Titratable acidity in wine and must, by Dr. Stührk's method:- 

The juice or wine to be analysed should be as clear as possible and free from carbon dioxide; if necessary it must be filtered or shaken until free from CO₂.

Fill the cylinder accurately up to the '0' mark (approx 10 ml). When doing this hold the cylinder at eye level and take care that the lowest part of the meniscus just touches the '0' mark.
The special 'acidity test' liquid is now added, drop by drop, shaking the cylinder constantly. Eventually a pale green colouration is obtained, a further 1 to 2 drops are then added carefully and the contents of the cylinder thoroughly mixed by applying the thumb to the top of the cylinder and inverting it. As soon as the colouration changes from green to green-blue the determination is complete.
The acidity in grams per litre ('tartaric equivalent') is then read off the scale; at the lowest part of the meniscus.

After the test the dropping bottle must be closed tightly with the sealing cap, otherwise the liquid can change composition.