Determining Sulfuric Acid Percentage in Anodizing Tank

A. Hi James!

I'm curious what "real time" means to you and/or your vendor, and how automated you are, and whether your operators run in-tank ASF meters and check build during the run.

To me, real time means a near continuous monitoring that would be useful if you are fully automated, running the same parts configuration on repeat, with a dosing system (don't even get me started on this; I come from aquaculture and auto-titrators and dosers kill fish when they get a dirty probe or suffer a brief power outage).

If you're running manually with traditional process controls, manual titration at a frequency based upon a ratio of tank volume to parts throughput, measured in total surface area including racks, is adequate. If you want to get a bit more refined, I'd propose looking at a ratio of amp-hours on the rectifier clock to documented tank adds, and calculating what you might need for a really conservative drip feed from the drum, using a peristaltic pump set to ratio input acid based upon the rectifier being live, and still have your lab tech run manual titration for both aluminum and sulfuric acid on a schedule that meets spec (no offspec readings allowed, so schedule it to catch drift before control limits are breached).

Can you see I'm quite distrustful of autotitrators?

Humorous aside, a couple years ago a shop tried to poach me, I went in for a "tour" and found the autotitrator broken and leaking reagents all over the place, and they had not replaced their lab tech nor learned to test manually. Hence the poach. My dear current colleague, who had actually set up their heavy equipment when they built from scratch, confided that one of the reasons he peaced out was because of that stupid $140k robo-labrat never being as trustworthy as a real set of hands, and it actually made running the line almost impossible because it was always wrong lol. And then it was just broken, and no one knew what was in the tanks.

Guess they just could have asked us ;)

So, am I putting in a shameless plug for having someone in the Quality or Engineering or Technical Rodent Department test your tanks? Heck yes. Because I've seen what happens when a plating shop or Barramundi Fish Farm depends on a robot, and forgets to train a human to double check the results.

Sorry for the rant. Just please be circumspect when dealing with salesfolk. Some are now my friends for life, and some will just sell a dream ... of their own timeshare in Tampa.

A. Marc,

If your solution 15% (weight/weight) corresponds to 165 g/l, it means that density is 1.1 g/cc (your 11, because to have g/l, you multiply also by 10), but a lower percentage as 12% has got for sure also a lower density due to the lower sulfuric acid content.

So you have to multiply % value by density (measured for instance weighing 100 cc of solution on a precise balance: if 100 are 108 g, the density is 1.08 g/cc).Don't forget that pure sulfuric (96-98%) has a density of 1.84 g/cc. Example: 12 * 1.08 * 10 = 129.6 g/l. I hope to read you again.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Sounds logical to me, but I'd take and obviously make up several different % baths and test your theory, but really, math is math, and that seems to be a logical way to start. How accurate it may be is questionable, but there is always a few % difference in this stuff anyway, just trust your instincts

A. Marc:

I don't think the conversion factor holds true. My source is the Chemical Rubber Company Handbook of Chemistry and Physics 50th Edition, 1963 (I am an old guy!). 12 % is 129.6 g/l. At 20 %, it is 227.9 and at 30%, it is 365.6 g/l.

Your best bet is to purchase a CRC Handbook of Chemistry & Physics [adv: on Amazon & AbeBooks & eBay affil links] or Lange's Handbook of Chemistry [adv: on Amazon, AbeBooks, or eBay affil links] . It has a wealth of useful tables of specific gravity for most common solutions along with % stated in g/l and lbs/gal. Both books also give you invaluable information on reagent makeup for analytical work, conversion from metric to English units and a full section of mathematical expressions not easily found.

I have had my CRC since high school. I used it through my undergraduate work and all my working career. Chuck Reichert

A. You can use this formula to get a more exact answer for H2SO4 (g/l) Concentration as a function of Pwt(%)

Concentration(g/l) = Qf * SG(H2SO4) * D(H2O)

Qf = [330.9(Pwt^3)+92.3(Pwt^2)+572.6(Pwt)]/1000

where:
SG = 1.84 :Specific Gravity
D = 1000 g/l :Density
Qf= formula :Conversion Factor
Pwt = 0-1 : Percent Wt of H2SO4 12% = 0.12

----------------------------------------------
For Pwt = 12%

Qf= [330.9*.12^3 + 92.3*.12^2 + 572.6*.12]/1000
Qf= [70.6129]/1000 = 0.0706129

Concentration(g/l) = [0.0706129]*1.84 *1000 g/l
Con = 129.93 g/l

This is all derived from the basic formula:

Density(final) =
[Mass(H2SO4) + Mass(H2O)]/[Volume(H2SO4) + Volume(H2O)]

Solving for Final density as a function of SG & Pwt:

Df = D(H2O)/[(Pwt/SG)+(1+Pwt)]

Concentration(g/l) = Df*Pwt

I plugged this formula into Excel and did some
*math magic* to get a trend-line that gives the formula for Qf. (R^2 = 1)

Hopefully this will be useful to someone.

• Topic 5168 "Best Titration Method for sulfuric Acid"