Analysis Methods Zinc-Nickel Plating Bath: Titration, AA

A. To find the Nickel concentration in an alkaline Zn-Ni bath by UV-Vis, you can use nioxime (1,2-cycloheanedione dioxime). A typical procedure for about 2 g/L nickel would be to acidify an aliquot to break the nickel complex. Add a complexing agent for interfering impurities, e.g. Fe, such as ammonium citrate. Add a pH 5.5 acetate buffer to control the pH. Add a supporting dispersant of 50 g/L Gum Arabic (about 1:99 volumes). Then add the nioxime reagent, prepared at 5 g/L in 9:1 aqueous ethanol, at 2 mL per 100 ml for a maximum 4 mg/L nickel solution. The red suspension absorbs at 550 nm. A calibration curve should be prepared.

A. Dear Daniel,

There may be a couple of alternatives to the AA for your application. The first method is an ion selective electrode. This technology has been used in the metal finishing industry as a cost alternative to more expensive and accurate equipment. Here is the drawback, these electrodes have many interferences and in some cases lack the necessary accuracy required for analysis. The second method is to use your aforementioned UV/Vis. There are a few different methods, of which the most reliable that I have found is the use of Dimethyl Glyoxime. This method uses forms a complex which is easily measured at 445 nm. The last method is a complexometric titration using automated titration equipment. The new advances in the titration equipment industry can allow the nickel to be determined separately from the zinc. It is preferred that you contact appropriate manufacturers of equipment to determine the proper fit to your need.

Best of luck.

A. Dear Daniel,

I currently have a plating operation where we are plating zinc-nickel, and have used several methods for measuring zinc and nickel solution concentration. Yes you can measure nickel concentration with spectrophotometry with less than 2% error. The procedure that I use takes forty minutes to perform. A good chemical supplier can provide with this test method. You can also measure both zinc and nickel concentration using a XRF analysis or X-ray thickness tester with a solution cup. I found that this method has an error about 8%; however, the analysis only takes 4 minutes to perform. In my shop, we used the XRF method 99.9% of the time because it is fast to do, baths can be checked more often, and we find the error acceptable.

A. Zinc: Dilute a 10 ml bath sample to 100 ml in a volumetric and pipette [pipettes on eBay or Amazon] a 10 ml aliquot of the dilution into a 250 ml flask and add 10 ml 10%NaCN solution. Add 100 ml DI H2O and 10 ml of pH 10 buffer (70 g NH4Cl, 570 ml 88 Be NH3, balance DI to 1000 ml in a volumetric). Add 10 ml 10%HCHO and .1g Eriochrome Black T [affil links] indicator. Titrate immediately with .1M EDTA to a blue endpoint. g/l Zn=(ml EDTA)*6.54

Nickel: Make the following:
Solution A: 30 g sodium pyrophosphate, 50 g NH4Cl, 2 g KI to 1000 ml total with DI.
Solution B: 25.5g NaCN, 1.7g AgNO3 to 1000 ml total volume in a volumetric flask with DI (transfer this to a dark glass container and store in the dark).

Pipette 5 ml bath into a 250 ml flask and add 50 ml solution A. Add 88 Be NH3 until the pH is >8.0. Titrate with solution B until the turbidity just disappears. g/l Ni=(ml solution B)*1.5

A. No, the cyanide masks the nickel. It forms a complex that is too strong for the formaldehyde to break.

Another idea I had is; dilute to a suitable concentration (10 - 20 ppm Ni) and analyze with the dimethylglyoxime colorimetric method, employing the method of additions. Or, make up Ni standards in a matched matrix, and read directly.

A. The amount of cyanide I said to use may be excessive - you would have to experiment. As long as there is an excess of formaldehyde the zinc will be unmasked.

Dimethylglyoxime method: Add enough 1:1 HCl to bring the pH of a 10 ml sample to approximately 5; then, add 1.0 ml more. Add roughly 0.3 g ammonium persulphate; shake till dissolved. Add 1.0 ml 10 % sodium citrate solution, shake. Now, add 1.0 ml 1 % dimethylglyoxime solution in alcohol, and immediately add 2.0 ml 1:1 aqua ammonia. Shake well. Wait 10 minutes, then measure the absorption at 420 nm.

A. If I recall correctly, standard aqua ammonia, or, "ammonium hydroxide", is 29% by weight.

Go ahead and use what you have, it should do.

A. The method suggested by Dave Wichern is the best for you. Take 10 mL of the solution to be analysed. Add 10mL 25% ammonia and 0.5 g murexide [affil links] indicator. If colour is not yellow or dark yellow, add more ammonia. Add 80mL water. Titrate against 0.05M EDTA from yellow to violet. This gives the total metal (Zn + Ni). Next, take another 10mL of the solution. Add 0.5 g KCN or NaCN. Add 0.5M buffer of pH 10.5 (NH4OH/ NH4Cl). Shake well. Add now 10mL Formaldehyde (40%). Add 0.5 g Eriochrome Black T indicator. Add 70mL water. Titrate against 0.05M EDTA from wine red to blue. This gives zinc. Difference in EDTA volumes give the volume consumed by Nickel. Calculate now nickel. You can have confidence in this method.

1. murexide [affil links] and Eriochrome Black T - Prepare 0.5 g in 100 g KNO3 or NaCl
2. If your company uses very strong complexing agents (not stronger than EDTA of course) then the solution upon addition of ammonia will not turn completely yellow. It will remain greenish yellow. If you can smell ammonia strongly that usually will do. Check pH. Should be around 11 - 12.
3. In case your company uses strong complexing agents, reaction with EDTA will be slow and drop-wise addition with 2 - 3 mins time between drops may be necessary.

I think I have covered most of the points.