Titration Ensuring good volumetric technique How do you ensure that you carry out an accurate titration so that the same results are achieved each time you do the titration? IF you use good volumetric technique you should get similar results to those of the teacher and your peers. You should research online how to do an accurate titration to minimise errors. In the following slides there are some examples of good volumetric technique. If you do use these slides make sure that you include this in your sources of information.

Making up a standard solution

Mass of weighing boat + Na2CO3 (g) Mass of empty weighing boat (g) Titration Weighing out the solid base Ensure that the balance is calibrated before use – follow the manufacturers instructions. It should have been electrically tested to ensure it is safe to use and in working order. Ensure that the top is clean. Some balances may be affected by a draft from an open window which could affect your results. The more decimal places the balance reads to the more accurate the weighings. You should weigh by difference – if you were to weigh the solid into a weighing boat then tip the solid into the beaker, some solid may have been left behind in the weighing boat. For this reason weighing by difference instead ensures better accuracy. How clean was the spatula used to weigh the base and the weighing boat? were these cleaned before use? Could any impurities have contaminated your base and therefore affected the accuracy of your titration results? Mass of weighing boat + Na2CO3 (g)   Mass of empty weighing boat (g) Mass of Na2CO3 used (g)

Titration Making up the solution of base How clean was the beaker and glass rod used to dissolve your base? Why do you use distilled water and not tap water to make up the base solution? If all of the solid is not fully dissolved this would mean less solid that expected in the water, meaning a weaker than expected basic solution. You must ensure all of the base has been fully dissolved for accuracy of results!

Titration Transferring the base to the volumetric flask You could add more to 250cm3 in the beaker but you do not because the graduations are not accurate enough. Therefore you must use a volumetric flask. How clean was the volumetric flask? It must be rinsed with distilled water before use. You need all of the basic solution transferred into the volumetric flask. This is done by using a clean funnel to ensure you do not spill any of the basic solution. You then must rinse the beaker and glass rod with distilled water into the volumetric flask to ensure every drop of the base has been transferred. If some is left behind in the beaker then you have lost some base and this would result in a weaker than expected base solution, affecting your titration results.

Titration Making up the standard solution in the volumetric flask Next, more water is added until the bottom of the meniscus is on the graduation line exactly. If the meniscus was below the line, less than 250cm3 has been added, making your basic solution more concentrated. If the meniscus goes past the line more than 250cm3 has ben added – your solution is too dilute and you must start again! Did you check the meniscus at eye level? If not there would have been a parallax error Did you thoroughly shake the volumetric flask to ensure a uniform solution. If not more concentrated base was at the bottom and less concentrated at the top of the volumetric flask, affecting your titration when you pipette out 25cm3 of the base.

Setting up the titration

Titration Transferring the base to the conical flask You ensured accuracy by using a pipette to measure out the 25cm3 of base and not a measuring cylinder which are not as accurate. Pipettes are already calibrated by the manufacturer. But you could calibrate the pipette to make sure you are using it correctly and also to ensure it does deliver the correct volume of liquid. How clean was the pipette before use? You should rinse it with water if required and then with some of the base from the volumetric flask before use. Did you ensure there were no air bubbles in your pipette when measuring the basic solution? Any air bubbles would mean less base measured out, requiring less acid to neutralise, affecting your titration results. Did you ensure the meniscus was on the line exactly and this was measured at eye level for accuracy? When transferred to the conical flask, how clean was the conical flask? The conical flask is Not rinsed with base before adding the base because then you would have too much base in the conical flask (more than 25cm3!). The conical flask is rinsed with distilled water only!

Titration Preparing the burette How clean was the burette before you used it? Rinse with distilled water if required and then some of the acid before adding any acid. Any remaingin water would dilute the acid, affecting your results! Ensure all of this is removed by the rinse with acid. Then add your acid. Did you ensure that you rinsed below the tap and also that there were no air bubbles? How would any air bubbles affect the volume of acid added and therefore the titration result?

The titration

Titration The titration Was the acid added from the burette at a reasonable rate? Were you going too fast? Was the conical flask being swirled at the same time? If you did not swirl the conical flask it may take time for the reaction to take place and any colour change to happen. You could miss the endpoint, affecting your titration results. Did you add the acid dropwise as you approached the endpoint? You followed the titration with an indicator. How clear was the colour change at the end point? Did you use a white tile to ensure that you could clearly see the colour change at the endpoint? Did you read the initial/final burette volumes from the bottom of the meniscus and at eye level? Did you record results to the nearest 0.05cm3? Did you do a trial titration first so that you knew where the end point would be and you also knew when to slow down and add the acid dropwise?

Titration The titration At the end of each titration, did you thoroughly rinse the conical flask with distilled water before adding another 25cm3 of base? If not any acid left behine would have neutralised your next batch of bas, affecting that titration result. Did you repeat the titration until you got concordant results for accuracy (two titres with 0.1cm3 of each other)?

Titration Second titration You should have done the same things for accuracy as in the first titration. Did you find the colour change easier to follow with the second titration where you used a different indicator?

Results

Titration Calculating the unknown concentration of the acid Did you ignore your trial titration? Did you achieve two titres within 0.1cm3 of each other? Did you double check your calculations? Did you round your values correctly? Did you give answers to 3 or more significant figures and not round up too much to ensure accurate calculations and results?

Titration Evaluating your results How did you ensure accuracy throughout? Where could you have made improvements? How are titrations carried out in industry (use of machines over manual methods). The technician suggested that the HCl and the NaOH solutions were both 0.10 M solutions. How does this compare with your results and the results of your peers? Discuss this in a meaningful and quantitative way. Give specific examples of how you ensured safe working throughout.