Practical investigations
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I'm doing my A2 aspirin investigation where a known excess of base (NaOH) is added to the aspirin and the excess is back-titrated with a standard solution of HCl. I'm using 0.1 M NaOH, but I need to make it up from 1 M NaOH. How do I do this? Do I need to standardize NaOH to achieve this? And if so, (I have never come across standardizing before) how do I standardize? Also the same question with the HCl (I need to get 0.1 M HCl from 1 M HCl too)
Thanks
Corrie writes ....
Diluting a solution by a factor of ten is easy - decide how much dilute solution you need and obtain a volumetric flask of that volume. Using a pipette (or burette) and your stock solution of acid (or alkali), transfer an amount of your stock (1 M) solution equal to one-tenth of the required volume into the flask. For instance, if you need 100 cm3 of diluted solution, measure out accurately 10 3 of the stock solution into a 100 3 volumetric flask.
Then make up the solution in the flask to 100 3 using distilled or deionised water and shake well to mix. You don't need to standardise your NaOH to achieve the dilution, but as NaOH solutions react with the carbon dioxide in the air, you would be well advised to standardise your diluted solution with 0.1 M standard acid solution. Standard acids are acids, such as potassium hydrogenphthalate, which are available as pure, stable solids which can be accurately weighed and therefore made up into solutions of accurately known concentrations. Search in React under 'potassium hydrogenphthalate' and you'll find past FAQs on this subject.
Having made your 'standard' acid solution, you then titrate it it against your sodium hydroxide solution in the usual way, and work out the accurate concentration of the alkali.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 09 September 2007
