Basic Laboratory Techniques, Chemistry tutorial

Introduction:

Basic laboratory techniques in chemistry contain an essential place in the training of a chemist. They give a good background for experimental skills and for subsequent independent research. Through assembling suitable apparatus, we can bear out reactions from starting materials to pure products. Sometimes such operations require particular laboratory reactors that work in well-defined operation conditions. At the end of  the  react ion, different   techniques  are  required  in  order  to  separate the  products  from  the  reaction mixture,  and then  the  final  compound will require to be purified via means of specific operations. The separation of reaction products is attained by techniques such as extraction through solvents, crystallization, and distillation. Such methods, that are almost standardized, are often able to purify organic compounds by separating the impurities. Chromatographic methods are very effective in separating more complex mixtures.

Basic laboratory techniques:

Measuring volume   

Medicine Droppers

A satisfactory, but frequently rough, technique of estimating volumes, the dropper is calibrated via counting the number of drops it generates to build up a milliliter. 

Beakers and Flasks

The volume on the side of the beaker or flask is only a rough approximation at best. Use this process only for crude approximations of volume.

Volumetric Glassware

This is the mainly precise and accurate technique of transferring and delivering liquids. It is significant that the volumetric glassware be extremely clean before make utilize of, as dirt and other chemicals will not only decrease precision due to improper draining but as well can contaminate the experiment. The best time to clean glassware is immediately after it's utilize. Special cleaning solutions are available, but soap, warm water, and a brush, followed via thorough rinsing-first by tap water and then through small amounts of distilled water-is frequently satisfactory. No drops of distilled water should adhere to the surface of clean glassware. To dry, the glassware is reversed onto a paper towel. Don't wipe or air-blow dry due to possible contamination.

In determining the volume of all volumetric glassware, it is significant to comprehend how to read the meniscus. The meniscus is the apparent downward curvature of the liquid mostly due to surface tension. It is needed to read the bottom of the meniscus by the eye horizontal to this surface. If it isn't read at eye level, errors in the reading will consequence.

Proper lighting is vital to see the meniscus clearly. To boost the meniscus, a little white card through a 1 x 1.5 black rectangle in the lower 1/3 of the card is situated behind the glassware. The card is slowly increased until the reflection of the black rectangle on the meniscus is seen. The bottom of the meniscus is now readily observable against the white background.

a. Graduated cylinders are the mainly ordinary place measuring instruments in the laboratory. A tall cylinder by a small diameter will be more accurate than a short one by a large diameter. 

b. Burettes are constructed so that it is feasible to determine and deliver accurate volumes. To clean a burette, clean by a soap solution. If a burette brush is utilized be sure not to scrape the sides of the burette wall by the metal handle. Rinse the soap solution from the burette several times through the stopcock, first by tap water and then through several small portions of distilled water. Be sure to roll the burette in a close to horizontal position to thoroughly wet the entire surface of the glass.

To activate the burette, close the stopcock and add a small portion of titrant (3-5 mL). Tilt the burette to an almost horizontal position and roll the burette so that the rinse comes in contact by the whole inner surface. Drain this material during the burette tip into a waste beaker. Repeat this process 3 times.

To fill and operate a burette, close the stopcock and, by a funnel, fill the burette to just above the zero mark. Be sure the burette is vertical and doesn't slant. Open the stopcock briefly to eliminate any air bubbles and drain the titrant to several points below the zero mark.

Record the starting volume and execute the titration. Be sure to record the final volume. Whenever the titration is terminated, drain the excess titrant and rinse the burette by a small amount of distilled water 2 to three times. Clean it if needed.

c. Volumetric pipettes, properly manipulated, can deliver volumes reliable to one part per thousand. Graduated pipettes aren't capable of such precision. In pipetting a liquid, oral suction must never be utilized. Keeping the pipette tip beneath the surface of the liquid and, using a pipette bulb, draw the liquid above the graduation mark. Slowly squeeze the release button to level off the amount. Any drops adhering to the bottom of the tip should be removed before delivering the volume. Hold the pipette vertically and let the liquid drain for 20 seconds after the liquid has been delivered; the tip is then touched to the wall of the receiver. The liquid in the tip of the pipette must not be removed; calibration of the pipette has allowed for this. 

d. Volumetric flasks are utilized to prepare solutions of a specified concentration. In making a solution the solute is placed in the flask first. Then a small amount of the solvent is added and the flask is vigorously shaken to dissolve or mix the solutions. Care should be taken as the flask and solution may get very warm.  Never use your fingers to cover the opening at the top. There will be an appropriate sized stopper to use. Add more solvent to just below the fiducial mark, stopper, and shake again. Let the flask come to room temperature and then fill the flask to the fiducial mark and stopper. Invert the stopper flask several times to thoroughly mix contents. Clean the glassware when finished. 

Glass Working:

Cutting Glass Tubing

Make a scratch across the glass tubing at the desired location by a single stroke of a triangular file. Situate a drop of water on the scratch through our finger. The tubing must always be held in a towel while pressure is being applied to stop injury to the hands. Situate both thumbs close mutually on the side of the tubing opposite the scratch, and snap the tubing in a direction away from us and others against the pressure of the thumbs.

Fire polishing glass tubing

All edges of glass tubing must be fire-shined to round off the sharp edges. Hold the sharp edges of the tubing at an angle in the flame and rotate the tubing until a bright yellow color is conveyed to the flame. Be certain to avoid over heating the glass tubing. 

Bending Glass Tubing

Attach a wing top to the burner to spread the flame. Hold a piece of tubing horizontally in the upper portion of the flame and gradually rotate to ensure regular heating. Continue rotation of the tubing until it happen to soft. Smoothly bend the glass tubing and allow it to cool on a fire opposing surface. 

Separating Liquids and Solids:

Decanting 

Permit the solid to settle in the beaker or test tube. Then transport the liquid, or supernatant, by the aid of a stirring rod. Hold the stirring rod against the lip of the beaker and pour the liquid down the rod that is touching the inner wall of the receiving vessel. Do this slowly so as not to disturb the settled solid. 

Gravity Filtration

Fold a piece of filter paper in ½ refold to within 10o of a 90o fold; tear off the corner unequally, and open. Situate the folded filter paper snuggly into the funnel. Moisten the filter paper by the solvent being utilized and press the filter paper against the funnels top wall to form a close. Hold a stirring rod against the lip of the beaker and pour the liquid down the rod that is touching the within of the funnel. The funnels tip should handle the inside wall of the receiving vessel to reduce splashing. Never fill the funnel more than two-thirds full. 

Vacuum Filtration 

A Buchner funnel fitted by a rubber stopper is inserted into a suction flask. The side arm of the flask is connected to a safety trap, which in turn is attached to the water aspirator by a short piece of pressure tubing. Filter paper, slightly smaller than the funnel diameter, is situated over the holes and moistened through the solvent. Turn the aspirator on full. 

Centrifugation

Whenever the centrifuge is utilized it must be balanced or it might happen to harmed. To do this fill, an identical tube by similar level of water in the position opposite the mixture to be separated. Close the cover and set the machine in motion.  Stay the cover closed and our hands away from the top of the centrifuge while it is rotating. 

Transferring liquids:

When the reagent is being moved from a reagent bottle, eliminate the glass stopper and hold it between the fingers of the hand utilized to grasp the reagent bottle. Never lay the stopper on the desktop; impurities might be picked up and contaminate the solution. Hold the stirring rod against the lip of the reagent bottle and pour the liquid down the rod that is touching the inner wall of the receiving vessel. This prevents splashing and losses of reagent down the side of the bottle. Never transfer more liquid than is needed and never return unused portions to the reagent bottle.

Testing Gases for Odour:

An educated nose is a significant and helpful asset to have in the laboratory. It must be utilized by caution since several gases are toxic or just irritant. Never hold our nose directly over the vessel from which the gas is coming from. Rather, fan several of the vapors toward our nose.

Testing with Litmus:

To test the acidity/basicity of a solution by litmus paper, insert a stirring rod into the solution, withdraw it, and touch it to a litmus paper that is resting on a clean, inverted watch glass. Not at all situate the litmus paper straight into the solution.

Keeping samples dry:

Materials might absorb water if left depiction to the air. This is to be avoided, particularly if the example of material is to be weighed precisely. The desiccator is the container to keep samples dry. It contains Drierite as a drying agent; blue Drierite turns pink when it is no longer valuable.

Heating:

Many of the operations carried out in a laboratory experiment need heating. Though there are several process available, reaction states and chemical and physical properties of the materials make certain heating methods preferable to others. Since most organic compounds, particularly the commonly utilized solvents these as hexane and ether, are flammable, a flameless method is generally preferred.

A steam bath is often utilized to heat solutions that boil below about 90oC, or to heat a mixture to approximately 100oC. The laboratories are supplied by steam from a central boiler and in this case use of the steam bath eliminates the hazards of a flame. Connect the steam line to the top inlet on the bath. The bottom inlet is joined to a hose, which drains into the sink. Any water that condenses in the bath while we are using it will drain out. Generally, steam baths contain concentric rings as covers. We can manage the 'size' of the bath via adding or removing rings.

Steam bath:

A steam bath is frequently utilized to heat solutions that boil below about 90oC, or to heat a mixture to just about 100oC. The laboratories are supplied by steam from a central boiler and in this case utilize of the steam bath removes the dangers of a flame. Connect the steam line to the top inlet on the bath. The bottom inlet is connected to a hose that drains into the sink. Any water that condenses in the bath while using it will drain out. Generally, steam baths have concentric rings as covers. We can manage the 'size' of the bath via adding or removing rings.

Hot water bath:

The hot water bath is a helpful device for heating at temperatures below 100oC - simply a beaker or steam bath full of hot water, sometimes from the faucet will be hot enough or it might be necessary to heat the water on a hot plate. A steam bath or hot water bath should be used with flammable substances whenever possible

Sand bath:

A sand bath is a suitable source of heat for micro-scale reactions. A sand bath warms up much more slowly than a steam bath. Unlike the hot water and steam baths, a sand bath can offer temperatures which range from near room temperature at the surface up to 200oC and above, deep in the sand. 

Oil bath:

An oil bath gives an even source of heat whose temperature can be directly controlled. A simple oil bath can be made via heating a dish of oil on a hotplate. The temperature of the oil can be observed via a thermometer. It is most appropriate, in combination via a hotplate, for heating something at a high temperature (depending on the oil utilized) or at steady temperature for an enlarged period of time. The drawback of an oil bath is the long time needed to fetch it to the needed temperature, splattering if it obtains contaminated by water and the possibility of spilling the hot oil.

Hot plate:

The hot plate can be utilized as a source of heat. It tends to be rather inefficient due to poor contact between the top of the hotplate and the bottom of the flask, and it gets a while to warm up. A hotplate is most helpful whenever something must be heated for a long time, and when it employed through an oil bath.

Heating mantle:

A helpful source of heat for reactions, particularly where temperatures over 100oC are needed, is the heating mantle. The heating mantle employed in such laboratories consists of a ceramic shell through embedded electric heating coils. The ceramic bowl will accommodate flasks by volume capacities up to 250mL. Through flasks smaller than 250mL, the mantle can be employed as an air bath or a sand bath, generally the latter. A small layer of sand in the bowl will serve as a medium for conducting heat to the reaction flask. The heating mantle is secure, since it doesn't generate flames. It is rapid. One must be careful, though, not to overheat the heating mantles. Heating mantles must be utilized through variable transformers.

Refluxing: 

Reflux is the procedure of boiling reactants while continually cooling the vapour going back it back to the flask as a liquid. It is utilized to heat a mixture for enlarged periods and at convinced temperatures. The reflux apparatus is given in Figure. A condenser is connected to the boiling flask, and cooling water is circulated to condense escaping vapours. One should always use a boiling stone or a magnetic stirrer to keep the boiling solution from "bumping."

If the heating rate has been correctly adjusted, the liquid being heated under reflux will travel only partly up the condenser tube before condensing. Below the condensation point, solvent will be seen running back into the flask; above it, the condenser will appear dry. The boundary between the 2 zones will be obviously demarcated, and a reflux ring or a ring of liquid will appear there. In heating under reflux, the rate of heating should be adjusted so that the reflux ring is not higher than a third to a half the distance to the top of the condenser. The temperature of a reaction in a refluxing mixture will be about the boiling point of the solvent utilized for the reaction.

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Fig: The Reflux Apparatus

Filtration:

Filtration engages the separation of insoluble solid materials from a liquid. In this operation, the liquid passes during a porous barrier (sintered glass or filter paper) and the solid is maintained via the barrier. The liquid can be made to pass through the barrier via gravity alone, in which case the procedure is termed a gravity filtration. Alternatively, the liquid can be caused to pass through by a combination of gravity and air pressure. Such an operation is called a vacuum or suction filtration. A piece of filter paper and a conical glass funnel to support it are all that are required for gravity filtration. In order to maximize the rate at which the liquid flows through the filter paper, the paper should be folded as indicated by the steps below:

  • The folded paper is then dropped into the funnel
  • The funnel is finest sustained in an iron ring, as given in the figure.
  • The material to be filtered is poured into the filter paper cone, in segments if needed.

For smaller amounts, as in microscale work, a Pasteur dropping pipette can be used instead of the glass funnel, and a tiny piece of cotton or glass wool pushed down into the narrow part of the  tip can serve as the filter.

Fig: The Filtration Method

Folding of filter paper for gravity filtration (Figure 1.3): 

(a)   Fold the filter paper circle (11 cm diameter) in half

(b)   Crease the half to divide it into eight pie-shaped sections; it is easiest to make the creases in the numerical order shown below

(c)   Turn the piece over and pleat it into a fan by folding each pie-shaped section in half in the direction opposite to the previous increases

(d)  Pull the two sides apart.

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Fig:  Steps involved in Folding Filter Paper for Gravity Filtration

Heating of the Reaction Mixture:

Several methods of heating are generally encountered in the laboratory, but  the  ready flammability  of  a broad  range  of  reagents,  coupled by their  volatility,  always  needs vigilance  whenever  heating.  For such causes open flames symbolize an obvious hazard, but hot metal surfaces can as well give increase to dangerous situations. Such can normally only be utilized for heating aqueous solution in open vessels.  Whenever heating beakers, flasks, or flat-bottom vessels by a burner, wire gauze should be situated between the vessel and the flame.  This provides both as a support and as a means of dispersing heat. Burners can as well be utilized for sanitization and reflux methods involving high-boiling-point materials, but in these cases care must be taken to ensure that no flammable vapors come into contact with the flame.

Heating Baths

For  temperatures up  to 100°C,  a water bath  or  steam bath  is generally employed, even though water condensing can be a problem if it is needed to  ensure  anhydrous circumstances within  the  reaction. Water is situated in the vessel, which is heated via means of the flame. For this cause it might be  utilized  for  non-inflammable  liquids  or  for refluxing of  low-boiling-point products, but in this case the presence of a naked flame introduces considerable  risks  of   fire. Such baths are normally equipped by a series of overlapping concentric rings, which can be eliminated to provide the right size of support for the particular vessel being heated.

If  the  laboratory  is  equipped through  a  steam  service,  it  is  suitable  to have  a  series  of steam  baths. On the other hand,  if  the  laboratory  has  no external steam service, to avoid naked flames, an electrically heated bath might  be  utilized  and  fitted  by  a  steady  level device. A resistance attached to temperature regulator heats water in the bath.

For temperatures above 100°C, oil baths are commonly utilized. The bath can be heated through a heating element or on a hotplate. Medical paraffin, glycerol, silicon oil, and cotton seed oil might be employed; it based on the work temperature. Silicon oils bear a low risk of inflammation; they don't provide of fun pleasant odours, and contain along service life. Synthetic thermal liquids are mostly generated on a hydrocarbon basis and exhibit a low viscosity within the suggested working temperature range. Mineral oil is generally utilized for the high temperature range. Unpleasant odours are minimized to the lowest level.

The silicon fluids are almost certainly the best liquids for oil baths, but they are extremely luxurious for common utilize. On the other hand, these fluids can be heated to up to 250°C with no loss or discolouration. In modern equipment, an immersion heating circulator is mounted on to the rear panel of the bath vessel (Figure). This merges a heater, a temperature control, and a circulating pump for temperature uniformity throughout the bath, which is of great benefit in temperature control. It can be shifted from one vessel to another and can be utilized by any tank. The heater and control sensors are sometimes located underneath the bath, therefore guaranteeing easy cleaning.

Heated baths feature external parts to circulate the fluid to an external system.  Heating circulators  (Figure)  are  generally  utilized  for temperature  controlling  external  systems, such  as  density  meters, reaction vessels,  autoclaves,  and viscometers. Powerful pumps provide good heat exchange and optimum temperature accuracy.

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Fig: Schematic (a) Heated Bath

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(b) Heated Circulator Bath

Higher  temperatures  might  be  attained  through  the  support  of  a  bath  of fusible metal alloys (for instance, Rose's metal:2 parts of Bi,1part of Pb, and1part of Sn, melting point 94°C; woods metal: 5parts of Bi, 2parts of Pb,1partof Sn, and1part of Cu, melting point 71°C; and a mixture of Pb  (37%) and Sn  (63%), melting point 183°C). such metal baths should not be employed at temperatures in excess of 350°C owing  to  the  quick oxidation  of  the  alloy.  Metal baths are solid at common temperatures, and for this cause the flask and the thermometer should be eliminated from the bath before the latter solidifies.  

Electric Hot Plates and Electric Heating Mantles

Such items of equipment might as well be employed for heating, even though their utilize for several tasks (for instance, distillation) is to be discouraged. The hot plate/magnetic stirrer are a single device that can heat liquids and stir them with a magnetic stirring bar.  One knob controls the rate of stirring and another controls heating. A stirrer hot plate keeps the solution at a steady temperature while stirring. The built- in magnetic stirrer  permits  efficient  agitation of non-viscous  solutions  by  adding  an appropriately sized magnetic stirrer bar  to  the  liquid  in  the container. It is designed for heating flat-bottomed vessels, such as flasks and beakers, in a temperature range from 40-200°C. Round-bottomed flasks might be heated using a stirrer hotplate through immersing the flask in a flat-bottomed oil bath.  The flat, exposed surface of the hot plate, designed for transferring heat speedily, makes it extremely dangerous when hot.

The electric heating mantel is an extremely convenient method of heating, especially for temperature above 100°C. It consists of an electric resistance embedded within a hemispherical knitted mantle, so that the heat supply is as close to the flask to be heated as possible.  Electric mantels are intended only for heating round-bottomed flasks and can accept a flask of a particular size. All heating mantels are mainly vulnerable to spillage of liquids, and by steady employ this can lay bare the wires within the heating element.

The Bunsen burner

A Bunsen burner, symbolized after Robert Bunsen, is a common piece of laboratory equipment that produces a single open gas flame, which is used for heating, sterilization, and combustion. The device in use today safely burns a continuous stream of a flammable gas such as natural gas (which is principally methane) or a liquefied petroleum gas such as propane, butane, or a mixture of both. The hose barb is attached to a gas nozzle on the laboratory bench by rubber tubing. Most laboratory benches are equipped by multiple gas nozzles connected to a central gas source, as well as vacuum, nitrogen, and steam nozzles. The gas then flows up through the base through a small hole at the bottom of the barrel and is directed upward. There are open slots in the side of the tube bottom to admit air into the stream via the Venturi effect, and the gas burns at the top of the tube once ignited by a flame or spark. The most common methods of lighting the burner are using a match or a spark lighter.

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Fig:  A Bunsen burner

How to use a Bunsen burner

i. Attach the Bunsen burner to a gas tap using a piece of rubber tubing.

ii. Close the needle valve (counter clockwise through burner in standing position) and the air vent (turn chimney clockwise until closed) 

iii. Open the gas valve.  

iv. Open the needle valve 1-2 full turns (clockwise in upright position), listening for the sound of escaping gas. 

v. Using the striker, light the burner.  

vi. The flame of the Bunsen burner should be yellow and irregular in shape.

vii. Open the air-hole gradually. The colour of the flame transforms to blue.

viii. Whenever not using the Bunsen burner for a while, close the air-hole. This will transform the blue flame back to a yellow flame. The yellow flame is the protection flame.

ix. Not at all leave the flame unattended

x. Turn off the gas valve after utilize.

Caution: It is possible for the burner to strike back and begin burning in the stack or hose.  Turn off the gas at the outlet valve, let the burner cool, then close the air vent and repeat the lighting process.

To Adjust the Flame:

1.  Use the needle valve to attain a yellow flame approximately 7.5 cm (3 in.) long.

2.  Open the air vents by turning the chimney counter-clockwise until the flame loses its yellow colour and transforms to a pale blue.

3.  Decrease the gas flow by the needle valve until the flame is about 2.5cm (1 inch) long high with a distinct inner core of a deeper blue.

To Turn Off the Burner:

Turn the handle on the gas outlet valve until it is perpendicular to the outlet nozzle.

Cooling of the Reaction Mixtures:

Methods of Cooling 

It is often needed to cool a reaction, particularly when a reaction is rapid and highly exothermic, or when an intermediate formed in a reaction is thermally labile. Temperatures near 0oC can be maintained through the utilize of an ice-water bath. Temperatures down to about -20oC can be maintained by the use of ice-salt baths formed through the addition of salt (sodium chloride) to ice. Lower temperatures can be sustained by the addition of dry ice (solid carbon dioxide) to various solvents; Very low temperatures can be maintained through the employ of liquid nitrogen (boiling point of -196oC). 

The addition of dry ice or liquid nitrogen to any solvent must be carried out by caution; the addition reasons excessive bubbling via the evaporation of gaseous carbon dioxide or nitrogen. The dry ice or liquid nitrogen is slowly adjoined to the solvent contained in a wide-mouth vacuum flask or any other suitably insulated container until a slush is shaped. Additional quantities of dry ice or liquid nitrogen are added to maintain the slush. Solvents of low flammability are recommended for utilize in such cooling baths, even though several of the most useful cooling baths employ highly flammable solvents since of the desired accessible temperatures and due precautions must be exercised. 

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Table: Useful combinations of Coolant and Solvent with their various Temperatures 

Caution must as well be utilized in the employed of liquid nitrogen in that if the cooled system is not shielded from the air, liquid oxygen ( with boiling point of -176oC) might be condensed in the apparatus which on warming may consequence in the formation of extremely high internal pressures. Useful combinations of coolant and solvent are listed in the table above.

Sometimes a heated circulator is combined with a refrigerator chapter. Powerful, quiet-running cooling compressors cool them. Refrigerated circulators  and  cryostats  are  mainly  used  when  below-ambient temperatures  must  be  reached  or  maintained  or  when it  is  wished  to cycle  between  two  temperatures  at  a  control  rate.  Refrigerated circulators and cryostats feature a wide temperature range (for example,-90°C to150°C).  They are especially suited for controlling open and closed circuits due to their extremely powerful pressure/suction pumps.

These coolers are appropriate for individual cooling applications:

  • For cooler through smaller volumes down to -90°C,
  • For removing reaction heat, and
  • For replacing tap-water cooling.

The  lowest  consistent  temperature  based on  the  quantity  of  liquid,  the sort of liquid and its viscosity, and the bath insulation.

Stirring:

Most of the chemical reactions require to be stirred to combine the reagents or to aid heat convey. There are 3 major ways to agitate a mixture: via hand, through a magnetic stirrer, and by a mechanical stirrer; but whenever a steady stirring is required for a maintained period, a stirrer motor should be used. Magnetic stirring has many applications, but the most important is probably stirring in closed systems. Magnetic stirrers are easy to utilize, and have the benefit that they are often joined through a hotplate. Hot-plate stirrers permit us to keep solutions at a steady temperature while stirring. A rotating field of magnetic force is employed to induce a variable-speed stirring action. The principle of magnetic stirring is following in Figure. The  stirring  is  accomplished through  the  assist  of  a small  magnetic  bar-coated  through  Teflon  or  Pyrex-  that  is  obtainable  in diverse sizes and forms depending on the volume and the viscosity of the liquid. Even though bar magnets can be attained by several sorts of coating but only Teflon-coated stirrers are universally helpful (Figure).

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Fig: Schematic (a) Magnetic Stirrer and (b) Magnetic Bars

There are 2 sorts of magnetic stirrers: mechanical and electronic. Most manufacturers of magnetic stirrers use a mechanical approach. They utilize steel and aluminum for the structural material and outdated techniques of controlling the speed. Adjusting the stirring speed with a mechanical stirrer is fairly inaccurate.

Electronic  controls permit  the  stirrer  to  control  the  speed  by  greater accuracy, and if  the motor  is  running at  the maximum operating  speed through  a  load,  and  the  load  is  suddenly eliminated,  the  circuitry  will  not permit the motor to amplify in speed, that would harm the chapter. It is beneficial to stir in 2 directions to gain homogeneous consequences. This can't be done through a mechanical stirrer. Liquid behaviour is important to consider when we begin a mixing project. If the reaction mixture is very viscous or heterogeneous- with a large  amount  of  suspended solid - the magnetic  stirrer motor,  through  its relative  low  torque, will  not  be  suitable  for the purpose, and in these cases a mechanical stirrer should be used.

The stirrer is basically attached to a motor via a flexible association made out of a kind of pressure tubing.  A typical collection of mechanical stirrers is demonstrated in the Figure. The stirrer - either rod or paddle - might be made of glass, metal, or Teflon, depending on the trait of the liquid to be stirred.

 

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Fig: Several Useful Types of Mechanical Stirrer

Proper selection of a suitable motor and type of stirrer requires that you know certain application variables:

  • Container volume
  • Liquid viscosity (the viscosity of most liquids varies inversely with temperature)
  • torque requirements (the rotation force required of the mixer computed in lbs*for N*m) Horse power (hp) must (the efficiency need of the mixer motor via regard to torque and to rotation speed)
  • Rotational speed (measured in rpm). The consequence of the liquid's viscosity can present difficulties whenever the liquid is subjected to a force. Different kinds of liquid display characteristics when force is applied. The most general kinds of viscous liquids are:

Newtonian liquids: viscosity remains steady regardless of transforms in shear rate or agitation. Liquids showing Newtonian behaviour enclose water, hydrocarbons, mineral oil, and syrup.

Pseudo plastic liquids: viscosity reduces as shear rate amplifies, but primary viscosity might be sufficiently great to prevent mixing. Typical pseudo plastic liquids are gels and latex paints.

Dilatant liquids: viscosity amplifies as shear rate rises. Mixers can bog down and stall after initially mixing such liquids. Dilatant liquids include slurries, clay, and candy compounds.

Thixotropic liquids: viscosity decreases as shear rate or agitation increases, but whenever agitation is prevented or reduced, hysteresis take place and viscosity increases. Often the viscosity will not return to the initial value. Thixotropic liquids include soaps, tars, vegetable oils, inks, glue, and peanut butter.

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