Determination

Sodium hydroxide and dissolved aluminum in caustic etch baths.

Application

In this example, we will explain the process of testing the sodium hydroxide and aluminum concentration in a caustic etch bath that might be used for etching aluminum prior to anodizing or chromate conversion (chemfilm). Various methods have been published for analysis of a solution with an unknown mixture of caustic and dissolved aluminum. The method shown in Reference 1 requires analysis of two bath samples: one with KF added to bind up aluminum and another without the KF. The two results can then be compared to determine the sodium hydroxide and aluminum concentration. Reference 2 describes a method where one sample can be analyzed to achieve the same result. Other vendors of caustic etch chemistries have published variations of the single-sample analysis with similar calculations. Reference 3 discusses the behavior of dissolved aluminum as a function of pH.

The following table shows concentrations of sodium hydroxide and aluminum for a typical caustic etch bath (Reference : BAC 5786 Inhibited Caustic Etch). Most chemical vendors allow higher concentrations of dissolved aluminum as long as the free caustic concentration is increased as the aluminum increases. While the Boeing specification is clear that aluminum metal concentration is specified, some vendor data sheets show the dissolved aluminum as sodium aluminate. Reference 3 shows that dissolved aluminum may be in the form of sodium aluminate (NaAlO2), sodium hydroxide (Al(OH)3), or a number of intermediate states depending on the pH of the solution. Sodium aluminate is 3.04 times heavier than Al, so 1.0 g/L of Al is equivalent to 3.0 g/L of sodium aluminate. The amount of NaF (or KF) used in the following method must be adjusted to ensure complete reaction with the dissolved aluminum. If you prefer ounces and gallons, convert g/L (grams per liter) to oz/gal (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
  42 g/L 49 g/L 56 g/L 
NaOH 5.5 oz/gal6.5 oz/gal7.5 oz/gal
Aluminum    
(dissolved) 4 g/L 8 g/L 12 g/L 
as NaAlO2 12 g/L 24 g/L 36 g/L 
  1.6 oz/gal3.2 oz/gal4.8 oz/gal

Method

1 Pipette a 10.0 mL sample from bath.

2 Add approximately 100 mL of DI water.

3 Add 3-5 drops of phenolphthalein.

4 Titrate with A mL of 1.0N HCl from pink to colorless endpoint.

Swirl flask and be sure pink does not reappear.

5 Add 5 grams NaF and dissolve.

If no pink appears, skip Step 6, and set B = 0.

If pink appears, reset (zero) the burette, and ...

6 Titrate with B mL of 1.0N HCl from pink to colorless endpoint.

Calculations

[ (A mL - (B mL / 3) ] x Factor = Free NaOH

  Sample Titrant Factor
NaOH solid (g/L) 10.0 mL  1.0N 4.00 
NaOH solid (oz/gal) 10.0 mL  1.0N 0.534 
50% NaOH (%v) 10.0 mL  1.0N 0.524 

B mL x Factor = dissolved aluminum

  Sample Titrant Factor
Al (g/L) 10.0 mL  1.0N 0.9 
Al (oz/gal) 10.0 mL  1.0N 0.12 
NaAlO2 (g/L) 10.0 mL  1.0N 2.74 
NaAlO2 (oz/gal) 10.0 mL  1.0N 0.37 

Explanation

Sodium hydroxide is a strong base with a single point of dissociation. Dissolved aluminum behaves as a weak base with neutralization of three hydroxide ions occurring at three distinctly different pH points. This is shown in the following curve (from Reference 3).

Al(OH)3 Titration Curve

As HCl is added to the sample, the pH is reduced to the first knee of the curve where all of the strong base (NaOH) and approximately 1/3 of the weak base has been neutralized. We cannot determine the amount of Free NaOH or Al at this time, so we just remember the amount of titrant used.

When NaF (sodium fluoride) is added, it binds up the aluminum in the sample as AlF3. This releases all of the sodium hydroxide that is locked up with the aluminum, if there is any aluminum in the sample. This will cause the pH of the sample to increase as a result of the increase in NaOH that is proportional to the amount of aluminum. Dissolved aluminum takes the form of sodium aluminate (NaAlO2) at high pH, transitions to Al(OH)3 as the pH nears 7.0 and precipitates as Al(OH)3 in a pH neutral solution. So, 3 moles of NaF react with 1 mole of Al.

Al(OH)3 + 3 NaF —> 3 NaOH + AlF3

NaF molecular weight is 41.99 amu, and Al molecular weight is 26.98 amu, so you will need 41.99 / 26.98 = 1.56 gram of NaF for every gram of dissolved aluminum that might be in the sample. The Isoprep 35 caustic etch bath contains a maximum 10 g/L of dissolved aluminum, so a 10 mL sample could contain as much as 0.1 gram of aluminum which would require 0.156 grams of NaF for complete reaction. The 5 grams specified in the MacDermid TDS will be sufficient to remove 3.2 grams of Al from the 10 mL sample which corresponds to 32 grams of dissolved Al or 96 grams of sodium aluminate (NaAlO2) from the bath.

After the addition of NaF, the solution is titrated again. Three OH- ions are released for every Al atom, so the amount of HCl required to neutralize the NaOH can be used to calculate the amount of Al in the sample.

In Step 5, all of the dissolved aluminum from the original sample is converted to AlF3, and the hydroxide that was bound with the aluminum will be returned to solution as NaOH. In Step 6, this additional NaOH is then neutralized with HCl, reducing the pH to bring the solution back to a clear endpoint. In summary ...

Step 4 titrant is A mL, and this neutralized all of the NaOH and 1/3 of the dissolved aluminum.

Step 5 returns the OH bound with Al to solution.

Step 6 titrant is B mL which is the amount required to neutralize 1/3 of the Al, since this was previously neutralized in Step 4.

So, Al in g/L = (mL of titrant) x 26.98 x 1/3 x 1.0N / (sample size in mL) = (mL of titrant) x 0.9 where 26.98 is the weight of aluminum in amu.

Note : This method defines the amount of titrant used in Step 6 as B mL, and it is determined by zeroing the buret prior to Step 6. Other methods might continue titration and calculate the amount used for Step 6 by subtracting the A mL used in Step 4 from the total amount used for the entire titration. Many technical data sheets do not explain this very well, and it can be confusing.

Accuracy

The pH change at the first hydroxide dissociation is gradual, and it is difficult to determine the exact point where it occurs. The accuracy of determining both free NaOH and Al concentration depends on estimating the point at which 1/3 of the Al(OH)3 has been neutralized. The phenolphthalein color change is an approximation. Other methods use the formation of the Al(OH)3 precipitate as indication of the endpoint, but the precipitate is difficult to see, and is also gradual. With either approach, it is impossible to determine exactly how much aluminum remains in solution at any time. One can accurately determine the total caustic; however, the important numbers are free caustic and dissolved aluminum, and these can only be estimated by any volumetric analysis. You may as well use this analysis because it can be done with one sample and works even if there is no dissolved aluminum present.

References

Volumetric Analysis of Metal Finishing Solutions

Andrew McFadyen, B.Sc

Finishing Publications Ltd, 1998

Isoprep 35 Technical Data Sheet

MacDermid Enthone PC 110531

Issue 12 February 1999

Form and Stability of Aluminum Hydroxide Complexes in Dilute Solution

J.D. Hem and C.E. Roberson

Geological Survey Water Supply Paper 1827-A

Thank you to DataLab in San Jose for running tests on pure sodium hydroxide with pure aluminum dissolved to confirm this method without the uncertainty of other additives that may be present in commercial etch chemistries.