Determination of boric acid in nickel plating baths

A. The endpoint is a very good one.
The manitol is added to a thin paste, just a little thinner than Elmers, because the dilute NaOH is going to add quite a bit of liquid.
Double the amount of BCP indicator that you are using. If that does not help, make up a new solution.
I normally used a glass stirring rod in a 25 ml beaker [beakers on eBay or Amazon]. A friend of mine used a slightly thinner solution and used a flea and a magnetic stirrer for the actual titration.
It is an excellent and very reproducable analytical procedure within plus or minus 0.1 ml of titrant.

Once you see it done , you will agree that it is a very good procedure.

PS: the amount of boric acid in your nickel solution is very dependent on temperature of the sample. Hot solution holds a lot more than a room temp one. Normal concentrations will salt out at room temp and give you a low and very erratic reading.

A. Mr. Watts's P.S. is an excellent lead-in into a less orthodox way of determining where you stand with boric acid concentration. Since the actual concentration of boric acid in the solution is not as important as that it is above a certain limit (usually above its solubility at room temperature), consider this "test" I have been using for years:

Fill a small (1 cc) test tube with your solution and let it cool down to room temperature. The amount of boric acid crystals formed is a pretty good indication of whether you need to make an add.

Once you use this method for a while, you will agree that it is the only procedure you need for boric acid analysis :-)

Good luck from PlaterB

Sounds good, but does not work until some homework is done. It takes quite a bit of volume to cover the pH probe. The normal way is to use a thin paste, which does not give much quantity. It would be more of a pain than it is worth. The indicator method works very well, is quick, easy and generates a lot less waste.

I am not sure that a large dilution will give the same answer.

A. Potentiometric titration with pH electrode provides very accurate determination of end point. Most probably, you already have necessary equipment: pH-meter and glass electrode.

A. I USED TO USE bromocresol purple [on eBay or Amazon] ,UNTIL I FOUND A BETTER WAY. MY PROCEDURE TO CHECK BORIC CONTENT IN NICKEL BATHS IS 5 ML SAMPLE IN ERLENMEYER FLASK,ADD 5 DROPS OF BORIC ACID INDICATOR, SWIRL WHILE TITRATING WITH 1.N NaOH [1N NaOH on Amazon] TILL BLUE RECORD THIS AS A. NEXT ADD A HEAPING TEASPOON OF MANITOL.(THIS WILL TURN BACK TO GREEN) SWIRL WHILE CONTINUING TITRATING WITH 1.N NaOH TILL SLURRY TURNS BLUE AGAIN.RECORD THIS NUMBER AS B. CALCULATION IS B-A X 1.65 GIVES YOU OZ/GAL BORIC.DO NOT REZERO BURET AFTER FIRST TITRATION. THIS HAS SEEMED TO BE VERY ACCURATE AND CONSISTENT FOR ME.

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Ed. note: Readers -- This is a large site with 60,000 finishing topics. If you are working with sulfamate nickel rather than a Watts bath, be sure to search the site for "sulfamate boric" or similar terms [maybe also insert 'Probert' in your search phrase because Robert is a sulfamate nickel specialist] for several threads about that because the situation is rather different.

A. My guess is that you have drag in and or breakdown products that are forming a buffer that interferes with your titration.

I used color indicators, but the result is pH sensitive so are somewhat equal. I never saw the phenomenon you speak of.

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 [on eBay & Amazon] , 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.

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.

A. Kevin,

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 [on eBay or Amazon] 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 [on eBay or Amazon] , 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.

A. Hi Julia,

You may try below alternative method:

Reagents:
1. Manitol Solution (Dissolve 100 g Manitol in D.I. water and make up to 1 litre)
2. B.C.P. (bromocresol purple [on eBay or Amazon] ) Indicator
3. 0.1 N Sodium Hydroxide Solution

Procedure:
1. Pipette [pipettes on eBay or Amazon] 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.

Calculation:
H3BO3 (g/l) = ml of NaOH x 6.18

Regards,
David

Thank you David,
Every method that I have so far lacks precision and accuracy.
I am going to try your method today.
Thanks again
Julia

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.

David,

Your procedure gave me the best results by far for clarity of the endpoint. I hope Julia found it the most helpful as well.