Determination of boric acid in nickel plating baths
Q. The problem being encountered with the boric acid titration is that we are not getting enough accuracy. Precision is good, but for a 30 g/L QC sample a typical value obtained would be around 34 g/L. Can anyone explain this inaccuracy? We don't use any indicators at present, when titrating the QC 0.5 mL are pipetted into a titration vessel, along with 20 mL DI water and 20 mL of a 50 g/500mL mannitol solution. Endpoint is determined using a pH electrode. Thoughts on this appreciated because the current method of ion chromatography is running into problems and we are not confident that the results generated are reliable. We hope to introduce the titration method in the near future.Kevin Colm Behan
- Leixlip, Co. Kildare, Ireland
A. Firstly, what are your problems with ion chromatography? In the past I have used it with great success, but you do need to use very dilute solutions. However, boric acid has a tendency to remain in the system as it is not easily fully flushed through. I also believe a similar thing happens with boron analysis by ICP.
My first thought is that the tank volume is varying; - make sure you keep the water level topped up otherwise you will be sampling a "too concentrated" bath! You do not give much information about the electrolyte; is there any chance of something interfering with the titration. Alternatively, since boric acid is a pretty weak trivalent acid, perhaps you are not giving the pH "end point" sufficient time to be reached and are hence overshooting. I have to admit I have not used the mannitol method, so I cannot speak about it from experience.
I generally analyze boric acid in nickel baths by taking 1ml of the bath and adding it to 25 mls of "boric acid mix". The text books say use 2 mls, but I have found this problematic. I even sometimes add another 9 mls of water to further dilute to green colour. I then titrate the aliquot against 0.1N NaOH, ensuring the liquid is thoroughly shaken throughout. The concentration of boric acid in the bath is then: [H3BO3]g/l = mls NaOH x 6.184
Obviously the sodium hydroxide needs standardizing and should not be stored too long, otherwise it absorbs CO2 and changes its concentration.
Boric acid mix is 600 mls of glyercol with 60g sodium citrate and 2g phenolphthalein, that has been dissolved in a minimal amount of hot ethanol, and the whole lot diluted to 1 litre. This solution is quite viscous and should be thoroughly shaken before use. It is also essential to thoroughly shake the titrating mixture because even that is viscous.
I know there are other methods for analyzing boric acid, but this seems to be the best I have found. The only problem is the end point; the liquid is green and the end point is when the solution goes violet/pink. However, it does not do this until after it has been through a blue colour and the transition from blue to violet/pink can be difficult to see. Hence, I use only 1 ml of the plating bath, as this helps me see the final end point; diluting it further with 9 mls of water makes it even easier to see.
R&D practical scientist
Chesham, Bucks, UK
A. I always did this analysis by the old time method; blending about 10 grams of mannitol with 5 - 10 mls of sample and just enough water to form a mobile slurry, titrating with 1.0 N NaOH to a bromcreosol purple endpoint, and multiplying the mls titrated by 0.824 (for a 10 ml sample) to get the boric acid number. I believe that if you took your boric standard, added the known required volume of NaOH to neutralize the boric under the conditions described above, and used this mixture as a guide to endpoint recognition in titrating your bath, that this would give you the needed accuracy and precision.
There is another method where the pH is adjusted to a fixed value, and the mls of standard alkali required to raise the pH by exactly 0.50 pH units recorded and used to calculate the answer. I have never used it, and don't have the procedure handy, but some vendors recommend it and I don't doubt you could locate it with a bit of searching.
Hope this has been of some help.
Consultant - The Bronx, New York
I analyze the boric acid in nickel bath by taking 2 ml of solution then add 5 ml distilled water, add 4 drops of bromothymol blue and bromocresol purple mixed indicator, add 5 grams mannitol powder and mix. Titrate 0.1N SODIUM HYDROXIDE TO THE BLUE END POINT.
Calculation : ml of 0.1N NaoH Used x 0.41 = oz/gal Boric acid ( 1 OZ/GAL = 7.5 Grams/liter )
A. I agree with Dave W. In fact I may have guided him in that direction from my early '80s work. This does make the color appearance more observable and accuracy and precision improve. Kevin is using a pH electrode for detection. It may be possible that while traversing the buffing region there may be a delay in response due to several factors. The boric may still be slightly polymerized, because it is only slowly hydrated, if indeed completely. Hydrogen (hydronium) ions also need to become hydrated and pass through the glass frit of electrode. It may also be possible that the boric associates with the (silicate) glass or even dissolve into the initial surface to cause a sluggish effect on proton mobility through the glass as would be expected in borosilicates. There may also be other fundamental principles to consider. pH titrations are logarithmic. They depend on the detection of potential gradients generated by changing hydroxyl and hydronium ion concentrations.
According to J. Jordon, J. Chem. Educ. V40 A5 1963, the weak acid, boric does not have a crisp endpoint as does hydrochloric acid. Boric needs an indicator which has a color change at c 10.3. Boric doesn't need mannitol added and in fact its addition makes the analysis less accurate. However, by detecting the heat (changes of 0.001°C) generated from reacting with caustic, e.g. 0.5 N NaOH, both hydrochloric and boric titrations are on an even playing field. Like hydrochloric, boric has then become detectable down to 0.002%. Introducing nickel mucks this up. Boric acid plus nickel, as in plating baths, appears to require mannitol addition. The reason may be that borate chelates or ligates nickel (remember the polymeric forms of boric). Results by detecting heats of reaction from caustic titration with and without mannitol show that a very low value for boric and very high value for nickel occurs without adding mannitol. Perhaps the heat delay is reserved for removing the borate sequestration of nickel ions during the normal hydroxo- formation. Adding mannitol has the effect of enhancing the weak proton (hydronium, or hydrogen ion) as needed, in the presence of nickel, by the displacement of water when mannitol reacts with borate to give the chelated structure of boron. It consists of two 5-membered rings formed doubly from two adjacent carbons' hydroxyl oxygens with the single boron ion and bears a single negative charge, easily correlated to boric acid concentration.
The heat of reaction between the mannitol and boric singly pales in comparison with the changed heat from no addition of mannitol with nickel present. Thus a single (linear) titration with only caustic, using mannitol, will accurately detect both boric acid and nickel. From an example, results in g/L are expressed as Average (Standard Deviation) for Boric, 36.7 (0.61) and Nickel, 77.52 (0.24) for five titrations using 1.95 N NaOH of 2.65 gm. plating solution (to 10 ml. water) and 10 ml. of 25% mannitol.Stephen Koelzer
- Sunnyvale, California, USA
March 18, 2014
Q. Hi Trevor,
Could you please let me know if the Boric acid mix is further diluted to 1 L with Ethanol, or with Deionized water?
- Danbury Connecticut USA
April 8, 2014
Q. I am experimenting with your method (Trevor) for the determination of Boric Acid but am having problems with the Boric Acid Mix.
1- I can't get the sodium citrate to dissolve even in hot ethanol,
2- It dissolves in water but not fully.
Can you tell me how do you get it to dissolve, and how do you fill it to 1000 ml? In Glycerol?
- Danbury Connecticut USA
April 9, 2014
A. Hi Julia,
You may try below alternative method:
1. Manitol Solution (Dissolve 100 g Manitol in D.I. water and make up to 1 litre)
2. B.C.P. (Bromo Cresol Purple) Indicator
3. 0.1 N Sodium Hydroxide Solution
1. Pipette 1.0 ml sample into a 250 ml Erlenmeyer flask.
2. Add 40 ml of Manitol solution into the flask.
3. Add 2 drops B.C.P. indicator to the flask.
4. Titrate with 0.1 N NaOH until solution turn blue from green yellow.
H3BO3 (g/l) = ml of NaOH x 6.18
April 10, 2014
Thank you David,
Every method that I have so far lacks precision and accuracy.
I am going to try your method today.
- Danbury Connecticut USA
April 10, 2014
A. I used someone's method of adding enough manitol to the sample to make a thin slurry. the 0.1N NaOH is important. 1N does not work. I either stirred this with a glass rod or used a magnetic flea. The end point is very repeatable.
You might try adding 1 more drop of indicator when you are very close to the end point. I actually could tell the imminent approach of the end point and then would add the indicator.
Strong NaOH or added far to fast will precipitate some of the nickel and give you an erratic false value.
- Navarre, Florida
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