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AIMS
1.Validationon the basic principles and fundament of chemistry,and then deepening and expanding the theoretical learning.
2.To master the basic skills of experimental operation.
3.To learn how to analyze and solve problems.
This experiment is designed to introduce you to some of the apparatus and proper analytical laboratory techniques you will be using during the remainder of this course,including the methods for evaluating analytical data treatment.As a chemist,it is imperative to gather the best possible data from your equipment.Therefore,it is important to use all equipment correctly,patiently and precisely.In addition,a good chemist will calibrate an “instrument”before they use it to assure that the data gathered is as accurate as possible.
(1)Never assume any glassware is clean unless you washed it yourself!!!
(2)Keep your bench area clean!
(3)Clean all the glassware with soap and tap water unless otherwise directed.
(4)Use distilled water to rinse your glassware,preferably a double or triple rinse.
(5)Glassware that will not come clean in soap and water should be cleaned by soaking in a solution of 1 M Nitric Acid or 1 M KOH in alcohol,followed by a soap and water wash and distilled water rinse.This is typically true of Burettes.
(6)Never heat any piece of volumetric glassware.Heating will cause changes inthe glassware that will cause it not to be volumetric anymore.
(7)All items in this laboratory will be massed using the analytical balance.These balances read to the nearest 0.1 mg(0.000 1 g).Thus,all masses recorded must be to the nearest 0.1 mg.(Do not record anything from the balance without all 4 places)
(8)You will be assigned a balance to use.You will use the same analytical balance for all measurements throughout the semester.This improves the precision of your measurements.(Plus I know who to penalize when it is trashed!)
(9)You must clean the analytical balance when you are finished with it!Failure to do so will result in a 5 point deduction from the laboratory reports of all the students assigned to that balance.
(10)Burettes must be washed at the end of every laboratory period (soap,water,rinsing with distilled water).Burettes should be placed in the burette holder to drain—upsidedown,with the stopcock open(Make sure you initial your burette).(11)Label all your glassware(on the glassware itself)as you use it and you should write the labels in your notebook so that you can keep track of what sample goes with what piece of data.
(12)Refer to guidelines about keeping a notebook and writing a report in the ACS format.
Figure2-1 Volumetric Flask
Figure 2-2 Pipettes
Figure 2-3 Burettete
In precise work it is never safe to assume that the volume delivered by or contained in any volumetric instrument is exactly the amount indicated by the calibration mark.Instead,recalibration is usually performed by weighing the amount of water delivered by or contained in the volumetric apparatus.This mass is then converted to the desired volume using the tabulated density of Water:
All volumetric glassware should be either purchased with a Calibration Certificate or calibrated by the analyst in this manner.
The volume occupied by a given mass of liquid varies with temperature,as does the volume of the device that holds the liquid.20℃ has been chosen as the normal temperature for calibration of many volumetric glassware.
Glass is a fortunate choice for volumetric ware as it has a relatively small coefficient of thermal expansion;a glass vessel which holds 1.000 00 L at 15℃holds 1.000 25 L at 25℃.If desired,the volume values( V )obtained at a temperature( t )can be corrected to 20℃by use of:
Where V 20 is the volume at 20℃, V is the volume value at a temperature t .
In most work,this correction is small enough it may be ignored.
However,the thermal expansion of the contained liquid is frequently of importance. Dilute aqueous solutions have a coefficient of thermal expansion of about 0.025％/℃.A liter of water at 15℃ will occupy 1.002 L at 25℃.A correction for this expansion must frequently be applied during calibration procedures.
Fiigure 2-4 Parallax
Parallax is another source of error when using volumetric ware.A correction for this expansion must frequently be applied during calibration procedures.Frequently,graduation marks encircle the apparatus to aid in this.
Readings which are either too high or too low will result otherwise.
The Pipette is used to transfer a volume of solution from one container to another.Most volumetric pipettes are calibrated to-deliver (TD);with a certain amount of the liquid remaining in the tip and as a film along the inner barrel after delivery of the liquid.The liquid in the tip should not be blown-out.Pipettes of the“blow-out”variety will usually have a ground glass ring at the top.And,drainage rates from the pipette must be carefully controlled so as to leave a uniform and reproducible film along the inner glass surface.Measuring pipettes will be gradatedin appropriate units.
Once the pipette is cleaned and ready to use,make sure the outside of the tip is dry.Then rinse the Pipette with the solution to be transferred.Insert the tip into the liquid to be used and draw enough liquid into the pipette to fill a small portion of the bulb.Hold the liquid in the bulb by placing your fore finger over the end of the stem.
Withdraw the pipette from the liquid and gently rotate it at an angle so as to wet all portions of the bulb.Drain out and discard the rinsing liquid.Repeat this once more.
Figure 2-5 Use of pipette
To fill the pipette,insert it vertically in the liquid,with the tip near the bottom of the container.Apply suction to draw the liquid above the graduation mark.Quickly place a fore finger over the end of the stem.Withdraw the pipette from the liquid and use a dry paper to wipe off the stem.Now place the tip of the pipette against the container from which the liquid has been withdrawn and drain the excess liquid such that the meniscus is at the graduation mark.
Move the pipette to the receiving container and allow the liquid to flow out(avoiding splashing)of the pipette freely.When most of the liquid has drained from the pipette,touch the tip to the wall of the container until the flow stops and for an additional count of 10.
The volumetric flask is used to prepare standard solutions or in diluting a sample.Most of these flasks are calibrated to-contain(TC)a given volume of liquid.When using a flask,the solution or solid to be diluted is added and solvent is added until the flask is about two-thirds full.It is important to rinse down any solid or liquid which has adhered to the neck.Swirl the solution until it is thoroughly mixed.Now add solvent until the meniscus is at the calibration mark.If any droplets of solvent adhere to the neck,use a piece of tissue to blot them out.Stopper the flask securely and invert the flask at least 10 times.
The Burette is used to accurately deliver a variable amount of liquid.Fill the burette to above the zero mark and open the stopcock to fill the tip.Work air bubbles out of the tip by rapidly squirting the liquid through the tip or tapping the tip while solution is draining.
The initial burette reading is taken a few seconds,ten to twenty,after the drainage of liquid has ceased.The meniscus can be highlighted by holding a whitepiece of paper with a heavy black mark on it behind the burette.
Place the flaskinto which the liquidis to be drained on a white piece of paper.(This is done during a titration to help visualize color changes which occur during the titration.)The flask is swirled with the right-hand while the stopcock is manipulated with the left-hand.
Figure 2-6 Use of burette
The burette should be opened and allowed to drain freely until near the point where liquid will no longer be added to the flask.Smaller additions are made as the end-point of the addition is neared.Allow a few seconds after closing the stopcock before making any readings.At the end-point,read the burette in a manner similar to that above.
Figure 2-7
As with pipettes,drainage rates must be controlled so as to provide a reproducible liquid film along the inner barrel of the Burette.
Cleaning of volumetric glassware is necessary to not only remove any contaminants,but also to ensure its accurate use.The film of water which adheres to the inner glass wall of a container as it is emptied must be uniform.
Two or three rinsings with tap water,a moderate amount of agitation with a dilute detergent solution,several rinsings with tap water,and two or three rinsings with distilled water are generally sufficient if the glassware is emptied and cleaned immediately after use.
If needed,use a warm detergent solution (60—70℃).A burette or test tube brush can be used in the cleaning of burettes and the neck of volumetric flasks.Volumetric flasks can be filled with cleaning solution directly.Pipettes and burettes should be filled by inverting them and drawing the cleaning solution into the device with suction.Avoid getting cleaning solution in the stopcock.Allow the warm cleaning solution to stand in the device for about 15 minutes;never longer than 20 minutes.Drain the cleaning solution and rinse thoroughly with tap water and finally 2—3 times with distilled water.
Pipettes and burettes should be rinsed at least once with the solution with which they are to be filled before use.
● Analytical balance,weighing bottles,400 mL beakers,100 mL plastic bottles with caps,5 mL and 25 mL volumetric pipettes,25 mL volumetric flasks,50 mL burettes,thermometer,crucible tong.
● Distilled and deionized water.
The analytical balances in the lab are probably the most precise,accurate and reliable pieces of equipment that you will use during the semester.Although there are inherent limits in the accuracy and precision of these balances most weighing errors are caused by incorrect handling of the sample.In the first part of this lab you will investigate several potential sources of error.
1.In this section you will determine the mass of a clean,dry weighing bottle under various conditions.Unless instructed otherwise,you should handle the bottle with your crucible tongs,gloves,or lint-free paper and measurements should be made to the nearest 0.1 mg.Begin by placing the weighing bottle and cap (with cap removed)in the oven for about 5 minutes.Remove and re-mass while warm.Follow the change in its apparent mass for several minutes,reweighing every thirty seconds.Record all masses including the final constant value.
2.After massing the weighing bottle,roll it around in your hand (handle the bottle with your fingers)and then re-mass and compare the two masses.
3.Next,wipe the bottle clean with a dry,lint-free cloth or tissue and reweigh.Record all observations.
4.Hold the weighing bottle and inch from your mouth and breathe on it several times.Re-mass and compare with previous data.
5.Discuss your results in your laboratory write-up.
As was noted above,volumetric glassware is calibrated by measuring the mass of water that is contained in or delivered by the device.
To obtain an accurate mass measurement,buoyancy effects must be corrected for.The amount of air displaced by the standard weights of the balance is somewhat different than the amount of air displaced by the weighed water.This difference leads to different buoyancies for these objects;meaning the balance levels at a point otherthan when the two objects are of the same mass.This can be corrected for use:
Where d s is the density of the standard weights(8.47 g/cm 3 ), d a is the density of air(0.001 2 g·cm 3 ),and d is the density of the object being measured.
This mass data is then converted to volume data using the tabulated density of Water(See Appendix)at the temperature of calibration.(In very accurate work,the thermometer must also be calibrated as an incorrect temperature reading will lead to the use of an incorrect density for water.This,in turn,will give an inaccurate volume calibration.)
Finally,this volume data is corrected to the standard temperature of 20℃.This can be accomplished using the thermal expansion coefficient of water:0.000 25/℃:
Thus,in this exercise we will calibrate a volumetric flask and a pipette and determine a burette correction factor by calibrating each of these devices with water.In each case,the measured mass of the calibrating water will be corrected for buoyancy effects and the resulting volume will be standardized to 20℃.
Begin by cleaning a 5 mL or 25 mL volumetric pipette,a 50 mL burette,and a 25 mL volumetric flask according to the procedure outlined above.It is imperative for the purposes of calibration that these glassware items be cleaned such that water drains uniformly and does not leave breaks or droplets on the walls of the glass.
If detergent solutions are not sufficient to clean your glassware,a cleaning solution (dichromate in conc.sulfuric acid)may be used.Consult you instructor before taking this step.
Once cleaned,the burette should be filled with distilled water and clamped in an upright position and stored in this manner until needed.The volumetric flask should be clamped in an inverted position so that it may dry.
Use your cleaned Pipette.Weigh a receiving container on the analytical balance:a 100 mL plastic beaker with an aluminum foil cover.Pipette distilled water into the plastic beaker and reweigh it.
Record the temperature of the water used.
Repeat the procedure at least 2 more times.Dry the plastic beaker and re-weighit for each replication.(Are you pipetteing consistently and correctly?)
Calculate the apparent mass and the buoyancy corrected mass of the water delivered for each time you pipette.From this mass,and the density of water at the given temperature (See Appendix),calculate the volume of the water delivered.Correct the volume to 20℃.Calculate the average,standard deviation,and 90％confidence interval for your calibration result.
Is your result within the listed tolerance for this pipette?(See Appendix.What is the better question to ask?)
Use your cleaned 50 mL burette.Fill the burette with water.Make sure the tip is free of bubbles.Drain into a waste beaker until it is at,or just below,the zero mark.Allow 10—20 seconds for drainage.Make an initial reading to a precision of at least 0.01 mL.Test for tightness of the stopcock by allowing the burette to stand for 5 minutes and then re-reading the volume.There should be no noticeable change in the reading.
Once the tightness of the stopcock is assured,refill the burette and again drain into a waste until it is at,or just below,the zero mark.Allow for drainage.Touch the tip of the burette to the wall of the waste beaker to remove the pendent drop of water.Make a volume reading.
Weigh a receiving container on the analytical balance;a 100 mL plastic beaker with aluminum foil cover.Drain about 5 mL of water from the burette into the beaker.Allow 10—20 seconds for drainage.Touch the tip of the burette to the wall of the beaker to remove the pendent drop again.Read the burette and weigh the water.
Calculate the actual volume of water delivered by the burette in the same manner as outlined above in the procedure on calibrating pipettes.Calculate the correction factor(CF)by subtracting the apparent volume delivered,as given by the burette readings,from the actual volume delivered.
Repeat the procedure at least once more.The two correction factors should agree within 0.04 mL.If they do not,repeat the procedure again.Report the average correction factor for 5 mL.
Repeat this process for 15 mL,25 mL,35 mL,and 45 mL delivered.
Plot the average burette correction factor vs.volume delivered using Excel or some other graphing software.
Label and store your burette properly;upright and filled with distilled water.This is the burette you will use for the remainder of the course.
Use your cleaned25mL volumetric flask.Weigh the flask empty.Fill the flask to the mark and re-weigh it.Measure the temperature of the water used.
Repeat the procedure at least twice.
Calculate the true volume of the flask using the method outlined above.Report the average,standard deviation,and 90％confidence interval for this result.
(Continued)
of burette vs.Volume.
1.How should weighing bottles be handled on a regular basis?
2.Are your results within the listed tolerance for pipettes,burettes,and flasks?(See Appendix.)Analysis the reasons.
Table 2-1 Density of Water
Table 2-2 olerances for Class A Volumetric Glassware at 20℃
