The use of volumetric glassware Objective of this lab is to practice volumetric measurements using various glass wares. The proper use of these units will be demonstrated by your instructor. Graduated cylinder or or Measuring Cylinder is is used to measure volume of liquid with ease. It is available in various sizes and made of different materials like glass and plastics. It has a lot of closer graduation markings on the cylinder wall, so more accurate volume measurements are generally achieved.
A Beaker is is a specialized measuring glassware used to pour and hold reagents and chemicals. Though it is graduated it is generally used to measure approximate volumetric quantities of liquid. It is designed to contain (TC). Buret is is a very long glass volumetric device with graduation markings on the wall and a tap at the base. It is designed to deliver (TD). The buret is securely held using buret clamps on a special stand. Most burets are designed to deliver to the nearest 0.01 mL. They are marked in tenths of a milliliter, so the user then estimates to the nearest hundredth of a milliliter.
Pipet is is a long glass tube to hold and to deliver (TD) quantitatively exact volumes of a liquid from one container to another. There are many type of pipets, but two of them commonly used in labs. Volumetric pipets has a single graduation mark (etch mark) calibrated at a particular temperature. It is used to deliver one specific volume accurately. Mohr or graduated pipet has multiple graduation marks so that it is useful to deliver a range of volumes.
The other lab-ware you are going to use in this lab
An electronic balance is used to measure mass (the quantity of matter).It is done by comparing an object of unknown mass with an object of known mass. The digital readout directly gives the mass of the object. Pipet pump is a mechanical safety device to lift liquid chemicals chemicals into the pipet. Rubber bulb and Pipet pump are commonly used in the laboratory.
Wash bottles are used to clean lab glassware and other general laboratory equipment. It is filled with water or a solvent, squeezed to pour over the glassware being cleaned. It is made up of special chemically inert plastics.
A funnel is is used to help transfer of liquids and fine powder chemicals from one container to another. It is conical in shape with a long stem is available in glass and plastic.
Measuring Mass: An electronic balance measures mass automatically. It is good practice to ‘tare’ or reset the
balance to read zero before any measurement is made.
One must remember a few points before using a balance.
It is an electronic, sensitive and expensive instrument. Always keep the balance clean and dry.
Never weighs or place any chemicals directly on the pan. It can react with pan and corrode it.
Gently place your object in the middle of the pan. No banging!! Make sure there is no water or chemicals stick at the bottom of the glassware or container.
Any spilled chemicals on the pan must be removed immediately using a brush or soft tissue. If necessary, take the assistance of the lab staff/Instructor to clean.
Never measure any hot objects on a balance. The heat can damage the electronic circuits and damage the balance. Hot objects can also cause air currents that can make the digital readout unstable.
There are two types of mass measurement methods generally used; measuring mass directly and determining the mass by difference. Measuring the mass directly . This is the simplest type of measurement where the object is placed directly on the
balance pan. The digital readout gives the mass in few seconds. For general use the direct reading of the mass readout is good enough. Measuring the mass by difference. For very accurate results ‘mass by difference’ method is used . In this method the
mass of the empty container is subtracted from the combined mass of the container and the substance. This helps to eliminate the calibration and systematic errors associated with the balance. To use an electronic balance, tare it to show a zero mass reading. Place the object gently in the middle of the pan, wait a few seconds for a stable reading and read the mass from the screen. Graduated Cylinder or Measuring Cylinder
Graduated cylinders are most used device to make moderately precise measurements of volume. To read the volume, lift the cylinder and hold at the top so it hangs straight down. To read the volume correctly, it is very important that your eye must be on the same level with the bottom of the meniscus. Meniscus is the curved surface caused by interaction between the molecules in the liquid and the molecules of the container wall. Beaker
Beakers are designed for low-volume applications to hold and heat liquids. Though there are graduation marks on it, but it is not generally used for volumetric applications. Buret
Buret is used to deliver variable, precisely-measured volumes of liquid. Buret is a long open tube with multiple graduation marks and a tap (stopcock ) at end of the tube. Buret is typically clamped onto a Buret stand for support. It is very easy to use. Buret is cleaned with distilled water, rinsed few times with the solution of interest and then filled. It is good practice to make sure to fill the buret without any bubbles or air voids inside the tip and to check the tap (stopcock ) for leaks before the start of the experiment.
Burets are read from top to bottom. As in the case of graduated cylinder, the buret is lifted to the eye level and bottom tip of the meniscus on the scale is recorded as initial level . The final level is recorded after the desired amount of liquid has been dropped out of buret. The difference between the two levels ( final level - initial level ) is the volume of liquid that has been delivered. Pipet
Pipet is a delicate volumetric device so handle it with care. Before using your pipet, be sure it is clean and without any defects. To ensure accurate volume measurements, rinse the pipet with distilled and then with the solution you will pipet. One must remember a few points before using a pipet.
One should never pipet a chemical directly from a reagent bottle.
Pipet tip must be clear and without any cracks. The pipet is always handled vertically.
Never fill a pipet by mouth. Always use a pipet bulb or pump to pull up liquids.
Do not force the bulb or pump onto the mouth of the pipet.
Do not allow the liquid to enter the pump.
Do not attempt to fill the pipet by running water from the tap directly into the mouth of the pipet.
Pipetting involves two steps. The first step is to draw liquid into a pipet and hold. The next step is to allow the liquid to drain from the pipet to the container in a controlled manner. Your Instructor will demonstrate the pipetting operation by pumps. A pump is used to fill the pipet . There are two types of pipet pumps are commonly used; the 3-way rubber bulb and the pipet pump. 3-Way rubber bulb: In rubber bulb pump the liquid flow is controlled with three valves marked A, S and E for aspiration, suction and emptying. Place the tip of the pipet below the solution's surface in the beaker.
Squeeze the rubber bulb while pressing valve- A to create vacuum inside the rubber bulb and place it over the mouth of the pipet. Do not force the bulb onto the mouth of the pipet. Gradually release vacuum by pressing the valve- S on the bulb and allow the liquid to be drawn up a little higher than the desired volume. By gentle pressing of the valve- E the solution can be emptied or adjusted to the etch-mark on the pipet. Pipet pump: The pipet pump works like a syringe in which the suction of the liquid is controlled by the rotation of
the piston wheel with a thumb. It is very easy to operate with one hand. Rotation of the thumbwheel draws or dispenses the liquid with precision and ease. To dispense the content the release lever on the side is pressed.
It is a good practice to touch the tip of the pipet to the inside of the beaker to remove the last drop hanging from the tip.
While drawing up solution, make certain that the pipet tip always remains below the surface of the liquid. If not, air can rush in and liquid will flow into the pipet bulb destroying it.
Make sure there is enough solution in y our beaker/container to completely fill the pipet.
Experimental Procedure and calculations
In this exercise, volume of a certain quantity of liquid is measured (observed volume) by using a graduated cylinder, beaker, buret and pipet. The mass of the liquid transferred to a container is determined in each case. The theoretical volume of the liquid transferred is calculated using the density at the room temperature. The absolute value of the difference between the theoretical and observed volume gives the volume error (∆V) for that particular measuring device. Volume = mass/density
∆V = |Volume (observed) – Volume(theoretical) |
Graduated cylinder:
1. Find out the mass of an empty measuring cylinder. 2. Add about 10mL of distilled water and record the actual reading. 3. Find out the mass of cylinder and water 4. From the mass of water, calculate the theoretical volume of the water present in the cylinder. (volume=mass/density ) Beaker: 1. Find out the mass of an empty beaker. 2. Add distilled water to one of the graduation mark and record the reading. 3. Find out the mass of beaker and water 4. From the mass of water, calculate the theoretical volume of the water present in the beaker. Pipet: 1. Find out the mass of an empty beak er. 2. Using a pipet transfer 10mL of distilled water to the beaker. 3. Find out the mass of beaker and water 4. From the mass of water, calculate the theoretical volume of the water transferred by pipet. Buret: 1. Find out the mass of an empty beaker. 2. Clean and fill a buret with distilled water. Record the initial buret reading. 3. From buret, transfer about 10 ml of distilled water to the beaker. 4. Find out the mass of beaker and water 5. From the mass of water, calculate the theoretical volume of the water transferred to the beaker.
Calculate the percentage volume error for each instrument.
Percentage volume error = 100 * [ (Vol. (observed) – Vol.( theoretical) ) / Vol.( theoretical) ]
The instrument with least volume error is considered the best for volumetric measurement. Based on your data arrange them in an order from the best to worse.
