A titration uses four kinds of glassware. A volumetric flask makes a solution of an exact volume, such as 100.0 mL, and a volumetric pipette measures out an exact volume, such as 10.00 mL.
A burette is a long graduated tube with a tap at the bottom. It delivers the solution of known concentration and shows how much has been added.
The solution being titrated is placed in a conical flask, whose sloping sides keep the liquid from splashing out when it is swirled.
Glassware is washed with pure water, but water left inside a volumetric pipette or a burette would dilute the solution put into it. These two are therefore rinsed several times with small amounts of the solution they will hold.
A volumetric flask and a conical flask can be used wet. Water in the volumetric flask does not matter because water is added up to the mark, and water in the conical flask does not change the amount of acid that the pipette delivers.
Glassware for measuring volumes is never dried by heating, because heating can change its volume slightly.
A volumetric pipette transfers 10.00 mL of the acid into a conical flask, and a few drops of phenolphthalein are added. The burette is filled with 0.100 mol/L sodium hydroxide solution and its first reading is noted.
The base is run in while the flask is swirled. Each drop turns the liquid pink where it lands, and the colour disappears at once at first but more and more slowly near the end.
The titration stops when a faint pink colour stays for about 30 seconds. This is the end point, and the volume of base added is found from the two burette readings. The titration is repeated until two results agree closely.
A burette is marked every 0.1 mL, and the reading is estimated to one tenth of a division, that is, to 0.01 mL. The eye must be level with the bottom of the curved liquid surface, the meniscus.
Unlike a measuring cylinder, a burette reads 0 at the top, so the numbers grow as the solution runs out. The volume added is the final reading minus the first: 8.45 mL minus 1.25 mL gives 7.20 mL.
Reading from above or below the level of the meniscus gives a wrong value, and so does an air bubble left in the tip below the tap.
To find the ethanoic acid in vinegar, 10.00 mL of vinegar is diluted to 100.0 mL in a volumetric flask, and 10.00 mL of this solution is titrated with 0.100 mol/L sodium hydroxide, which takes 7.20 mL.
Ethanoic acid gives one hydrogen ion, so the diluted solution has a concentration of 0.100 mol/L × 7.20 mL ÷ 10.00 mL = 0.0720 mol/L, and the vinegar 0.720 mol/L, or about 43 g of ethanoic acid per litre.
Solid sodium hydroxide absorbs water and carbon dioxide from the air, so its solution cannot be made to an exact concentration by weighing. Its concentration is first found by titrating a solution of ethanedioic acid (oxalic acid) dihydrate, which can be weighed accurately.