Filtration  is a process used to separate solids from liquids or gases using a filter medium that allows the fluid to pass through but not the solid. The term “filtration” applies whether the filter is mechanical, biological, or physical. The fluid that passes through the filter is called the filtrate. The filter medium may be a surface filter, which is a solid that traps solid particles, or a depth filter, which is a bed of material that traps the solid.

Filtration is typically an imperfect process. Some fluid remains on the feed side of the filter or embedded in the filter media and some small solid particulates find their way through the filter.

Filtration Methods

There are different types of filtration. Which method is used depends largely on whether the solid is a particulate (suspended) or dissolved in the fluid.

1. General Filtration: The most basic form of filtration is using gravity to filter a mixture. The mixture is poured from above onto a filter medium (e.g., filter paper) and gravity pulls the liquid down. The solid is left on the filter, while the liquid flows below it.

How to use filter paper

First,  make  sure  you  use  the appropriate  size  filter  paper.   This  is  most  important  for  vacuum filtration  where  the  paper  should  smaller  in  diameter  than  the  base  of  Buchner  or  Hirsch funnel (but  it must cover all the holes) and should sit flat on the bottom of the funnel with no creases or folds.

Second, folding  the  filter  paper.   There are  two  ways  to  fold  filter  papers,  the  “conventional” method and “fluted”.

Conventional   filter paper 

Fluted filter paper

Fluting the filter paper maximises the rate at which the liquid may flow through the filter paper by increasing the surface area and by allowing air to enter the flask along its sides to permit rapid pressure equalisation

Vacuum filtration

Vacuum filtration is typically a fast and efficient way of filtering. In a vacuum filtration, the solution  to  be  filtered  is drawn through the filter paper by applying a vacuum to a filter flask with a side arm adaptor (also known as a Buchner flask).

This  typically  comprises  a Büchner funnel fitted with the appropriate size filter paper;   a   clamped   filter   flask   with   conical   filter adapter,  and  a  vacuum  applied  to  the  side  arm  of the filter flask (see left).

The crystals are collected by swirling the mixture of the solid and  liquid  and then pouring quickly it  into the  filtration  apparatus.  

If  smaller  quantities  are  to  be  filtered  the  Hirsch funnel and a small filter paper should  be  used  instead.   If needed,  the  filter  flask can be replaced by a test tube with side arm. Again, the tube should be clamped and the vacuum applied at  the  side  arm  using  the  thick  walled  vacuum tubing.

When  using  vacuum  filtration,  it  is  very  important that the correct size of filter paper be used. The filter paper should be flat (i.e. not folded up at the edges) and  should  cover  all  the  holes  in  the  base  of  the funnel.    It  is  also  important  that  the  apparatus  be clamped  since  it  is  very  easily  tipped  over  usually resulting in loss of the sample.

2. Cold Filtration:Cold filtration is used to quickly cool a solution, prompting the formation of small crystals. This is a method used when the solid is initially dissolved. A common method is to place the container with the solution in an ice bath prior to filtration.
3. Hot Filtration:In hot filtration, the solution, filter, and funnel are heated to minimize crystal formation during filtration. Stemless funnels are useful because there is less surface area for crystal growth. This method is used when crystals would clog the funnel or prevent crystallization of the second component in a mixture.

Sometimes filter aids are used to improve flow through a filter. Examples of filter aids are silica, diatomaceous earth, perlite, and cellulose. Filter aids may be placed on the filter prior to filtration or mixed with the liquid. The aids can help prevent the filter from clogging and can increase the porosity of the “cake” or feed into the filter.

The hot filtration process is best carried out using a fluted filter paper and a stemless filter funnel.

Membrane filtration

.  Membrane filters are used extensively in the laboratory and in industry to sterilize fluid materials. Membrane filters have a known uniform porosity of predetermined size (generally 0.45 µm ) sufficiently small to trap microorganisms. Using the membrane filter technique, sample is passed through the membrane using a filter funnel and vacuum system. Any organisms in the sample are concentrated on the surface of the membrane Membrane filter technique is an effective, accepted technique for testing fluid samples for microbiological contamination. It involves less preparation than many traditional methods, and is one of a few methods that will allow the isolation and enumeration of microorganisms

The membrane, with its trapped bacteria, is then placed in a special plate containing a pad saturated with the appropriate medium. The passage of nutrients through the filter during incubation facilitates the growth of organisms in the form of colonies, on the upper surface of the membrane. Discrete colonies thus formed  can be easily transferred to confirmation media.

The selection of the appropriate method is typically dictated by the nature of the experimental situation.  The answers to each these questions help dictate what type of set up is required.

1.    Whether you are collecting the solid or filtrate(solution) If you are collecting the solid, vacuum methods are likely best as they are faster.  If you are collecting the filtrate (solution), gravity filtration methods are often preferred.
2.    How much solution is there to filter.  If you have a large volume to filter, gravity filtration is probably the better choice.   
3.    How much solid is being collected.   If only a small amount is being collected,  a vacuum  filtration using the Hirsch funnel is preferred.
4.    How fine is the solid.  If it’s a fine solid, regular filtration might be very slow, so use vacuum filtration.
5.    IF  unwantedcrystallisation can occur in the filter funnel.  If yes, use hot filtration
6. Decantation      

Decantation is the process of separation of liquid from solid and other immiscible (non-mixing) liquids, by removing the liquid layer at the top from the layer of solid or liquid below

The process can be carried out by tilting the mixture after pouring out the top layer. This process can also be used to separate two liquids that do not mix with each other for e.g., oil and water When we leave the mixture of oil and water, two separate layers are formed, with water at the bottom and oil, being lighter, at the top…

We can remove the oil layer from the top by pouring it into another vessel, which leaves us with the water layer at the bottom. Usually this process is not very efficient in this separation, as thin layer of the remaining oil cannot be easily procured from the mixture. In order to make the procurement easier and the separation efficient, we use a separating funnel, as shown in the next figure.

Here, the bottom layer is collected first and the layer above it is made to remain in the vessel with the help of a stop cock, as shown which can be procured later.

Hence, separation of mixtures by this process is highly efficient if one of the components of mixture settles easily. But what can we do for the mixture where the particles do not settle. For e.g., for a mud water mixture, even after the process of decantation is complete, the water sample still looks slightly muddy. For such mixtures we use the process of loading.

Loading Definition

Loading is the process by which the mixture of liquids and liquids containing tiny impurities is separated by adding a chemical that sticks to the impurities and makes them heavier e.g., if we take a slightly muddy sample and add some alum to it, after some time we observe that a layer of mud settles at the bottom of the vessel and water becomes clear. Here, the alum got adhered to the small bud particles and made it heavier, thus causing the mud to settle at the bottom. This process is used during the filtration of water in water filtration tanks.