下载掌阅APP，畅读海量书库
立即打开
1. To understand the principles of acid-base titration.
2. To learn the titration procedure.
3. To properly determine the endpoint.
Titrimetric analysis is one of the most basic quantitative analysis,which is fast,accurate and widely used.In general,the standard solution of known concentration is put into burette as titrant,which is dripped into the analyte system to have quantitative reaction with the analyte according to the specific stoichiometric relationship.Measure the volume of titrant required for the complete reaction with the tested substance,and then calculate the quantity of the tested substance.
Acid burette;Alkali burette;Pipette;Volumetric flask.
NaOH;HCl;phenolphthalein indicator;methyl orange indicator.
Figure 2-1 Pipette(a)and graduated tube(b)
In order to accurately move a certain volume of solution from one container to another,pipette(Figure 2-1 a)and graduated tube(Figure 2-1 b)are often used.The pipette is used to accurately transfer a fixed volume of solution,while the graduated tube is used to accurately transfer a non fixed volume of solution.
At the temperature indicated on the pipette and graduated tube,suck the solution and adjust the meniscus liquid level of the solution to be tangent to the scale line of the pipe,and discharge the solution in the specified way.The volume of the solution discharged is the same as that indicated on the pipe.Ifthe actual transfer temperature is different from the indicated temperature,or when the non-aqueous solvent is absorbed,the volume of the solution will be slightly different.Pipette and graduated tube can be corrected if necessary.
When the pipette and graduated tube are used,the right hand holds the part above the scale line of the pipette,the tip of the pipette is inserted into the solution to be sucked,and the left hand controls the rubber suction bulb,so that the rubber suction bulb is closely connected with the upper mouth of the pipette for suction.The insertion depth of the pipe tip shall be appropriate,generally 1~2 cm below the liquid level.The tip of the tube should drop as the liquid level drops during suction.
Before taking the solution,wash the pipette and graduated tube with tap water.If water drops are found on the inner wall and the lower part of the outer wall,wash them with detergent.First,remove the residual water,suck the lotion to about 1/4 of the ball part of the pipette,or 1/4 of the volume of the graduated tube.Remove the rubber suction bulb,press the right index finger tightly on the nozzle,and remove the lotion.Lay the tube flat,hold the middle of the tube with left hand,release the right index finger,rotate the tube,and let the inner wall of the tube be moistened by the lotion.Pour the lotion back to the original bottle from the top of the tube,wash it thoroughly with tap water,and then wash it with pure water three times.Every time the water is absorbed to about 1/4 of the ball part of the pipette,lay the tube flat and rotate to wash the inner wall,and then remove it.The lower part of the outer wall of the pipe should also be blown clean with pure water.
Before transferring the solution,use the filter paper to absorb all the water inside and outside the pipe tip,so as to ensure the constant solution concentration.Then,moisten the pipette with the solution for 2~3 times.Moistening method:pour a small amount of transferred solution into a small dry beaker,insert the tube into the beaker to absorb the solution,when it is sucked to about 1/4 of the ball part of the pipette,or filled with 1/4 of the total volume of graduated tube,immediately press the nozzle with the right index finger,take out the tube,slowly lay the tube flat and rotate,so that the liquid will wet the whole tube wall.Then discard the solution from the tip of the tube.Repeat it for 2~3 times.
When transferring the solution,hold the upper part of the neck line with the right thumb and middle finger,insert the lower part of the pipe to 1~2 cm below the liquid level,and hold the rubber suction bulb with the left hand in the form of a clenched fist.First,discharge the air in the rubber suction bulb,and then place the rubber suction bulb tip on the upper mouth of the pipe,slowly release the left finger,so that the solution to be transferred can be inhaled into the pipe.When the liquid level rises above the mark,remove the rubber suction bulb and quickly plug the nozzle with the right index finger.Lift the lower mouth of the tube to above the liquid level,but still lean on the wall of the device,slightly loosen the index finger,slowly rotate the tube with the thumb and middle finger.
Figure 2-2 Usage of pipette
When the liquid level slowly drops to the bottom of the meniscus tangent to the marking line,remove the rubber suction bulb,immediately press the tube mouth with the index finger to make the solution no longer flow out.Take the pipette out of the container,use the filter paper to dry the solution on the outer wall of its lower end,but do not touch the lower mouth.Take the container with the left hand,incline about 45°,keep the pipette vertical and move it into the container with the right hand,and the lower end of the pipe is close to the inner wall of the container.Release the index finger so that the solution flows down the inner wall of the receiving container.After the solution flow is completed,stay for 10~15 s,and then take out the pipette.As shown in Figure 2-2.
Except for the pipe marked with“blow”on the pipe body,a small amount of solution remaining on the inner wall of the pipe tip should not be blown into the receiving container.The reason is that the volume of the solution remaining on the inner wall of the tip has been taken into account when calibrating the pipette.
After use,wash the pipette and place it on the pipette holder.
Graduated tubes are used in the same way as pipettes,but when the solution is discharged,it should be lowered from one scale to another and the end scale should be avoided.
A volumetric flask is a pear shaped,flat bottomed glass bottle with a ground glass or plastic stopper.There is a mark on its neck.At the specified temperature,when the solution is filled to the bottom of the meniscus tangent to the mark,the volume of the solution contained is equal to the volume marked on the bottle.The volumes of commonly used volumetric flask are 10mL,25mL,50mL,100mL,250mL,500mL,1000mL,etc.The main purpose of volumetric flask is to prepare accurate concentration solution,including preparing standard solution by direct method or diluting accurate concentration and volume of concentrated solution to accurate concentration and volume of dilute solution.
Check for water leakage before using the volumetric flask.The specific operation is as follows:add water to the volumetric flask near the marking line,and tighten the cork.One hand holds the cork tightly above the marking line of the bottle neck,and the other hand holds the bottom of the bottle.Invert the volumetric flask for about 2 min,and observe whether there is water leakage.If there is no water leakage,rotate the cork for 180°and check again.It can be used only when there is no water leakage in both inspections.
The washing of the volumetric flask is similar to that of the pipette.
If the standard solution is prepared with solid substances,put the accurately weighed solid reagent into a small beaker,add some pure water,and stir to make it dissolve(if it is difficult to dissolve,cover the surface dish and heat it slightly,but it must be cooled before transferring).Quantitatively transfer the solution to the volumetric flask with the glass rod,and then blow the wall of the beaker with the washing bottle for 3 to 4 times,and transfer the washing solution into the volumetric flask according to the same method.Add distilled water to dilute.When diluting to 2/3 of the volume of the volumetric flask,shake the volumetric flask several times in the horizontal direction(do not reverse),so as to preliminarily mix the solution.Then,place the volumetric flask flat on the table,continue to add distilled water to 1~2 cm away from the mark line,wait for 1~2 min,so that the solution attached to the inner wall of the bottleneck flows down.Finally,add distilled water drop by drop with a rubber head dropper until the bottom of the solution meniscus is tangent to the marking line.Close the cork,press the cork with the index finger of left hand,hold the bottom of the bottle with the fingers of right hand,turn the volumetric flask upside down,and make the bubble in the bottle rise to the top.Turn it over and over several times so that the solution is well mixed.As shown in Figure 2-3.
If diluting the solution with a volumetric flask,use a pipette(graduated tube)to transfer a certain volume of concentrated solution into the volumetric flask,add water according to the above method to dilute to the marking line,and shake well.
Figure 2-3 Usage of volumetric flask
There are two types of burette:acid burette and alkali burette(Figure 2-4 a and b).The commonly used burette solvent is 25mL and 50mL.The universal burette with polytetrafluoroethylene piston has been widely used.
(1)Preparation of burette before use
Before use,check whether the burette cock rotates flexibly and whether the burette cock matches with the plug groove,otherwise it is easy to cause leakage.For the matching burette,in order to make the piston rotate flexibly without leakage,oil(Vaseline or vacuum grease)can be applied to the part of the cock,and then leak detection.The operation is as follows.
Pour out the water in the burette,lay it on the test bench,draw out the cock,use the filter paper to suck up the water in the cock and the cock groove,dip a little grease in the finger,evenly apply a thin layer on both ends of the cock(Figure 2-5 A and B).Insert the plug into the plug groove and rotate in the same direction until the plug is transparent except for one turn of the plug hole.Grease shall not be applied too much,otherwise it is easy to block the plug hole.However,if it is applied too little,the plug will not rotate flexibly and even leak liquid.In case of these situations,the plug and plug groove must be wiped clean and then greased again.Finally,counter the large end of the cock and put a rubber band on the other end.
Figure 2-4 Acid burette(a)and alkali burette(b)
Figure 2-5 Apply grease
During leak detection,fill the burette with water,discharge the air bubble in the outlet pipe,close the cock,clamp the burette on the burette frame,and stand upright for 2 min.If there is no water drop leaking,rotate the cock 180°for static inspection.If water leakage is found,grease shall be applied again.
After the burette is coated with grease for leakage detection,it shall be fully cleaned to ensure that the inner wall of the burette is evenly wetted by water to form a water film without water drops.When washing,according to the degree of contamination in the pipe,tap water or detergent can be used respectively.
The lower end of the Alkali burette is connected with a latex tube embedded with glass beads to control the flow rate.The Alkali burette should not be filled with oxidizing solution.
Before using the Alkali burette,check whether the latex tube is aging and whether the glass bead size is appropriate,otherwise it should be replaced.
The washing method of Alkali burette is basically the same as that of acid burette.
(2)Loading of standard solution
The prepared standard solution shall be directly poured into the burette without using other containers(such as beaker,funnel,etc.)for transfer.Before pouring into the solution,shake the solution in the reagent bottle well,so that the water drops condensed on the bottle wall are mixed into the solution.Then,hold the top without scale of the burette with the first three fingers of the left hand,tilt the body of the burette slightly or make the burette naturally vertical,take the reagent bottle(label up)with the right hand and pour it slowly.
First,moisten the inner wall of the burette with solution for three times(about 5~10mL each time).Pay attention to wash all the inner walls every time to ensure that the concentration of the solution is constant after loading.For acid burette,open the cock,wash the outlet pipe and drain the residual liquid.
Hold the top without scale of the burette with the right hand,open the cock quickly with the left hand,let the solution wash down,and drive out the bubbles.For the Alkali burette,bend the lower latex tube to make the outlet tube tip up,extrude the latex tube on one side of the glass bead to let the solution spray out,and bring out bubbles(Figure 2-6).After the bubbles are exhausted,the solution is discharged and the latex tube is straightened at the same time.Finally,wipe the outer wall of the burette dry and clamp it on the burette frame for use.
Figure 2-6 Bubble discharge from Alkali burette
(3)Burette reading
The following principles should be observed.
①After the solution is put in or discharged,it must wait for 1~2 min,and read after the solution attached to the inner wall of the pipe flows down fully.If the solution is discharged slowly(for example,only one drop or half a drop of solution is added at a time near the end of titration),it can be read only after 30~60 s.
②Before each reading,check whether there are water drops on the inner wall of burette and whether there are bubbles on the tip of burette.The reading is valid only when there is no water drop
Figure 2-7 Burette reading
on the inner wall and no bubble on the pipe tip.
③When reading,the burette can be clamped on the burette holder,or it can be removed.Hold the top without scale of the burette for reading,but pay attention to keep the burette vertical.
④When reading,the line of sight should be level with the liquid level,otherwise the reading will be large or small(Figure 2-7).When the burette is filled with colorless or light color solution,the scale at the intersection of the line of sight and the lowest point of the meniscus liquid level should be read.For dark solution,when the lower edge of meniscus cannot be seen clearly,the readable line of sight is horizontal with the highest point on both sides of meniscus.
⑤The reading must be read to the second decimal place,i.e.0.01mL.
⑥To facilitate reading,a white reading card with a black rectangle in the middle can be used.When reading,press the reading card against the back of the burette,so that the upper edge of the black square is about 1 mm below the meniscus liquid level.At this time,black is reflected on the meniscus,which is clear and easy to distinguish.The reading is based on the lowest point of the lower edge of the black meniscus.When reading the highest points on both sides of the dark solution liquid level,the white card should be used as the background.The same method should be used to read the readings.
⑦For burette with blue line backing of opal board,the scale corresponding to the upper and lower tips and intersection points formed by blue line at liquid level should be read.
⑧The initial reading should be adjusted at or near the“0”line.When taking the initial reading,remove the liquid drop hanging from the pipe tip at the same time.When the final reading is taken,the tip of the tube should not hang liquid.If there is a drop,the drop has been included in the titration volume,and it should be put into the solution to be dropped.If it is excessive after dripping,it must be redone.
(4)Titration
The burette should be clamped vertically on the burette holder for titration.
Figure 2-8 Titration used by acid burette
Use left hand to control cocks when using acid burette.The left ring finger and small finger point to the palm of the hand and bend,gently stick to the outlet pipe,and use the other three fingers to turn the cock.Do not push outwards when turning.The palm should not touch the small end of the cocks to prevent the cocks from being pushed loose and liquid leakage.Do not push too much inward so that the cocks cannot rotate flexibly.As shown in Figure 2-8.
When using the Alkali burette,the left ring finger and little finger clamp the glass outlet tube,and the thumb and index finger squeeze the latex tube on one side of the glass bead to make the solution flow out of the gap.In order to avoid leakage or air bubbles entering back,attention should be paid to:first,the glass bead should not be displaced up and down;second,do not pinch the latex tube under the glass bead;third,when stopping titration,loosen the thumb and index finger first,then the ring finger and small finger.
No matter which burette is used,the following three methods of dropping solution must be mastered:continuous dropping of solution(liquid flow cannot be linear);adding only one drop of solution;dropping half a drop or even half a drop,that is,let the solution hang on the tip of the tube without falling,contact the conical bottle with the mouth of the tube,make the drop flow out,and wash it down with pure water.
The titration is generally carried out in an Erlenmeyer flask,or in a beaker if necessary,and the test bench should be white background.
If an Erlenmeyer flask is used for titration,hold the bottle neck with the first three fingers of the right hand to make the bottom of the flask about 2 cm away from the table top.At the same time,adjust the height of burette so that the tip of burette is about 1 cm into the bottle mouth.Operate the burette with the left hand according to the above method,and add the solution while rotating the conical flask with the right hand.
The following points should be noted in titration:
(1)When rotating the conical flask,move the wrist joint slightly to make the solution move in a circle in the same direction(clockwise or anticlockwise),but do not shake back and forth to prevent splashing.The mouth of the bottle should not touch the tip of the burette when it is rotated.
(2)During titration,the left hand cannot leave the cock and let the solution to flow by itself.
(3)At the beginning of titration,the speed of adding solution can be slightly faster.During this period,pay attention to observe the color change of solution at the dropping point of the solution to determine whether it is close to the end point.When approaching the end point,the speed of adding solution should be slowed down.Add one drop,half drop or even a small half drop,shake it for several times after dropping,and blow the bottle wall with a little pure water.When the color of the solution changes obviously,close the cock quickly and stop titration,which is the end point of titration.
(4)When adding half a drop with an Alkali burette,it is necessary to avoid bubbles at the tip of the burette.After the extruded solution is hung on the tube tip,first release the thumb and index finger,then touch the liquid drop with the bottle wall,introduce it into the bottle,finally release the ring finger and small finger,and blow the bottle wall with a little pure water.
Use a clean small beaker to weigh 2.0 g NaOH on the platform scale,add 50mL pure water to make it dissolve completely,transfer it into a 500mL reagent bottle,rinse the small beaker with a small amount of pure water for several times,transfer the washing solution into the reagent bottle together,add water to the total volume of about 500mL,cover the rubber plug,and shake well.
Take 4.2~4.5mL concentrated hydrochloric acid with a 10mL clean measuring cylinder in the fume hood,pour it into a 500mL reagent bottle containing water,add water to dilute it to about 500mL,cover it with a glass stopper,and shake it well.
(1)Prepare one acid burette and one alkali burette.Rinse the acid burette and the basic burette with 5~10mL HCl and NaOH solution for 2~3 times respectively.Add HCl and NaOH solution respectively,discharge bubbles,and adjust the liquid level to zero scale or slightly lower.Record the initial reading in Table 2-1 after standing for 1 min.
(2)Phenolphthalein was used as indicator to titrate HCl with NaOH solution.Drain about 10mL HCl from the acid burette into a conical flask,add 10mL pure water and 1~2 drops of phenolphthalein.Shake continuously.Titrate with NaOH solution until the end point,record the final reading in Table 2-1.Repeat this three times.
(3)Methyl orange was used as indicator to titrate NaOH with HCl solution.Drain about 10mL NaOH from the basic burette into the conical flask,add 10mL pure water and 1~2 drops of methyl orange.Shake continuously.Titrate with HCl solution until the end point,record the final reading in Table 2-1.Repeat this three times.
Table 2-1 Basic operation data processing of titration analysis
1. The pipette should not be used to transfer too cold or too hot liquid.It should not be dried in the oven.In order to avoid errors,the same pipette should be used in the same experiment.
2. The volumetric flask can only be used to prepare the solution,not to store the solution for a long time.If the solute is exothermic during dissolution,it should be transferred after the solution is cooled.The solution to be kept away from light should be prepared in a brown volumetric flask.
3. Acid burette with glass piston at the lower end should not be filled with alkali liquor.
4. When washing with water,pay attention to the cleaning of the corner under the glass bead.When extruding the latex tube to release the solution,change the extruding direction constantly so that the glass bead can be washed around.
5. After titration,the remaining solution in the burette should be discarded and never be returned to the original bottle.
1. Is it necessary to moisten the volumetric flask with solution to be tested when preparing the solution?Why?
2. Can HCl and NaOH solution directly prepare accurate concentration?Why?
continued
