Acidex  is a double-salt seeded calcium carbonate designed to reduce both tartaric and malic acids in juice or wine.  Potassium bicarbonate just reduces tartaric.

(Acidex is no longer available, but some winemakerw still need the instructions
for use after fermentation for acid reduction.)
  

Before making any adjustments to acidity it is imperative that you use an
accurate acid testing kit to measure and record the initial acidity. Without it you
can only make matters worse by guessing. Acidex is a double-salt seeded
calcium carbonate designed to reduce both tartaric and malic acids in juice
or wine. This is unlike conventional calcium carbonate, which reduces only the
tartaric acid. It is not recommended for use on other fruits than grapes as there
is not enough tartaric available. The process was developed in Germany where
the cool growing season often produces grapes of high acidity. The process is
considered by many knowledgeable winemakers to be the best technique for
excess acid reduction.

The following is a simplified technique for the use of Acidex.
1. Determine the acidity of the juice or wine (expressed as tartaric).

2. Decide what final acidity is desired.

3. Next determine the amount of Acidex to be used as follows: Total
volume in gallons X (times) acidity reduction required X (times) 5* = Acidex in
tablespoons (level, not packed). OR Total volume in gallons X (times) acidity
reduction required X (times) 26* = Acidex in grams.
     5 gallons X .4 x 5 = 10 tablespoons of Acidex
     5 gallons X .4 x 26 = 52 grams of Acidex
EXAMPLE: For example we willl use a 5-gallon carboy of juice (wine) at acidity
of 1.2 with a desired acidity of .8, for a reduction of .4. 

4.  Treat only a PORTION of the juice or wine with the Acidex initially.
This part volume can be determined as follows: Total volume in gallons X (times) (the acid reduction required divided by the starting acidity minus 0.2) equals the portion to be treated.
EXAMPLE: 5 gallons juice at acidity of 1.2 with desired acidity of 0.8.
(Five) 5 gallons X (times) .4 (2.0) divided by 1.2 minus .2 (1) = 2 gallons to be
treated.

NOTE***

***The amount of Acidex determined in step 3 should be placed in a container
large enough to accommodate the portion of wine to be treated determined
in step 4.

***Always add the untreated wine to the Acidex or its suspension, mixing
continually. NEVER add the Acidex suspension to the quantity being treated.
Add 10% of the portion to be treated to the amount of Acidex,
mixing well as you add.
Slowly add the remaining 90% ( of the volume to be treated) and continue stirring until the foaming subsides.

Allow to settle. Settling will take from 2 to 24 hours. Separation by filtration
immediately is advisable. If filtration is not possible, rack, following with
subsequent racking as needed.

Removal of the lees into a smaller container for a second and third racking will
help salvage more of the treated portion.

Pump or siphon the treated portion into the untreated portion and
stir until thoroughly mixed.

Allow the de-acidified wine to stand at least three months before bottling. Give the wine time to rid itself of the dissolved CO2, which resulted from treatment.

NOTE: Acid reduction with Calcium Carbonate, Potassium Bicarbonate,
OR Acidex can not exceed more than a .4% total reduction either by combined
or any one application.

5.2 grams of Acidex will reduce the acidity of 1 gallon about 0.2
5.2 grams of Acidex = 1 level tablespoon, not packed