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Titration questions for copper and chrome plating

1) Why is methyl orange [on eBay or Amazon] indicator used in the titration to find conc. of H2SO4 in copper plating electrolyte ?
2) Why is Ammonium bifluoride used in the titration to find conc. of chromic Acid in Chrome plating electrolyte ?
3) Why is potassium iodide used in the titration to find conc. of chromic Acid in Chrome plating electrolyte ?
4) Why is starch is used in the titration to find conc. of chromic Acid in Chrome plating electrolyte?
5) Why is H2SO4 used in the titration to find conc. of chromic Acid in Chrome plating electrolyte?

KOMAL KUMAR
plating shop employee - Rudrapur, Uttarakhand, India
June 9, 2010

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A. You really need a good book on the subject. Methyl orange [on eBay or Amazon] is used because the pH that it changes color at is low enough that you will not precipitate metal ions. phenolphthalein [on eBay & Amazon] is the normal high school chemistry indicator, but at that high a pH, you will precipitate most of the metals in the sample solution and will have a terrible answer. The fluoride will tie up any aluminum in the solution. Starch is used as you near the end point because it is a good color change. It is a very difficult endpoint that few can do without the starch. It is because the visual endpoint is massively complicated if there are other trash metals in the solution if you do not use the starch. Follow the instructions. They were developed by talented people. James Watts - Navarre, Florida A. 1) The methyl orange [on eBay or Amazon] color change occurs at a convenient pH for the determination. There are others you could use that change color at a similar pH, but methyl orange [on eBay or Amazon] is the one that is typically selected 2) Ammonium bifluoride masks the dissolved iron in the solution and keeps it from interfering with the titration. 3) The determination is based on a class of titrations called iodometry. In this case, chromic acid reacts with excess iodide to produce an anione called triodide. The triodide is then titrated with the sodium thiosulphate [on eBay or Amazon] . 4) The starch indicator makes the endpoint easier in all iodometric titrations. 5) Acid is also consumed in the reaction from iodide to tri-odide. It just needs to be in significant excess. The best book explaining the details of the analysis of chrome plating is by Terrance Irvine, The Chemical Analysis of Electroplating Solutions [affil link on Amazon or eBay or AbeBooks]. I wish that I still had my copy. I'm sure it is out of print, but you can probably find it used. Jon Barrows, MSF, EHSSC GOAD Company Independence, Missouri

Q. 1) why are ammonium solution, ammonium purpurate and EDTA used to find the concentration of CuSO4 in copper plating electrolyte?

Komal Kumar [returning]
- Rudrapur, Uttarakhand, India
June 11, 2010

Q. I'm so thankful to Mr. James Watts & Mr. Jon Barrows to provide the answers of my questions actually I work for plating shop but chemistry is not my strong subject.
Please provide the answer of my previous question.

Komal Kumar
- Rudrapur, Uttarakhand, India
July 2, 2010

A. ammonia is used for complexing the copper ion so you have less interference from ions like iron. EDTA is a better complexer, so it will pull the copper from the ammonia complex. It also is a terrible end point to find if you do not have an indicator. The indicator that you mention also has a couple of trade names as well as a very specific chemical name. As in my first post, you need to get a book on chemical analyzes of plating solutions. A college chemistry book would not hurt also. James Watts - Navarre, Florida A. Ammonium purpurate, more commonly known as murexide [affil links], is just an indicator that is common for complexometric titrations involving EDTA. The indicator, when bonded to the free metal ion in your solution, will be purple. When you titrate, the EDTA forms a more favorable complex with the metal ion and displaces the indicator from its complex. When all of the indicator has been displaced by the EDTA, you visually see the color change as a disappearance of purple. If it is important for you to understand these basic mechanisms of your titrations, then I would suggest you get a used textbook in quantitative chemical analysis in addition to the book that I have already mentioned. Older editions of textbooks can be found for very little money. Daniel C. Harris, 4th or 5th edition would be one acceptable reference. Jon Barrows, MSF, EHSSC GOAD Company Independence, Missouri

I generally use PAN as an indicator in EDTA titrations of copper solutions. Nice, sharp endpoint.

It is helpful to, after the addition of the ammonia, to filter through Whatman #4 to get rid of the Fe(OH)3 before titrating. Makes the endpoint easier to see.

Dave Wichern
Consultant - The Bronx, New York

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I'm again thankful to Mr. James Watts & Mr. Jon Barrows
to provide the answers of my questions.

I have two more Questions Related to Nickel electrolyte titration.

1) why potassium chromate solution and silver nitrate solution [on eBay or Amazon] are used to find the cons. of NiCl2 in Nickel plating electrolyte?

2) why mannitol powder, bromocresol purple [on eBay or Amazon] and NaOH are used to find the cons. of Boric Acid in Nickel plating electrolyte?

Thanks in advance

Komal Kumar
- Rudrapur, Uttarakhand, India
July 30, 2010

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August 3, 2010

Komal-
You really need to get some of those reference books...
1) Silver reacts with chloride ion to form an insoluble silver chloride which is a white precipitate that is seen as cloudiness. The silver also reacts with chromate to precipitate a red-orange silver chromate, but the reaction with chloride is thermodynamically favored which causes the chromate reaction to be put on hold until there is no more chloride left in the solution. So, once you see the permanent darker color in the precipitate begin to form, you know that your titration is complete.
2)Boric acid is a weak acid which means that it doesn't dissociate very well into acidic hydrogen cations and a borate anion. In order to perform an acid-base titration between boric acid and NaOH, we need to have acidic hydrogen ions. Mannitol is just the thing that gives that to us. It reacts with the boric acid to fully free up one of the three hydrogens in boric acid. We then titrate that free ion with NaOH. Bromocresol blue is just a color indicator that changes when the pH is high enough to signal that the reaction between the H+ and the OH- is complete. Good luck.

Jon Barrows, MSF, EHSSC
GOAD Company

Independence, Missouri



August 5, 2010

A side note to what Jon said. Make a solution of boric acid in DI water and let it set overnight. Very slowly titrate a sample of this with a 0.1N NaOH [on eBay or Amazon] solution with a pH meter. on the graph you will see 3 fairly distinct inflection points in the curve where each H comes off. In titrating the boric acid in the nickel solution, most of us go too fast and would drop out some of the nickel which would give a very false analysis. As Jon said, the manitol frees up the first H ion and it can be titrated reasonably rapidly. As you should have noticed, care must be taken to exactly duplicate an analysis. Ie, do not rush it. I used a tiny spin bar in a small beaker [beakers on eBay or Amazon] (25 mls) at a fairly high speed and a slow drip of the hydroxide. The great part about this analysis is that it does not have to be ultra accurate. Close is good enough.

James Watts
- Navarre, Florida

Q. Hi,
I have a question related Effluent treatment.
Why below given chemicals are used in effluent treatment and how work individually?
1) polyelectrolyte
2) lime
3) calcium carbonate
4) poly aluminium chloride
5) alum
6) caustic soda [affil links]
7) amol gazol

thanks

Komal Kumar [returning]
- Rudrapur, Uttarakhand, India
September 21, 2010