# How to make and standardize sodium hydroxide solutions for determination of titratable acidity in winemaking

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 later sections 3 & 4) say by about 5%.

For example, based on use of a wine bottle calibrated to 750 g (0.75L) by weighing water (density 1g/mL) into it:

Weigh out an appropriate mass* of sodium hydroxide, and wash it through a funnel into your calibrated wine bottle filling it not quite full to allow for mixing.  When all the solid has dissolved just add water to make the volume up.

Note it is wise to wear eye and skin protection when handling Na OH and its solutions.

The formula weight of NaOH is 23 + 16 + 1 = 40 g/mol so 40 g (1 mol) in 1 L makes a 1M (=1N ) solution.

Some users want a 0.1M solution, others 0.133M or 0.2M. To get arbitrary volumes of less concentrated solutions just multiply 40 g by the desired concentration(mol/L) and volume in L.

So for only 0.75 L you need only 0.75 mol for 1M; that is 30 g.

This table* gives information for the lesser concentrations and their useage for measuring wine TA (g/L) by the three methods commonly used:

For 0.75 L of :                         Useage:

0.1M  weigh 0.1 x 30  = 3 g   Titrate 15 mL wine:    TA(g/L) = 2 mLNaOH

0.133M  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

The tartaric acid we use is very pure and can be used to make a solution of a known TA very easily. This is called a standard solution. 8.0 g/L is a good choice as it gives a larger titre which is therefore more precise while providing some room for a larger titre when titrating which can occur if the NaOH concentration is too low. Ideally the tartaric acid will be weighed using a scale accurate to 0.1g or even 0.01 g (if you don’t have one, check the Internet for a “jewellery scale” (they are cheap and accurate).

If such a scale is not available, make up the solution at ten times the concentration and then dilute by a factor of ten.

Measure the real concentration of the NaOH solution by titrating to pH 8.2, 10.0 mL (or if you wish, 15.0 mL for methods 1 & 3) of the standard solution (in this example TAstandard = 8.0 g/L). (Note that this will predict a 16 mL titre for Method 1. Such users may wish to make a 4 g/L standard or titrate 5 mL and avoid refilling a syringe).

Then calculate the true Na OH concentration using for Methods 1&3 or 2:

1&3: MNaOH= (15mL/mLNaOH)*2*TAstandard/150

= TAstandard/5mLNaOH

= 1.6/mLNaOH

2: MNaOH= (10mL/mLNaOH)*2*TAstandard/150

= TAstandard/7.5mLNaOH

= 1.0667/mLNaOH

As an example let’s say you were aiming for 0.133M NaOH but step 3 shows it is really 0.147M.  You have two options:

You can correct each subsequent measured TA by the appropriate percentage.  Take the ratio 0.147/0.133= 1.105.

This means that your NaOH is 10.5% too concentrated so all your measurements of must TA using this NaOH solution are too low and should be multiplied by 1.105.  Similarly if the NaOH is too dilute your TA measurements are too high so multiply them by the ratio that is less than one.

If the ratio is one, congratulations!

If the NaOH is too concentrated and you want to make it really 0.133 then measure the volume of your remaining solution (by weighing again?) and add to it 10.5 % water.  Then check it against the standard TA tartaric solution to make sure you got it right.

If it is too dilute you can add some solid Na OH and try again.

STORE YOUR NaOH SOLID and SOLUTION IN AIR-TIGHT PLASTIC BOTTLES.

Contributed by Sandy Kirk

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