FermCalc - Acidity Calculations

Introduction

The Acidity Calculations panel performs two types of calculations:  titration and acid balancing.  The titration calculator determines the acidity from the results of a titration with NaOH.  The acid balancing calculator determines the amount of acid to add to a given must or wine to raise the acidity to a specified level, and it can also estimate the amount of calcium carbonate, potassium carbonate, or potassium bicarbonate required to reduce the acidity to a target level.

Making a Calculation

To make a calculation, follow these steps:

1. Select Calculation > Acidity Calculations from the menu, or select the Acid tab at the top of the main window.
2. Select the type of acidity calculation by selecting either the Titration tab or the Acid Balancing tab.
3. Select the appropriate units for the input fields using the drop-down menus in the right-hand column.
4. If Titration was selected:
   1. Enter the Wine Sample Volume, Volume of NaOH Added, and NaOH Normality in the appropriate fields.
   2. The calculated acidity appear in the Titratable Acidity (TA) field.
5. If Acid Balancing was selected:
   1. Enter the Wine Volume, Initial Acidity, and Target Acidity in the appropriate fields.
   2. If the target acidity is greater than the initial acidity, FermCalc will show a dropdown list of acid types for increasing the acidity. Select the Acid Type from the drop-down menu.
   3. If the target acidity is less than the initial acidity, FermCalc will show a dropdown list of additives for reducing the acidity. Select the Deacidifier from the drop-down menu.
   4. The calculated amount of acid or deacidifier required to achieve the target acidity will appear in the Acid Required or the Deacidifier Required field.

Titration Calculation Details

Acid titration makes use of the neutralization reaction between NaOH and the acids present in wine. In this reaction, the OH^- ions donated by the NaOH and the H⁺ ions donated by the wine acids combine to form water, H₂O. The reaction is complete when all of the H⁺ ions have been neutralized by OH^- ions.  Since each OH^- ion neutralizes one H⁺ ion, we can write a balance between the H⁺ and OH^- ions as:

ia(ma/mwa) = vnNn

(1)

where

i_a = number of H⁺ ions donated by each molecule of acid
mw_a = molecular weight of acid, grams/mole
m_a = mass of acid, grams
v_n = volume of NaOH added, liters
N_n = Normality of NaOH solution, moles/liter

The mass of the acid in solution is simply the acidity multiplied by the volume or:

ma = a·vw

(2)

where

a = acidity, grams/liter
v_w = volume of wine sample, liters

Combining equations (1) and (2) and solving for acidity we get:

a = (mwa/ia)vnNn / vw

(3)

By titrating for acidity, all we've really determined is the number of available H⁺ ions in the solution and not the types of acid present.  For this reason we must choose an acid as a reference in order to express the acidity as a concentration. Tartaric acid is frequently used as a reference, and is the default acid reference in FermCalc. For tartaric acid, mw_a is 150.09, and i_a is equal to 2, so equation (3) becomes:

a = 75.0·vnNn / vw

(4)

Acid Balancing Calculation Details

There are two scenarios we need to consider for this calculation.

1. Initial acidity is less than the target acidity.
2. Initial acidity is greater than the target acidity.

For the first scenario, FermCalc will calculate the amount of acid required to increase the acidity to the specified target. For the second case FermCalc will calculate the amount of deacidifier to add.

If you are diluting a must with water, blending juices, or blending wines to adjust acidity, use the Blending panel to calculate the proportions required to achieve the desired acidity.

Case 1: Initial Acidity is Less Than the Target Acidity

For this case we'll be adding acid to increase the acidity. FermCalc first converts the initial and target acidity values to the tartaric reference using the conversion factors explained in the Acidity Conversions discussion.

To calculate the required addition of the selected acid we need to write a molar balance equation for the H⁺ ions in the wine.  The final (or target) number of H⁺ ions must equal the initial number of H⁺ ions plus the number of H⁺ ions added, or:

itatv/mwt = itaiv/mwt + iama/mwa

(5)

where

i_t = number of H⁺ ions per molecule for tartaric acid
a_t = target acidity, grams/liter tartaric
a_i = initial acidity, grams/liter tartaric
v = volume of wine, liters
mw_t = molecular weight of tartaric acid, grams/mole
i_a = number of H⁺ ions per molecule for the acid being added
m_a = mass of acid required, grams
mw_a = molecular weight of the acid being added, grams/mole

Rearranging to solve for the mass of acid to add we get:

ma = v(at - ai)(it/mwt) / (ia/mwa)

(6)

Values of molecular weight and number of H⁺ ions per molecule are shown in the table below.

For Acid Blend, the composition is assumed to be 40% tartaric, 40% malic, and 20% citric acids.

Case 2: Initial Acidity is Greater Than the Target Acidity

For this case we'll need to add either calcium carbonate (CaCO₃), potassium carbonate (K₂CO₃), potassium bicarbonate (KHCO₃), or potassium hydroxide (KOH) to reduce the acidity. All of these work by neutralizing tartaric acid, or H₂T, where T represents a tartrate ion (C₄H₄O₆). The reaction equations are (based on Margalit, 2004, and Beelman et. al., 1979):

CaCO3 + H2T  --->  CaT + H2O + CO2	(7)
K2CO3 + 2H2T  --->  2KHT + H2O + CO2	(8)
KHCO3 + H2T  --->  KHT + H2O + CO2	(9)
KOH + H2T  --->  KHT + H2O	(10)

To calculate the required amount of any of these additives, we need to know their molecular weights as well as the number of H₂T molecules that are neutralized by each molecule of deacidifier. These parameters are tabulated below.

We can calculate the required mass of deacidifier to add by balancing the initial and final quantities of H₂T molecules present against the number of molecules neutralized, or:

n·mr/mwr = v(ai - at)/mwt

(11)

where

n = molecules of H₂T neutralized per molecule of acidity reducer
m_r = mass of acidity reducer added, grams
mw_r = molecular weight of the acidity reducer being added, grams/mole
v = volume of wine, liters
a_t = target acidity, grams/liter tartaric
a_i = initial acidity, grams/liter tartaric
mw_t = molecular weight of tartaric acid, grams/mole

Solving for the amount of acid reducer we get:

mr = v(ai - at)mwr/(n·mwt)

(12)

Based on equation 12 above, FermCalc calculates the following addition rates to reduce acidity by 0.1% (1 g/l) tartaric.

For additional information on using these additives, see this article by Bill Collings.

© 2007-2010 Steve Gross
Last updated 30 April 2010.