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Decanting, gravity filtration and liquid transferring

G.Patton

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Introduction.

In this topic, I'll overview some important laboratory techniques. Hard to overestimate the useful and helpful importance of these operations. Decanting, gravity filtration and liquid transferring are simple but take some attention and a little laboratory skill. Practicing them is the best way to learn how to behave in laboratory and do the simplest manipulations. After you can meet with more complicated themes, such as Suction filtration, Recrystallization and hot filtration, and, finally, Distillation and distillation systems. If you have low laboratory knowledge, and you have a problem with names of glassware in this or linked above themes, you may use this topic as a tip.​

Decanting.

When there is a need to separate a solid-liquid mixture, occasionally it is possible to pour off the liquid while leaving the solid behind. This process is called decanting, and is the simplest separation method. Decanting is often used to remove hydrated sodium sulfate (Na2SO4) from an organic solution. The sodium sulfate frequently clings to the glassware (Fig.1 a), enabling the liquid to be poured off (Fig.1 b). If liquid is to be poured into a small vessel, a funnel could be used or liquid poured down a glass stirring rod to direct the flow (Fig.1 c). Unfortunately, there are many mixtures that do not decant well.​
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a) Sodium sulfate sticking to the glassware, b) Decanting a solid-liquid mixture, c) Using a glass stirring rod during decanting.​

Decantation is a process that separates components of a mixture based on differences in density. You may encounter decantation in everyday life with wine or spirits, but it’s also a powerful technique in chemistry for separating a solid from a liquid or isolating two immiscible liquids. Decantation is easy, but one disadvantage is that it doesn’t allow for perfect separation of mixture components. A small amount of one component gets lost when collecting the other component, or else the collection goes too far and the collection gets contaminated with the second component.

How Decantation Works.
Decantation involves two steps:​
  • Sedimentation: Sedimentation uses gravity or a centrifuge to separate mixture components based on density.​
  • Decanting: Decanting is pouring or siphoning off the top component of a mixture or draining the bottom component.​
A solid component is called “sediment” (or “pellet” when centrifugation is used). The liquid component that is collected is called “decant.”​
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The basic principle of decantation is that heavier (denser) substances sink, while lighter (less dense) substances float. In its simplest form, decantation uses gravity to separate a solid and liquid or two immiscible liquids. The lighter component is poured or siphoned off the top of the mixture. Alternatively, a separatory funnel drains the heavier component.

Small volumes are decanted using test tubes inclined at 45-degrees in a test tube rack. The angle lets heavier particles slide down the tube, while lighter particles rise to the top. The angle also makes pouring the lighter component easier. Pouring off the liquid is easier if it’s poured along a stirring rod. The decantation process is slower if the test tubes are kept vertical because the heavier component can form a plug and block lighter particles from rising.

Centrifugation speeds decantation by applying centrifugal and centripetal force. Basically, the artificial gravity separates mixture components more quickly. Centrifugation compacts solid components into a pellet. Pouring the liquid away from the pellet results in less loss than in simple decantation. A separatory funnel decants components of mixtures of immiscible liquids. One component floats on top of the other. The funnel drains the component at the bottom of the funnel.​

Filtering Methods.

There are many methods used to separate a mixture containing a solid and liquid. If the solid settles well, the liquid can sometimes be poured off (decanted). If the solid has very small-sized particles or forms a cloudy mixture, the mixture can sometimes be centrifuged or passed through a filter pipette (on the microscale, < 5ml). The most common methods of solid-liquid separation in the organic lab are gravity and suction filtration. Gravity filtration refers to pouring a solid-liquid mixture through a funnel containing a filter paper, allowing the liquid to seep through while trapping the solid on the paper (Fig.1 a). Suction filtration is a similar process, with the difference being the application of a vacuum beneath the funnel to pull liquid through the filter paper with suction (Fig.1 b).​
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a) Gravity filtration, b) Suction filtration.​

Gravity and suction filtration have pros and cons, but what helps decide which method to use is generally whether the solid or filtrate is to be retained. The "filtrate" refers to the liquid that has passed through a filter paper (as indicated in Fig.1 a). Gravity filtration is typically used when the filtrate is retained, while suction filtration is used when the solid is retained. Gravity filtration is preferred when the filtrate is retained as suction has the potential of pulling small solid particles through the filter paper pores, potentially producing a filtrate contaminated with the solid compound. Suction filtration is preferred when the solid is retained, as gravity filtration is much less efficient at removing residual liquid from the solid on the filter paper.​

Gravity Filtration.

When there is a need to separate a solid-liquid mixture, it is common that the particles are so fine that they swirl and disperse when the flask is tilted. These mixtures cannot be decanted, and an alternative method is gravity filtration. Gravity filtration is generally used when the filtrate (liquid that has passed through the filter paper) will be retained, while the solid on the filter paper will be discarded. A common use for gravity filtration is for separating anhydrous magnesium sulfate (MgSO4) from an organic solution that it has dried (Fig. b). Anhydrous magnesium sulfate is powdery, and with swirling in an organic solvent creates a fine dispersal of particles like a snow globe.​
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a) An organic solution dried with anhydrous magnesium sulfate, b) Gravity filtration of this solution.​

To gravity filter a mixture, pour the mixture through a quadrant-folded filter paper (Fig. 4) or fluted filter paper in a funnel and allow the liquid to filter using only the force of gravity (Fig. 3 c). It is best to pour as if attempting to decant, meaning to keep the solid settled in the flask for as long as possible. When solid begins to pour onto the filter paper, it has the possibility of clogging the filter paper pores or slowing filtration. After finished pouring, rinse the solid on the filter paper (and in the flask) with a few portions of fresh solvent to remove residual compound adhering to the solid.​
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Transferring liquids.

Pouring Liquids.
When transferring liquids with volumes greater than 5ml, they can be poured directly into vessels. Graduated cylinders and beakers have an indentation in their mouth, so they can be poured controllably as long as the two pieces of glass touch one another (Fig.5 a). If pouring from an Erlenmeyer flask, or transferring a liquid into a vessel containing a narrow mouth (e.g. a round bottomed flask), a funnel should be used. Funnels can be securely held with a ring clamp (Fig.5 b), or held with one hand while pouring with the other (Fig.5 c).​
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a) Pouring liquid, b) Pouring into a funnel held with ring clamp, c) Pouring into a funnel held by hand.​

Comments Regarding Measurements.
To determine a meaningful yield for a chemical reaction, it is important to have precise measurements on the limiting reactant. It is less significant to be precise when manipulating a reagent that is in excess, especially if the reagent is in several times excess.

A portion of the liquid measured by a graduated cylinder always clings to the glassware after pouring, meaning that the true volume dispensed is never equivalent to the markings on the cylinder. Therefore, graduated cylinders can be used for dispensing solvents or liquids that are in excess, while more accurate methods (e.g. mass, calibrated pipettes or syringes) should be used when dispensing or measuring the limiting reactant. A graduated cylinder may be used to dispense a limiting reactant if a subsequent mass will be determined to find the precise quantity actually dispensed.​
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a) Cork ring on an analytical balance, b) Beaker on a pan balance.​

When determining the mass of a vessel on a balance, it's best to not include the mass of a cork ring (Fig.6 a) or other support (e.g. the beaker in Fig.6 b). A cork ring might get wet, have reagents spilled on it, or have pieces of cork fall out, leading to changes in mass that cannot be accounted for. Beakers used to support flasks can get mixed up, and every 100 ml beaker does not have the same mass. It is also best to transport vessels containing chemicals to the balance in sealed containers, to minimize vapors and prevent possible spillage during transport.

Using Pasteur Pipettes.
Pasteur pipettes (or pipets) are the most commonly used tool for transferring small volumes of liquids (< 5ml) from one container to another. They are considered disposable, although some institutions may clean and reuse them if they have a method for preventing the fragile tips from breaking.​
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a) Short and long pipettes, b) 1ml marked on a pipette with a permanent marker.​

Pasteur pipettes come in two sizes (Fig.7 a): short (5.75") and long (9"). Each can hold about 1.5ml of liquid, although the volume delivered is dependent on the size of the dropper bulb. The general guideline that "1mL is equivalent to 20 drops" does not always hold for Pasteur pipettes, and may be inconsistent between different pipettes. The drop ratio for a certain pipette and solution can be determined by counting drops until 1mL is accumulated in a graduated cylinder. Alternatively, a pipette can be roughly calibrated by withdrawing 1mL of liquid from a graduated cylinder and marking the volume line with a permanent marker (Fig.7 b).​
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a and b) Creating suction with a Pasteur pipette, c) Delivering liquid from a Pasteur pipette, d) Incorrect delivering of reagent (liquid should not touch the sides of the glass).​

To use a pipette, attach a dropper bulb and place the pipette tip into a liquid. Squeeze then release the bulb to create suction, which will cause liquid to withdraw into the pipette (Fig.8 a and b). Keeping the pipette vertical, bring it to the flask where it is to be transferred, and position the pipette tip below the joint of the flask but not touching the sides before depressing the bulb to deliver the material to the flask (Fig.7 c). The bulb can be squeezed a few times afterward to "blow out" residual liquid from the pipette.

If the receiving flask has a ground glass joint, the pipette tip should be below the joint while delivering so that liquid does not splash onto the joint, which sometimes causes pieces to freeze together when connected. If the pipette is to be reused (for example is the designated pipette for a reagent bottle), the pipette should be held, so it does not touch the glassware, where it may become contaminated by other reagents in the flask (Fig.7 d).​

Using Calibrated Pipettes.

Calibrated Plastic Pipettes.
When some precision is needed in dispensing small volumes of liquid (1-2 ml), a graduated cylinder is not ideal as the pouring action results in a significant loss of material. Calibrated plastic pipettes have markings at 0.25 ml increments for a 1 ml pipette, and are economical ways to dispense relatively accurate volumes.​
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a) 1ml calibrated plastic pipette, b) Suction of liquid, c) Depressing bulb to required volume (the arrow points to the 1ml mark), d and e) Transfer of liquid.​

To use a calibrated plastic pipette, withdraw some liquid to be transferred into the bulb as usual (Fig.9 b). Then squeeze the bulb just enough so that the liquid drains to the desired volume (Fig.9 c), and maintain your position. While keeping the bulb depressed, so the liquid still reads to the desired volume, quickly move the pipette to the transfer flask (Fig.9 d), and depress the bulb further to deliver liquid to the flask (Fig.9 e).

Calibrated Glass Pipettes.
When a high level of precision is needed while dispensing liquids, calibrated glass pipettes (volumetric or graduated) can be used. Volumetric pipettes have a glass bulb at the top of their neck, and are capable of dispensing only one certain volume (for example, the top pipette in Fig. 10 is a 10.00 ml pipette). Graduated pipettes (Mohr pipettes) have markings that allow them to deliver many volumes. Both pipettes need to be connected to a pipette bulb to provide suction.​
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The volume markings on a graduated pipette indicate the delivered volume, which may seem a bit "backward" at first. For example, when a graduated pipette is held vertically, the highest marking is 0.0 ml, which indicates that no volume has been delivered when the pipette is still full. As liquid is drained into a vessel, the volume markings increase on the pipette, with the lowest marking often being the total capacity of the pipette (e.g. 1.0 ml for a 1.0 ml pipette).

Graduated pipettes can deliver any volume of liquid, made possible by differences in the volume markings. For example, a 1.0 ml pipette could be used to deliver 0.4 ml of liquid: a) Withdrawing liquid to the 0.0 ml mark, then draining and delivering liquid to the 0.4 ml mark, or b) Withdrawing liquid to the 0.2 ml mark and draining and delivering liquid to the 0.6 ml mark (or any combination where the difference in volumes is 0.4 ml).

It is important to look carefully at the markings on a graduated pipette. Three different 1ml pipettes are shown in Fig.11 a. The left-most pipette has markings every 0.1 ml, but no intermediary markings, so are less precise than the other two pipettes in Fig.11 a. The other two pipettes differ in the markings on the bottom. The lowest mark on the middle pipette is 1 ml, while the lowest mark on the right-most pipette is 0.9 ml. To deliver 1.00 ml with the middle pipette, the liquid must be drained from the 0.00 ml to the 1.00 ml mark, and the final inch of liquid should be retained. To deliver 1.00 ml with the right-most pipette, liquid must be drained from the 0.00 ml mark completely out the tip, with the intent to deliver its total capacity.​
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a) Bottom of pipettes, b) Top of pipettes​

Pipettes are calibrated "to-deliver" (TD) or "to-contain" (TC) to the marked volume. Pipettes are marked with T.C. or T.D. to differentiate between these two kinds, and to-deliver pipettes are also marked with a double ring near the top (Fig.12 b). After draining a "to-deliver" pipette, the tip should be touched to the side of the flask to withdraw any clinging drops, and a small amount of residual liquid will remain in the tip. A "to-deliver" pipette is calibrated to deliver only the liquid that freely drains from the tip. However, after draining a "to-contain" pipette, the residual liquid in the tip should be "blown out" with pressure from a pipette bulb. "To-contain" pipettes may be useful for dispensing viscous liquids, where solvent can be used to wash out the entire contents.​
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a and b) Applying suction to the pipette, c) Liquid withdrawn above the desired volume, d) The bulb is released and tip of the pipette sealed with a finger to maintain the liquid's position.​

In this section are described methods on how to use a calibrated glass pipette. These methods are for use with a clean and dry pipette. If residual liquid is in the tip of the pipette from water or from previous use with an alternative solution, a fresh pipette should be used. Alternatively, if the reagent is not particularly expensive or reactive, the pipette can be "conditioned" with the reagent to remove residual liquid. To condition a pipette, rinse the pipette twice with a full volume of the reagent and collect the rinsing in a waste container. After two rinses, any residual liquid in the pipette will have been replaced by the reagent. When the reagent is then withdrawn into the pipette, it will not be diluted or altered in any way.

To use a calibrated glass pipette:
  1. Place the pipette tip in the reagent, squeeze the bulb and connect it to the top of the pipette (Fig.12 a and b).​
  2. Partially release pressure on the bulb to create suction, but do not fully release your hand, or you may create too great a vacuum, causing liquid to be violently withdrawn into the pipette bulb. Suction should be applied until the liquid rises to just past the desired mark (Fig.12 c).​
  3. Break the seal and remove the pipette bulb, then quickly place your finger atop the pipette to prevent the liquid from draining (Fig.12 d).​
  4. With a slight wiggling motion or slight release of pressure from your finger, allow tiny amounts of air to be let into the top of the pipette in order to slowly and controllably drain the liquid until the meniscus is at the desired volume (Fig.13 a shows a volume of 0.00ml).​
  5. Holding the top of the pipette tightly with your finger, bring the pipette to the flask where the liquid is to be delivered and again allow tiny amounts of air into the top of the pipette to slowly drain the liquid to the desired mark (Fig.13 b and c shows the delivered volume to be slightly below 0.20ml).​
  6. Touch the pipette tip to the side of the container to dislodge any hanging drops and remove the pipette.​
  7. If liquid was drained to the bottom of the pipette with a T.C. pipette, use pressure from a pipette bulb to blow out the residual drop. Do not blow out the residual drop when using a T.D. pipette.​
  8. If a volumetric pipette is used, the liquid should be withdrawn with suction to the marked line above the glass bulb (indicated in Fig.13 d). The liquid can be drained into the new container with your finger fully released from the top. When the liquid stops draining, the tip should be touched to the side of the flask to withdraw any clinging drops, but the residual drop should not be forced out (similar to a T.D. pipette).​
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a) Red liquid to the 0ml mark, b) delivering reagent, c) Final volume, d) Volumetric pipette (arrow indicates the fill mark).​

Dispensing Highly Volatile Liquids.
When attempting to dispense highly volatile liquids (e.g. diethyl ether) via pipette, it is very common that liquid drips out of the pipette even without pressure from the dropper bulb! This occurs as the liquid evaporates into the pipette's headspace, and the additional vapor causes the headspace pressure to exceed the atmospheric pressure. To prevent a pipette from dripping, withdraw and expunge the liquid into the pipette several times. Once the headspace is saturated with solvent vapors, the pipette will no longer drip.​

Pouring Hot Liquids.
It may be difficult to manipulate a vessel of hot liquid with your bare hands. If pouring a hot liquid from a beaker, a silicone hot hand protector can be used (Fig.14 a) or beaker tongs (Fig.14 b and c).​
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a) hot hand protector, b and c) Beaker tongs, d) paper towel holder.​

When pouring a hot liquid from an Erlenmeyer flask, hot hand protectors can also be used, but do not hold the awkward shape of the flask very securely. Pouring from hot Erlenmeyer flasks can be accomplished using a makeshift "paper towel holder". A long section of paper towel is folded several times in one direction to the thickness of approximately one inch (and secured with lab tape if desired, Fig.15 a). This folded paper towel can be wrapped around the top of a beaker or Erlenmeyer flask and pinched to hold the flask (Fig.14 d and Fig. 15 b).

When pouring hot liquid from an Erlenmeyer flask, the paper towel holder should be narrow enough that the towel does not reach the top of the flask. If it does, liquid will wick toward the paper as it is poured, thus weakening the holder and also removing possibly valuable solution (Fig. 15 c). When the paper towel is a distance away from the top of the flask, liquid can be poured from the flask without absorbing the liquid (Fig. 15 d).​
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a) Paper towel holder, b) Holding an Erlenmeyer flask with a paper towel holder, c) A too-wide holder, causing liquid to wick onto the paper as it is poured, d) A narrower holder, which pours without wicking.​

Conclusion.

Hope this manual gave you necessary information which you had been looking for. I described three methods as good as I can. If you still have questions, you may ask me here.​
 
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