Dr. Sandy Kirk Nov 2018
1. Prepare a calibrated vessel for making up solutions.
Most of us don’t have accurate volumetric flasks but a wine bottle can be used instead. A clear glass 1 L bottle is ideal but as we all have lots of 750’s I’ll choose that as our example.
Weigh the bottle or tare it out on a scale reading to 1 g or better. Using a funnel, add room temperature water until the mass increases by 750 g, equivalent to 750 mL. This should fill the bottle into the narrow part of the neck. Place a mark or a piece of tape so that it shows the underside of the meniscus. This is now your 750 mL volumetric flask.
You can calibrate syringes and other containers in the same way. For smaller volumes just deliver the volume into a tared container ten or more times and divide the total by ten or more to get higher accuracy.
2. Make sodium hydoxide solution. Weigh out an appropriate amount of sodium hydroxide, and wash it through a funnel into your calibrated wine bottle filling it not quite full. When all the solid has dissolved just add water carefully to make the volume up to exactly 750 mL.
The formula weight of NaOH is 23 + 16 + 1 = 40 g/mol so 40 g in 1 L makes a 1M (=1N ) soln.
For only 750 mL you need only three quarters of this for 1M; that is 30 g. For lesser concentrations:
For 750 mL of : Useage:
0.1M weigh 0.1 x 30 = 3 g Titrate 15 mL wine: TA(g/L) = 2 mLNaOH
0.1333M 0.133 x 30 = 4 g Titrate 10 mL wine: TA(g/L) = mLNaOH
0.2M weigh 0.2 x 30 = 6 g Titrate 15 mL wine: TA(g/L) = mLNaOH
3. Standarize your solution. Sodium hydroxide is only about 97% pure and it forms sodium carbonate and bicarbonate by reaction with moist air as it ages (there are ways to make it up removing these impurities but it is not worth the bother for winemakers). Moreover it comes in the form of pellets so that it is almost impossible to make up a solution to a particular concentration 100% accurately. It is therefore simpler to make it up approximately, preferably a bit stronger than the target value (see next section) say by about 5%.
Measure the real concentration by titrating to pH 8.2, 10.0 mL of a tartaric acid solution made up to a known TA(g/L). Ideally the tartraic acid will be weighed using a scale accurate to 0.1g or better. If not, make up the solution at ten times the concentration and then dilute by a factor of ten. The solution provided in the test is 7.1 g/L.
Then calculate the NaOH concentration using the equation
4. Correct your results or adjust your solution if you want. Let’s say you were aiming for 0.1333M NaOH but step 4 shows it is really 0.147M. All is not lost.
A. You can correct each measured TA by the appropriate %age. Take the ratio 0.147/0.133= 1.105.
This means that your NaOH is 10.5% too strong so all your measurements of TA should be multiplied by this 1.105. Similarly if it is too weak and the ratio turns out less than one.
B. If it is too strong and you want to make it really 0.133 then measure the volume of your remaining NaOH (by weighing again?) and add to it 10.5 % water. Then check it against the standard TA solution to make sure you got it right. STORE YOUR NaOH IN PLASTIC BOTTLES.