Acid-base titrations or neutralization titrations depend upon the neutralization of an unknown quantity of acid with a known quantity of base, or vice versa. These titrations are used to measure acid concentrations in acid etch tanks, anodizing tanks, and plating tanks, and to measure base concentration in caustic etch tanks. As an example, let’s go through the process of testing the sulfuric acid and aluminum concentration in a bath used for MIL-A-8625 Type II anodizing. A similar titration may be used for a MIL-A-8625 Type III anodizing bath; although the concentrations of sulfuric and aluminum are normally different. The target range of acid concentration for a typical (Type II) sulfuric anodizing tank is 180 to 200 g/L, and dissolved aluminum in normally held between 4 and 12 g/L. If you prefer ounces and gallons, convert g/L (grams per liter) to oz/g (ounces per gallon) by dividing by 7.5. The factor of 7.5 is (28.375 grams/ounce)/(3.7854 liters/gallon).

  min mid max
Sulfuric acid 180 g/L 190 g/L200 g/L
(66°Be H2SO4)     
Aluminum 4 g/L 8 g/L12 g/L
(dissolved)     

NOTE : Aluminum hydroxide, Al(OH)3, is soluble in acid (low pH) but insoluble in water (neutral pH). So, when there is dissolved aluminum in a bath, the common terminology of "total acid" and "combined acid" is misleading. As the pH of a sample increases, the precipitation of OH ions from the bath as solid Al(OH)3 acts as a buffer preventing the pH from rising as more NaOH is added. This buffering effect continues until all of the Al(OH)3 is removed, and then the pH rises very quickly upon further addition of NaOH. Finally, at higher pH, the Al(OH)3 redissolves and drives the pH even higher. If the only chemicals in solution are strong and weak acids, free acid can be measured by titrating to a low pH, and total acid can be measured by titrating to a higher pH. However, when dissolved aluminum is present, the amount of NaOH required to reach a higher pH is not a measure of total acid because of the buffering effect of dissolved aluminum.

1 Pipette a 5.0 mL sample from bath.

This amount is critical since we will measure the number of molecules in this sample and use the sample size to compute the weight of those molecules (grams) per unit of volume (liter). Any error in the sample size will cause an error in the computation of both acid and dissolved metal concentration.

2 Dilute with 50 mL of DI water.

This measurement is not critical because the critical number of molecules of each measured substance is unaffected by the dilution.

3 Add a few drops of methyl orange.

The aluminum hydroxide will remain in solution as long as the pH is low so we cannot use phenolphthalein for this part of the analysis. Methyl orange transitions from orange color to yellow color at pH of 3.6: this color change occurs when the NaOH neutralizes the first H+ from the H2SO4, before there is sufficient NaOH to cause precipitation of the dissolved aluminum or to dissociate the second H+. The first dissociation of the H+ from the H2SO4 has occurred at pH 3.6, so the amount of titrant used in this first part of the analysis can give us a measure of the free acid (or the total amount of H2SO4).

4 Titrate with B mL of 1.0N NaOH from orange to a yellow endpoint.

As the sodium hydroxide is added, sodium sulfate and water is formed. The reaction is: H2SO4 + 2 NaOH —> Na2SO4 + 2 H2O

From the reaction equation: two moles of NaOH have reacted with one mole of H2SO4, when the solution begins to transition from acid to base (pH of 3.6) so: (98.073 grams) x (1 mole H2SO4/2 moles NaOH) = 49.04 grams of H2SO4 is neutralized by one mole (1 liter of 1.0N) of NaOH.

Since the bath sample is 5.0 mL, 49.04/5.0 = 9.807 grams per mL titrant. This is the number we need for the first part of the titration:
B mL of 1.0N NaOH x 9.807 = grams/liter of H2SO4.

5 Add a few drops of phenolphthalein indicator.

At this point in the titration the H2SO4 has been neutralized by the NaOH, but there is insufficient NaOH in solution to precipitate the aluminum hydroxide. Phenolphthalein changes from clear to a pink color at pH of 8.6. This pH indicates near completion of the reaction when the solution becomes highly alkaline due to an abundance of free OH- ions.

6 Continue titration to total of A mL of 1.0N NaOH from colorless to pink endpoint.

As the sodium hydroxide continues to be added, the pH increases. Since aluminum hydroxide is soluble in acid but insoluble in neutral solutions, it precipitates and removes OH ions, thereby acting as a buffer: the NaOH increases the pH, but the precipitating Al(OH)3 buffers the change as long as there is Al(OH)3 remaining in solution. For every 3 molecules of NaOH, one molecule of Al(OH)3 is formed. The reaction is:
Al2(SO4)3 + 6 NaOH —> 3Na2SO4 + 2Al(OH)3

From the reaction equation: six moles of NaOH have reacted with two moles of Al when the solution begins to transition from neutral to base (pH of 8.6) so: 2 x (26.982 grams) x (2 moles Al/6 moles NaOH) = 8.985 grams of Al reacts with one mole (1 liter of 1.0N) of NaOH.

Since the bath sample is 5.0 mL, 8.985/5.0 = 1.797 grams per mL of additional titrant, where the additional titrant is A mL - B mL.

This is the number we need for the second part of the titration:
A mL - B mL of 1.0N NaOH x 1.797 = grams/liter of dissolved Al.

This method analyzes acid and aluminum using one bath sample, and the anodizing baths in the ChemTrak template library calculate the concentrations of both components when the two titrant amounts are entered. A similar technique is used to speed up titrations for caustic etch tanks requiring analysis of NaOH (or free caustic) and dissolved aluminum.

A different approach to analyzing for free acid and dissolved aluminum is to add KF (potassium fluoride) to one sample thereby removing the dissolved aluminum as AlF3. With the dissolved aluminum removed, the free acid can be accurately determined. After addition of KF, the choice of indicators is less important because methyl orange or phenolphthalein will both change color at almost the same equivalence point; however, phenolphthalein is the better choice because it insures the total dissociation of the acid and you will compare the titrant amounts from two samples using the same indicator. Another sample is then evaluated without the KF addition using phenolphthalein as indicator to determine the total acid (i.e. to neutralize the sulfuric acid and precipitate the aluminum hydroxide). The difference between the titrant amounts used in these two samples shows the amount of dissolved aluminum. Theoretically, these two approaches should give equivalent results; however, we have experimentally determined that adding KF to remove the aluminum gives a more accurate analysis of free acid and dissolved aluminum; this is likely because the second dissociation of the H2SO4 is removed as a variable. However, you must be careful to add enough KF to remove ALL of the dissolved aluminum. If you are designing a method for a bath where the dissolved aluminum can reach 20 g/L, a 5 mL sample could contain as much as 0.1 gram of aluminum. In this case you should add at least 0.25 grams of KF (KF molecular weight is 58.1 amu; Al molecular weight is 26.98 amu; 0.1 gram of Al x 58.1/26.98 = 0.215 gram of KF).