‘The Relative Strength of an Unknown Acid’.

Titration

Before I start my titration I must make sure all my equipment is rinsed properly with distilled water.  To clean the burette I will run distilled water through it, I will do this by setting the burette up with the use of a stand.  I will close the tap at the bottom of the burette and fill the burette up to the top, I will then place a beaker under the burette and open the tap, letting the distilled water run out.  When all the distilled water has run out of the burette I will close the tap and refill it with sodium hydroxide, using a funnel (which will have been rinsed with distilled water).  I must always remember to remove the funnel as it can affect the results.  Before I run the sodium hydroxide through the burette I must make sure I shake the flask, to be sure it is all dissolved.  I will again place the beaker under the burette and let the sodium hydroxide run through the burette.  I will then again fill the burette with sodium hydroxide, making sure it goes above the zero mark.  This time I will only run out the sodium hydroxide until the meniscus of it is touching zero.  This will also ensure that the jet space of the burette is full of sodium hydroxide as it counts towards the measurement.  I will then remove the drop remaining on the tip of the burette as this may affect results.  Next I need to clean out a conical flask; I do this by again rinsing with distilled water.  I will also need to swill out a 25cm3 pipette, again using distilled water.  After this is completed I have to shake the volumetric flask of succinic acid, I do this by turning it upside down and shaking the contents, returning it upright, repeating this ten times, I must do this each time before I use the acid. After that I will use the 25cm3 pipette with a pipette filler to suck up some succinic acid.  I will fill it up over the 25cm3 mark and then drain it out under gravity into a beaker, making sure the tip does not touch the liquid as it will not run out as good.  Next I need to again shake the succinic acid.  I will then fill the pipette up again to the 25.0cm3 mark, making sure the meniscus of the acid touches the 25.0cm3 mark.  I will then transfer this to the conical flask.  I will then add 4 drops of phenolphthalein indicator to the acid, which will turn the solution pink.  To begin with the tap should be opened quite slowly allowing the sodium hydroxide to run into the conical flask slowly.  One hand should be turning the tap and checking the flow while the other hand should hold the conical flask and be constantly swirling it, once it is noticeable that the pink tinge is starting to disappear and turn clear the tap should be reduced to only dropping slowly.  This should be slowed down till its just drop by drop, and the swirling should be continued.  One drop could make the difference so this has to be done one drop at a time, slowly.  I must always touch the side of the flask with the tip of the burette to make sure the last drop lands in the conical flask, as this could affect the results.  To make it easier to decide the colour of the solution a white tile is placed under the conical flask.  The solution is always looked down upon, because it is easier to distinguish the colour of the solution, the tile helps this.  Sodium hydroxide should be added until the solution has completely disappeared even after swirling for ten seconds.  This is the end point and the reading of the burette should be recorded to the nearest 0.05cm3.  White paper placed behind the burette can help to read the result on the burette.  This first titration is described as a rough titration and is used as a guide for the rest of the titrations.  All equipment can be used again, the conical flask will be washed out with distilled water, and the burette may need to be topped up.  The recording of two concordant results or two recordings within 0.05cm3 of each other marks the end of the titration.

Hazards and safety precautions

I must be careful throughout preparing the solution and the titration because the acid and alkali are corrosive and toxic, and can be especially damaging to eyes.  Any spillages must be cleaned up immediately; water can be used for this.  To be careful, I must wear my lab coat and goggles at all times, gloves may also be helpful as most times.

Below is a table of my hypothetical results:

Limitations and errors in volumetric analysis

First I must take into account the accuracy of the balance, if the balance reads to three decimal places, then the third place is only approximate.  Therefore a reading of 1.324g indicates that the weight of the material is between 1.3235g and 1.3245g.  So hence if the balance reading is accurately recorded, it may still have an error of 0.0005g.  Because the mass of the substance is so small a greater error may result, an error of 0.05% could result from weighing a 1.00g sample.  Here the percentage error, which may exist, is 0.04%.  

The next error I must take into account is the accuracy of the burette.  The burette delivers a drop of liquid of approximately 0.05cm3 volume.  Therefore all burette readings should include a second decimal place, either a 0 or a 5.  This second decimal place of a burette reading is only approximate and therefore is an error.  An error of one drop in a volume of 25.20cm3 represents an error of 2%, but this has to be doubled because two burette readings are taken for each titration.  Two concordant results reduce errors.

Then the next error I must take into account is the accuracy of the pipette.  When the pipette is filled up to the 25cm3 mark, the pipette actually contains a little more than 25cm3 solution.  It is important that the pipette is allowed to drain under gravity and retain the last drop.  When the pipette is used properly, it should not introduce an error of more than 0.2%.

After that the next error I must take into account is the accuracy of the volumetric flask.  When filled up to the 250cm3 correctly, a volumetric flask is calibrated to be accurate to within 0.6cm3 and therefore should not introduce an error of more than 0.15%.

Afterwards the next error I must take into account is the accurate determination of end-points.  The end-point of a titration can only be determined accurately if the solution from the burette is added drop by drop as the end-point is approached.  The conical flask should be constantly swirled after each drop is added, so that the colour change at the end of the reaction can be determined without any excess solution.  A rough titration should be performed first as a preliminary test so that the titration’s to be carried out can be as accurately as possible.

Lastly the error I need to take into account is the accuracy is the use of indicators to determine the end-point of an acid-alkali titration.  It is important that only a few drops of indicator are added because excess indicator will result in a different volume of solution to be added from a burette before the required colour change is obtained.

Now to combine all the errors worked out so I can get the total percentage of errors of the whole experiment.

0.04 + 2 + 2 + 0.2 + 0.15 = 4.39%

From combining all the error rates I estimate the overall error rate for the experiment to be ±4.39%.  To make this clearer I will turn the percentage into a measurement:

25.2 x 4.39 = 1.10628cm3    Therefore the observed result was 25.2cm3 ± 1.12cm3.

100

From my hypothetical results I am going to use the mean titre of 25.2cm3 of sodium hydroxide and the 25cm3 of succinic acid to work out the relative formula mass of succinic acid.

(Concentration x Volume) NaOH  = 2    (CV) NaOH  = 2   =  0.1 x 25.2

  (Concentration x Volume) H2A      1      (CV) H2A       1      2 x 25 x C

H2A = 0.0504

Number of moles = Concentration x Volume    n = 0.0504 x 250   n = 0.0126

1000. 1000

Number of moles = mass taken    0.0126 = 1.32   Mr =   1.32       

                           Mr                           Mr            0.0126

Mr = 104.7619046

Mr = 104.8

Looking back at the relative molecular mass of succinic acid I can see that when the Mr is 104, n = 1.