Basics Review and Calibration of Volumetric Glassware

In this experiment you will be exposed to a variety of important concepts related to quantitative experimentation, including the proper use of glassware, analytical balances and statistics. You will calibrate a 25 mL pipet, that is, you will experimentally determine what volume a pipet really delivers. You will also calibrate two 100 mL volumetric flasks. You will use these pieces of calibrated glassware and their calibrated values throughout the remainder of the quarter. Construct a table at the front of your laboratory notebook to record this data.

1. The Analytical Balance

1.1 Electronic Analytical Balances

The user of an analytical balance should know how to determine whether it is operating properly and how to make the simple adjustments that are occasionally necessary. A brief overview of analytical balances can be found in Section 2-3 of Harris. The newer Mettler balances which we will be using in Chem 251 should not require adjustment by the student. If you notice any inconsistencies in the performance of the balances, please notify either the instructor or the teaching assistant. It is important that an analytical balance be treated with the care due any precision instrument.

1.2 Routine Weighing on an Analytical Balance

Weighing by difference will be the weighing technique used in this class. To use this technique, put sufficient material for all the samples to be weighed into a vessel. Weigh the vessel and contents and record the reading. Remove the vessel from the balance. Using a spatula, quantitatively transfer small amounts of material to a clean beaker or flask and weigh the vessel precisely again. The difference between the two weighings is the weight for the first sample. The weight of the second sample is obtained in the same manner as that of the first, but since the initial weight is already known, only one additional weighing is needed. Thus, three samples require only four weighings.

Some balances, such as the Mettler balances you will use, are equipped with a built-in compensator (taring mechanism) that enables the operator to set the balance to zero with an empty vessel on the pan. In this way, a sample can be weighed directly into a beaker or flask with only one weighing (excluding the zero reading) per sample. While this is the most efficient method, it is also less precise. In addition, weighing by difference enables hygroscopic or volatile samples to be weighed with little exposure to the atmosphere (the cap remains on the weighing bottle except for the time it takes to rapidly transfer some to the empty flask). Furthermore, weighing by difference prevents any spillage of chemicals to ever occur INSIDE the balance. Beware that the TA has been instructed to deduct points from a laboratory report grade if they see a student trying to use a spatula to place chemicals in a weighing container that is inside a balance.

Potential sources of error in weighing with a constant-load balance include inaccurate weights, shifts in balance zero or sensitivity, and changes in the sample. Errors from weight changes in the sample include absorption of moisture (especially serious when a hygroscopic material is being weighed), volatilization, air currents from a hot sample or container, and static charge on containers.

Exercise I: Use of the Balance

Never change balances in the middle of an experiment. Write the serial number down (located on a plastic "pull-up" flap at the top of the balance) and complete the following operations.

Weighing errors: Weighing errors are often brought about by evaporation of water from wet samples, incorporation of water in hygroscopic samples, and mishandling of objects (fingerprints do have weight!).
1.    Weigh a small beaker containing a few milliliters of water on the Mettler electronic balance. Measure the mass of the water (and beaker) by observing the reading at 30 second intervals for four minutes (do not wait for the reading to "stabilize").

2.    Alternatively, obtain a hygroscopic sample and watch the uptake of water from the air.
 

2. The power of spreadsheets for data manipulation and graphing: microsoft excel

In this exercise you will use a Microsoft Excel spreadsheet to graph the water evaporation data obtained above and determine the rate of evaporation (in milligrams per minute).
1.    Read Section 2-10 in Quantitative Chemical Analysis to introduce yourself to Excel if you are unfamiliar with this software.

2.    Enter the water evaporation data by placing the time (in seconds) in Column B of the spreadsheet and labeling the column appropriately.

3.    Label the adjacent Column C as time (minutes). In the cell adjacent to the "0" entry in Column B type the formula "=B5/60" if, for example, the "0" entry was made in cell B5.

4.    Copy this cell using the "Copy" command under the "Edit" option on the toolbar. Next highlight the remaining cells in Column C and use the "Paste" command, also found under the "Edit" option on the toolbar, to "fill-in" these cells. You should have now successfully converted the time in seconds into time in minutes (check to make sure the numbers are correct).

5.    Enter the mass data in Column D and label the column "Mass (milligrams)".

6.    Make a scatter plot of mass as a function of time by:

• Highlighting the data in Columns C and D
• From the Insert menu select "Chart"
• From "Chart Type" select "XY (Scatter)" then click the "Next" button
• Work your way through the Chart Wizard creating labels for your axes and a title for your graph
• When finished, the graph should appear in your spreadsheet

7.    Determine the linear fit to the data by:

• Single-clicking the mouse on the chart
• From the Chart menu select "Add Trendline"
• Under the Type page select "Linear"
• Under the Options page click on "Display equation on chart"

8.    From the linear fit provided by Excel determine the rate of water evaporation in milligrams per minute.

 

4. Volumetric Measurements
For making accurate measurements in analytical procedures, next in importance to the balance is volumetric equipment. In this handout, volumetric flasks, pipets, burets, and graduated cylinders are discussed. The experimental procedure includes gravimetric calibration of a pipet to provide checks on the accuracy of the markings as well as on proper technique. The few minutes required to learn the correct use of this equipment will save time in the long run and give better results. For example, a typical experienced person using incorrect technique takes about 10 sec to read a buret and will have a standard deviation in the readings of 0.02 mL or more. A person of the same experience using correct technique takes about 6 sec and will have a standard deviation in the readings of 0.01 mL.

Whenever you make chemical measurements, you need to understand the performance of your instruments and equipment. For example, although a pipet may be labeled "1 mL", this does not guarantee that it will deliver exactly 1mL. If you do not know the actual volume, you will be introducing an error into every operation that you perform with that pipet. Pipets are marked to deliver a known volume of water at a defined temperature, usually 20 or 25 degrees C.

4.1 Volumetric Flasks

Volumetric flasks are designed to contain a specified volume of liquid. Rather than technique, the principal source of error is variation in temperature, which causes enough expansion or contraction of aqueous solutions to give errors on the order of 0.1% per 5 degrees C.

Before use, a volumetric flask should be cleaned thoroughly. The solution to be diluted or the solid to be dissolved is then transferred to it with the aid of a funnel. Often it is more convenient for a solid to be dissolved in a beaker or a conical flask first, and then the resulting solution transferred quantitatively. In the most accurate work, the flask is filled to just below the mark and immersed in a water bath at the calibration temperature before final volume adjustment. Alternatively, the temperature of the solution may be taken and a correction made. In careful work droplets of solvent above the meniscus may be removed with a lintless towel after the meniscus has been adjusted but before mixing. Finally, the solution is mixed thoroughly by inverting, shaking, and turning upright at least 10 times.

4.2 Volumetric Pipets

Volumetric pipets are used to deliver precisely a single, definite volume of liquid. The tip must meet stringent requirements because drainage time is controlled by the diameter of the tip. The amount of liquid delivered depends on how a pipet is used; accuracies of 1 part per 1000 can be attained readily, provided the pipets are used in a reproducible manner.

Keep in mind the following points. First clean the pipet so that water drains smoothly from the interior surface. Rinse the interior by drawing a portion of the liquid to be pipetted into the pipet with the aid of a suction bulb, and then tilt and turn the pipet until all of the inner surface has been wetted. Discard this portion of solution and repeat the operation. Then draw solution above the mark, wipe the tip and stem of the pipet carefully to remove external droplets, touch the tip to the side of the beaker wall and allow the solution to drain until the bottom of the meniscus is even with the calibration mark. Touching the beaker wall draws liquid out of the pipet without leaving part of a drop hanging from the pipet when the liquid reaches the calibration mark.

Transfer the pipet to a receiving vessel and drain it by gravity while holding the tip against the wall of the vessel. The pipet should be held vertically, with the tip in contact with the container. Keep the tip in contact for about 5 sec after free solution flow has stopped and then remove it. The remaining liquid in the tip is supposed to be left there; do not blow out this portion.

Consult the instructor or a teaching assistant if you need refreshed on proper pipetting technique. The index finger (not the thumb) is used to cover the end. If difficulty is experienced at first with a leaky index finger, try moistening your finger slightly; too much moisture, however, makes fine control of the outflow rate difficult. After a little practice pipetting will become both rapid and accurate.

A rubber suction bulb is necessary for all pipetting. DO NOT PIPET ANYTHING BY MOUTH. Rinse the pipet thoroughly after use. Avoid drawing liquid into the bulb. If the bulb is contaminated accidentally, thoroughly rinse and dry it before re-use. Likewise, whenever a buret or pipet is used to deliver a measured volume of solution, the liquid in that item of glassware prior to measurement should have the same composition as the solution to be dispensed. As a result, always rinse your pipet with the solution to be measured prior to using it for the actual transfer.

4.3 Burets

Burets are designed to accurately deliver measurable volumes of liquid, particularly for titrations. A 50-mL buret, the most common size, has 0.1-mL graduations along its length and can be read by interpolation to the nearest 0.01 mL. Parallax errors in reading are minimized by extension of every tenth graduation around the tube. For reproducible delivery, proper design of the tip is important. Changes affecting the orifice will affect reproducibility; a buret with a chipped or fire-polished tip should not be employed for accurate work. Drainage errors are usually minimized if the tip is constricted so that the meniscus falls at a rate not exceeding 0.5 cm/sec.

The accuracy of graduation marks on volumetric burets depends on the diameter of the bore being uniform. The most convenient way to determine actual volumes is to weigh the amount of water delivered. Burets can be purchased already calibrated. In this course we will not calibrate the burets.

Because graduations on a 50-mL buret are 0.1 mL apart, volumes between the marks must be estimated. In this estimation the width of the lines must be taken into account. The thickness of a line on a 50-mL buret is usually equivalent to about 0.02 mL and so takes up one fifth of the distance from one mark to the next. It is preferable to read the bottom of a meniscus. Parallax, another source of error in reading a buret, occurs if the eye is above or below the level of the meniscus. This error can be minimized by use of the encircling markings on the buret as guides to keep the eye level with the meniscus.

The apparent position of the meniscus is significantly affected by the way it is illuminated. Lighting errors can be minimized by use of a white index card with a thick black line. The card is placed behind and against the buret so that the top of the black portion is flush with, or no more than a scale division below, the bottom edge of the meniscus. For some solutions, such as permanganate and iodine, the bottom of the meniscus may be difficult to see; in such cases the top may be read.

Before using a buret, clean it thoroughly with dilute acid, taking care not to scratch the interior surface. Rinse it well with distilled water; the buret is clean when water drains from the inside surface uniformly without the formation of droplets. Before use rinse the buret at least once with the titrant solution. After filling with titrant, make certain no air bubbles are present in the tip, especially just under the stopcock. Bring the solution level to slightly below the zero mark, remove the drop adhering to the tip by touching it to a clean piece of glass, and wait a few seconds for solution above the meniscus to drain before taking the initial reading. To provide effective control of the delivery rate when titrating, operate the stopcock with the left hand around the barrel (if right-handed). The other hand is free to swirl the titration flask.

Exercise: Buret Reading

You will have available in the laboratory, six alphabetized burets filled to some level with water and sealed. Read the volume of each with the aid of the buret and record the results in your notebook to 0.01mL along with your observations.

4.4 Calibration of Volumetric Equipment

Calibration of volumetric equipment is carried out by weighing the amount of water contained or delivered. Since the density, or weight per unit volume, of water changes about 0.03% per degree C at 25^oC, the temperature of the water used in the calibration must be known. The actual volume delivered can be obtained from a table relating temperature to volume, such as Table 1.1 below.

Table 1.1: Volume occupied by one gram of water at various temperatures, corrected for buoyancy.

Although errors in buret and pipet markings by the manufacturer are usually small, they cannot be neglected in careful work. For example, if a 10-mL pipet actually delivers 9.990 mL, an error of one part per thousand will be introduced unless the true value is employed.

Exercise: Calibration of Volumetric Equipment

Apparatus:

Two 250 mL Erlenmeyer Flasks and Nalgene Caps from the 1L storage bottles

25 mL Volumetric Pipet

Two 100 mL Volumetric Flasks and Stoppers

Wax Pencil

Analytical Balance

Thermometer

Chemicals:

Distilled water from the white tap
 
 

Glassware preparation:

Before you begin, make sure your glassware, especially your pipet is clean. Water should run out of the pipet smoothly, leaving a thin film on the inside of the glass. It should not "bead" on the inside. See a TA for instructions on use of the cleaning solution (DANGER!) if you believe your pipet is dirty.

Pipet Calibration:

1) Make sure you understand the proper use of the analytical balances and volumetric pipets. Ask questions if you are unsure.

2) Place about 450 mL of deionized water in a 600 mL beaker. Allow the temperature of the water to equilibrate to room temperature.

3) Place the Nalgene cap on the Erlenmeyer flask and weigh the flask to the nearest mg; record this value in your notebook. Note the specification given for the maximum weight allowance of the balance.

4) Record the temperature of the water. Carefully fill the pipet with water up to the graduated mark and deliver the water to the flask in the appropriate manner. Place the cap on the flask and record the mass.

5) Repeat step 4 five additional times. Depending on the initial weight of your Erlenmeyer flask, you should be able to make 2-3 weighings in each flask without discarding the water after each addition. Using the information in step 3, determine how many consecutive weighings you can safely perform without exceeding the maximum allowable weight limit on the balance.

6) Use the difference in mass between each set of two consecutive mass measurements to determine the mass of water delivered in each trial (thus the origin of the name "weighing by difference").

7) Calculate the true volume as in the example for a 10 mL volumetric pipet that follows. Duplicate calibrations should agree to within 0.01 mL. Use the average value for all subsequent measurements with this pipet.

Example for a 10-mL volumetric pipet:

 

At 26 ^oC, 1 g of water occupies 1.0043 mL (Quantitative Chemical Analysis Table 2.6). The true volume of 9.975 g of water at 26 ^oC is

9.975 g x 1.0043 mL/g = 10.02 mL

(Note: Confirm the proper number of significant figures.)

Volumetric Flask Calibration:

1. Label each of the 100 mL volumetric flasks with a wax pencil.
2. Weigh the first clean, dry, stoppered 100 mL volumetric flask to the nearest milligram on an analytical balance.
3. Insert a clean, dry funnel into the flask so that the stem extends below the calibration mark of the flask. Fill the flask with water at room temperature to just below the mark.
4. Carefully remove the funnel and add water with a disposable pipet until the bottom of the meniscus coincides with the calibration line. Remove droplets of water present above the line with a lintless towel or a strip of filter paper.
5. Stopper the flask and reweigh it. Immediately after weighing, measure and record the temperature of the water in the flask.
6. Calculate the true volume of the flask in the same way as for the volumetric pipet. Obtain three measurements by emptying the flask and drying it between measurements. Air drying is best if time permits; alternatively, filtered (clean, grease-free) compressed air can be used. If you are careful in keeping track you can alternate flask weighings to help in the drying process.

Calculations:

1. Enter the calculated masses and volumes in your laboratory notebook.
2. Calculate:

• the mean (average) volume
• the standard deviation
• the relative standard deviation (%RSD)
• the 95% confidence interval
• the % relative error (assume the theoretical volume is exactly 25 mL for the pipet and 100 mL for the volumetric flask)

        3.    Construct a reference table of this data and place it on the 2^nd or 3^rd page of your lab notebook, leaving space for your Table of Contents. You will use the mean and         standard deviation of your calibrated glassware throughout the rest of the quarter.

        4.    Report the above information as well as your buret readings on your 3" x 5" index card.