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Sulfuric acid

In this article, we explain the basics of sulfuric acid-based anodizing baths and provide tips on how to determine and adjust the concentration.

Contents

1. Basics of choosing the acid concentration in the anodizing bath

The acid concentration – along with other parameters – has a significant influence on layer formation. As explained below, two competing processes generally occur in the bath, and by carefully choosing the concentration, one process can be favored over the other.

During layer formation, several processes occur simultaneously in the bath, of which only the two most important are relevant here:

  • The layer growth with the formation of anodic ceramic by the reaction of electrolytically generated oxygen with the surface aluminum.
  • The partial layer dissolution through the dissolution of parts of the formed layer by the existing sulfuric acid.
Both processes are necessary to ensure the appropriate layer thickness and pore formation: the dissolution process by the acid ensures that through the resulting "holes," electrolyte can reach the aluminum surface, preventing the current flow from breaking (which would inevitably occur if we had a closed anodic layer with very high electrical resistance).

This leads to:

  • The higher the acid concentration, the larger the pores in the layer, the slower the layer grows, and the softer it becomes.
  • The lower the acid concentration, the smaller the pores, the faster the layer grows, and the harder it becomes.
This has consequences for the dye absorption capacity of the layer (which directly depends on the pores):
  • The smaller the pores and the fewer they are, the worse the dye absorption.
  • The larger the pores and the more they are, the better the dye absorption.
The adjacent diagram illustrates these relationships again.

The consequences are:

If a hard, thick layer is desired (hard anodizing), the acid concentration is set lower.
If the best possible dye absorption is desired – mainly for decorative purposes – a higher acid concentration is chosen.

Note:
By using our „hard anodizing additive“, very good hard anodic layers can also be produced in baths with higher acid concentrations. More details can be found in our „TM-7 Hard Anodizing“ in the Download Area.

2. Measuring the acid concentration

2.1  Hydrometer / Sink Scale

The simplest, fastest, and most commonly used method is to determine the density of the acid bath using a hydrometer (sink scale). This uses the simple principle that a body sinks into a liquid until the weight of the displaced liquid equals the weight of the body itself. If the body is marked appropriately, the density can be read directly from these marks.

Usually, sealed glass hydrometers are used, which contain metal beads as weight and are calibrated accordingly. Since the density of liquids also depends on temperature, the values read on a hydrometer are always valid for a specific temperature, usually indicated on the scale.

Depending on the liquid to be measured, hydrometers are available for different density ranges: for light liquids like alcohol (density approx. 0.79 g/cm³, 790 g/L) dissolved in water, one uses a range from 0.79 (pure alcohol) to 1.00 (pure water = 1.00 g/cm³) to determine the concentration of pure alcohol in spirits. The closer the minimum and maximum values, the more precise the reading, but the narrower the range in which the hydrometer can be used. One should consider the possible highest or lowest densities and choose the hydrometer accordingly – or obtain several hydrometers with consecutive ranges to measure very precisely.

For diluted sulfuric acid in the anodizing range, a hydrometer with a measurement range from 1.00 (pure water = 0% acid) to 1.20 (approx. 28% acid) is usually used. This allows all common acid concentrations to be determined.

Measuring with a hydrometer is simple: the hydrometer is slowly lowered into the liquid to be measured, and the value at the liquid level is read. This density value is then converted to a percentage using a table.

2.2  Weight determination from volume

This is another simple method. A precise volume of the anodizing bath is measured, and its weight is determined. Using the simple formula

Density = Mass / Volume

the density can be calculated, and from the table, the acid concentration can be determined.

Example:
50 cm³ of the anodizing bath is taken using a volumetric flask and weighed. With a result of 55.2 g, the density is calculated using the formula 55.2 g / 50 cm³ = 1.104 g/cm³. The table then gives an acid concentration of slightly over 15%.

Note that other substances such as dissolved aluminum or additives also affect the density, slightly distorting the result. However, due to the much higher sulfuric acid concentration, the deviations are usually negligible. For precise acid determination, a so-called titration must be carried out, as briefly described in the next section.

2.3 Titration

Titration determines the amount of an unknown acid by adding a base of known concentration until the solution reacts neutrally (pH 7). The pH is monitored either with special dye indicators, which change color at neutrality, or (more accurately today) with electronic pH meters.

Titration is much more accurate than the previously described density measurement methods, as only the actual acid is measured, not dissolved aluminum.
However, it is more labor-intensive and usually disproportionate if only refreshing an existing bath. Therefore, we omit a detailed procedure here. Numerous resources are easily found online for those interested.

3. Calculation of dilution

Since commercially available acid often has a different (=higher) concentration than needed, an important practical question is:
"How much water must I add to acid of concentration X% to obtain acid of concentration Y%?"
The following simple steps explain this and demonstrate it with an example.

When diluting liquids, the calculation is practically always done using masses (or – incorrectly – "weight") of the components. This is because mixing volumes can change (non-linearly), while masses remain constant: 1 liter water + 1 liter acid does not yield exactly 2 liters of diluted acid, but 1 kg acid + 1 kg water still weighs exactly 2 kg. This phenomenon is called volume contraction.

To obtain a desired concentration, proceed as follows:

  1. Find the corresponding amount of sulfuric acid (column 3, "g H2SO4/l") for the desired concentration in the table and multiply it by the required volume in liters.
  2. Find similarly the amount of sulfuric acid per liter in the available acid.
  3. Divide the required amount (from step 1) by the amount available per liter (from step 2) to find how many liters of starting acid are needed.
  4. Finally, add distilled water to reach the desired total volume.

All acids heat up significantly when diluted, so they should be added slowly to plenty of cold water, never the reverse! For the above procedure, this means the calculated amount of acid (step 3) is first added slowly to a suitable amount of cold water, then the remainder is added to reach the desired final volume.
Do not forget: wear protective gloves, safety goggles, and old clothing!

Example: We have 38% battery acid and want to make 10 liters of 15% acid from it.

  1. Step 1: Look up the required amount of sulfuric acid per liter for 15%: 165.30 g. For 10 liters, multiply by 10:
    10 x 165.30 g = 1653.0 g
  2. Step 2: Find the amount of sulfuric acid per liter in our 38% starting acid: 488.50 g
  3. Step 3: Divide the required amount (1653.0 g) by the available amount (488.50 g/L) to get the needed volume of battery acid:
    1653.0 g / (488.50 g/L) = 3.38 liters
    Measure this volume in a container.
  4. Step 4: To make 10 liters of acid but only need 3.38 liters of acid, first pour e.g., 5 liters of water, then slowly stir in the measured acid (3.38 liters). This yields about 8 liters of acid bath, then carefully add water to reach 10 liters – done!

4. Disposal of used anodizing baths

Disposal instructions for used anodizing baths can be found in our „TM-5 Disposal of Chemicals“ in the Download Area.

5. Table of sulfuric acid concentrations

Below is a table of sulfuric acid concentrations. Under "Notes," the recommended ranges for each acid concentration are indicated.


H2SO4
in %		 Density ρ (+20°C)
in g/cm³		 H2SO4
in g/l		 H2SO4
in mol/l		 Notes
1 1,0051 10,05 0,103
2 1,0118 20,24 0,206
3 1,0184 30,55 0,312
4 1,0250 41,00 0,418
5 1,0317 51,59 0,526 Hard Anodising (*1)
6 1,0385 62,31 0,635
7 1,0453 73,17 0,746
8 1,0522 84,18 0,858
9 1,0591 95,32 0,972
10 1,0661 106,60 1,087
11 1,0731 118,00 1,203
12 1,0802 129,60 1,321 Decorative
Anodizing
13 1,0874 141,40 1,442
14 1,0947 153,30 1,563
15 1,1020 165,30 1,685
16 1,1094 177,50 1,810
17 1,1168 189,90 1,936
18 1,1234 202,40 2,063
19 1,1318 215,00 2,192
20 1,1394 227,90 2,324
21 1,1471 240,90 2,456
22 1,1548 254,10 2,591
23 1,1626 267,40 2,726
24 1,1704 280,90 2,864
25 1,1783 294,60 3,004
26 1,1862 308,40 3,144
27 1,1942 322,40 3,287
28 1,2023 336,60 3,432
29 1,2104 351,00 3,579
30 1,2185 365,60 3,728
31 1,2267 380,30 3,878
32 1,2349 395,20 4,029
33 1,2432 410,30 4,183
34 1,2515 425,50 4,338
35 1,2599 441,00 4,496
36 1,2684 456,60 4,656
37 1,2769 472,50 4,818 Battery acid
38 1,2855 488,50 4,981
39 1,2941 504,70 5,146
40 1,3028 521,10 5,313
41 1,3116 537,80 5,483
42 1,3205 554,60 5,655
43 1,3294 571,60 5,828

(*1) Using our hard anodizing additive, even baths for decorative (=colored) anodizing (12-20% acid) can be used for hard anodizing, so only one anodizing bath is needed.