Complexing or complexometric titrations use the precipitation or color of chemical compounds to detect endpoints of chemical reactions. This type of titration is used to measure nickel concentration in electroless nickel baths, electro-deposited nickel baths, and nickel-acetate seal baths. It is also used to analyze other metals in various plating baths. This analysis method uses EDTA (ethylenediaminetetraacetic acid) in either direct or back titration. With EDTA, free metal ions are removed from solution as they are changed into complex ions. The pM (negative log of metal ion concentration) vs volume of titrant in EDTA titrations is similar to pH vs volume of titrant in acid-base titrations. At the end point of an EDTA titration, the pM rapidly increases due to the removal of metal ions from solution by EDTA.

Any method which can measure disappearance of free metal ions can be used to detect the end point in EDTA titrations. A common indicator is eriochrome black T which undergoes a change in color from red, in the presence of metallic salts with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti Co, Pt, and Ni, to blue when the metal has been complexed. Murexide is another indicator which changes from violet to blue as Cu or Co salts are removed from solution. Murexide complexes Ni to form a yellow compound and the solution color changes to purple when the nickel is complexed with EDTA.

We will also explain a complexometric titration involving color changes as nickel chromate (NiCrO4) is displaced by silver chromate (Ag2CrO4) as it reacts with silver nitrate (AgNO3). As the first example of complexometric titration, let’s go through the process of testing the total nickel metal concentration in a plating bath. Variations of this method can be used for sulfamate nickel or Watts nickel per QQ-N-290, AMS 2403, AMS 2423, and AMS 2424. as well as for an electroless nickel bath used for plating per MIL-C-26074, ASTM B733 or AMS 2404. For electroless nickel baths, the concentration of total nickel is directly useful, since there is only one nickel measurement needed for the bath control. In the case of sulfamate and Watts baths, the total nickel is one step in the process because there are two nickel concentrations that must be measured to control the bath - this is explained later. The titration steps for total nickel and explanation follow. This method can be used to determine nickel chloride concentration in either a sulfamate nickel bath or a Watts nickel bath, and we will show how it works for the Watts bath.

  Molecular
Weight
Nickel Metal 58.710
Potassium chromate (K2CrO4) 194.190
Silver Nitrate (AgNO3) 169.873
Nickel Chloride (NiCl2) 129.616
Nickel Sulfate (NiSO4-6H2O) 262.858
Nickel Chloride (NiCl2-6H2O) 237.706

1 Pipette a 10.0 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 nickel metal concentration.

2 Dilute with ~50 mL of DI water.

This measurement is not critical because the amount of nickel in the sample is unaffected by the dilution.

3 Add 10 mL of concentrated ammonium hydroxide.

Ammonium hydroxide serves as a buffer in the pH range of 9 to 10. The pH of the solution is critical since H+ ions compete with the nickel ions to complex with the EDTA molecule.

4 Add a pinch of murexide indicator to pale straw color.

100 grams of murexide indicator is a mix of 1 gram murexide and 99 grams NaCl. The murexide complexes with nickel ions to form the yellowish nickel-murexide complex.

5 Titrate with 0.1M EDTA to a purple endpoint.

At the endpoint, the nickel ions are complexed with the EDTA leaving the purple murexide free. Each EDTA molecule wraps up a Ni-- ion, taking it out of solution. The total nickel per liter of 1.0M EDTA is then 58.71 grams - the molecular weight of Ni. Conversion to our sample size of 10 mL and titrant molarity gives 0.1 x 58.71 g/L / 10 mL = .5871 g/L nickel per mL of 0.1M EDTA titrant.

Since the ratio between titrant molarity and sample size is determined by the chemical reactions, this analysis can be used for other similar sample sizes and titrant molarities. For example: 0.1M EDTA and sample size of 10 mL gives 0.1 x 58.71 grams / 10 mL = .5871 g/L nickel per mL
0.1M EDTA and sample size of 2 mL gives 0.1 x 58.71 grams / 2 mL = 2.935 g/L nickel per mL
0.575M EDTA and sample size of 2 mL gives 0.575 x 58.71 grams / 5 mL = 6.75 g/L nickel per mL

The following titration can be used to determine the concentration of nickel chloride, thereby allowing the nickel sulfate (or nickel sulfamate) concentration to be calculated from the total nickel concentration. Here’s how it works:

1 Pipette a 2.00 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 nickel chloride concentration.

2 Dilute with ~100 mL of DI water.

This measurement is not critical because the amount of nickel chloride in the sample is unaffected by the dilution.

3 Dissolve ~0.2 gram of K2CrO4 in 3 mL of DI water and add to solution. The solution will turn yellow-green.

This amount must be sufficient to allow complete reaction: NiCl2 + K2CrO4 —> NiCrO4 + 2 KCl. This reaction shows a 1:1 ratio of NiCl2 to K2CrO4. The 2.0 mL bath sample has a maximum of 53 g/L x 2.00 mL = .106 grams of NiCl2 which requires (194.190/129.616) x .106 = .159 grams of K2CrO4 The yellowish color is due to the formation of NiCrO4 in solution.

4 Titrate with 0.1M AgNO3 to a reddish-brown endpoint.

The reaction is: NiCrO4 + AgNO3 —> Ni(NO3)2 + Ag2CrO4. The endpoint is determined by the reddish-brown color of Ag2CrO4. (Almost all chromate salts are yellow, like NiCrO4, the red color of Ag2CrO4 is an exception.) The reaction shows a 2:1 ratio of the AgNO3 molecule in the titrant to the original NiCl2-6H2O in the solution. So, for each molecule of Ag2NO3 in the used titrant there is 0.5 x 237.706 g/L = 118.853 g/L of NiCl2-6H2O in the original solution. Conversion for the sample size and molarity of the titrant gives 0.1M x 118.853 g/L / 2 mL = 5.943 g/L per mL of titrant.

The Watts bath in the ChemTrak library is setup to titrate first for NiCl2-6H2O followed by a titration for total nickel as described earlier. The titrant amounts used in each of these titrations are combined with a bit of arithmetic to calculate the concentration of NiSO4-6H2O instead of the total nickel. If the titrant amount from the NiCl2-6H2O titration is called A_mL and the titrant amount from the total nickel titration is called B_mL, the concentration of NiSO4-6H2O in the Watts bath can be shown to be [A_mL - B_mL x 0.5] x 13.16 g/L for the sample sizes and titrant molarities shown in the example. Similar calculations are done for the sulfamate nickel bath in the ChemTrak template library to allow NiCl2-6H2O and Ni(SO3NH2)-4H2O to be computed using similar titrations (the sample sizes and molarities are optimized for the sulfamate bath concentrations).