Sciencemadness Discussion Board - Gentian Violet/Malachite Green (first transition)

Gentian Violet/Malachite Green (first transition)

CHRIS25 - 10-7-2012 at 15:57

I am completely unfamiliar with Ph indicators. having read some information about what they do I still have a couple of questions. They concern the Gentian Violet and the Malachite Green (first transition).

I am looking for a ph indicator that will have a gradual transition change of colour between 1 and 3.0. Or do the above indicators have a rapid and abrupt colour change?

I will be using it to determine the HCl Ph of my copper chloride solutions since the titrating with sodium hydroxide is inaccurate and often confusing.

I just need to determine Hcl content in CuCl solutions before and after etching. Weeks of studying and hair pulling about ions and correct nomenclature and cu(-) and Cu +2 and titrations have left me exhausted and full of theory; but the practical simple question about how to accurately determine amount of free Hcl in a copper chloride solution is still a mystery, and I have aquired two chemistry books per people's advice, been through dozens of internet sources. Theory theory theory. I just need to be told exactly what to do - I do not see any harm in that, my appreciation and understanding would undoubtedly benefit enormously and I could finally put the paracetomol away.

(I guess this frustration ignited when I finished a perfect 6 hour etch, had titrated before and after; I then proceeded with a subsequent etch the following day and it had dissolved half of my same thickness and size copper in the same amount of time: This was ridiculous)

So I need desperately to MEASURE both the amount of soluble Cu2Cl in my solutions and amount of free Hcl. Thankyou guys. I would appreciate somebody's precious time.

Magpie - 10-7-2012 at 17:49

Quote: Originally posted by CHRIS25

I am looking for a ph indicator that will have a gradual transition change of colour between 1 and 3.0.

Usually you want an abrupt color change when titrating.

Quote: Originally posted by CHRIS25

I will be using it to determine the HCl Ph of my copper chloride solutions since the titrating with sodium hydroxide is inaccurate and often confusing.

Why not use pH paper to measure pH? Much easier.

Quote: Originally posted by CHRIS25

I just need to determine Hcl content in CuCl solutions before and after etching.

So I need desperately to MEASURE both the amount of soluble Cu2Cl in my solutions and amount of free Hcl.

Is it CuCl? Cu2Cl does not exist.

One reason it is difficult to answer your questions directly is that we don't know exactly what you want or where you are starting. I know you are trying your best and have put a lot of effort into this.

CHRIS25 - 10-7-2012 at 23:57

Hallo Magpie, thankyou for taking the time to respond. Ok....regards this "Why not use pH paper to measure pH? Much easier", two problems, first I find in this particular situation the ph paper turns consistently red even when the Hcl was used up. Then I discovered that ph paper is deceptive because there is Hcl tied up with the Cu+1 in solution keeping it dissolved, so in essence it was measuring the Copper chloride solution and not the free acid (of which IN Practise there was none). I came up with the Gentian Violet/Malachite Green idea because I read that it simply measured ph between 1.0 and 3.5 which would have given me an idea of overall acid content in a more precise way, albeit not ideal, but better than what I have.

Ok...."Is it CuCl?"

When the Cu+2 attacks the copper surface at the same time that the HCl keeps the oxide away the solution becomes heavy with Cu+1 (ions) or (Cu+1Cl), (or cuprous) (I won't even bother mentioning other ways of expressing this), ( but being fed up and tired of the confusing way chemists write and their lack of consistent nomenclature amongst chemist's papers I will stick to calling things copper 1 and copper 2), Cu+1 is then kept dissolved in solution and converted back to Cu+2. The Hcl keeps this happening and importantly the constant addition of dissolved oxygen via air pump and sometimes H2O2. If Cu+1 concentrations become high then we get a very slow etch or it stops altogether. The Cu+1 concentrations need to be kept below 4g/pl. I can do this by keeping high amounts of dissolved oxygen in solution, aeration. But this does not measure anything - it's guess work at the moment. If the free unreacted Hcl is depleted the same thing happens, if the Hcl is too high we have a fast and inaccurate etch rate.

As a result I need to keep Cu+1 ions monitored (at the moment I do this by dropping a droplet of the etch solution onto white paper, if it is light green or light blue green then I'm ok, the darker it becomes the higher the Cu+1 ions in solution, and are and not converted to Cu+2. I forgot to add here that a specific gravity of between 1.2 and 1.3 (or 125gpl to 175gpl) is recommended here for measuring Cu+0 (ordinary copper content). But I have yet to acquire a hydrometer.

I keep the Hcl monitored with NaOH titration, adding 1ml of 1 mole NaOH to 1ml of CuCl solution that has just been syringed into 30ml of water. If the copper hydroxide dissolves immediately and the solutioon remains clear then there is sufficient Free Hcl in solution, BUT how much? And this is my problem, I guess everything when I add more of anything, and this led to my disaster yesterday, which in itself, thinking about it las night must have been a result of increased and very high concentration of Cu+2Cl from previous etch, and not my over generosity with the acid and peroxide that I needed to add, as first thought/feared.

Lastly...I played around with adding more NaOH to the same 1ml of Copper chloride in the water solution and I thought that the blue through to green colour changes that I saw in the NaOH titration tests were a good indicator of Cu+1 and +2 concentrations especially when accompanied with the solid precipitate of Copper Hydroxide, sometimes blue sometimes green. The more I played around with all this and then dropping in miniscule amounts of acid and adding more NaOH - the more confusing things became because it is impossible to find information in any books about this sort of thing, and weeks of web searching, while turning up one set of photographs from "air regenerated etching pdf", have revealed nothing. The frustration here amounts to weeks and weeks trying to get answers to questions about something that I KNOW I could understand in 35 minutes from a teacher.

Thankyou for allowing me to indulge my outburst...I actually do learn other chemistry, but at the moment this situation needs to be resolved.

[Edited on 11-7-2012 by CHRIS25]

[Edited on 11-7-2012 by CHRIS25]

[Edited on 11-7-2012 by CHRIS25]

Magpie - 11-7-2012 at 09:16

Let's start over.

1. Provide a concise statement of the objective of this etching process.

2. Provide the recipe for your etch solution, in quantitative terms.

3. Provide a step-by-step procedure of your etching process, again as quantitatively as possible.

4. State again what help you want us to provide.

CHRIS25 - 11-7-2012 at 10:43

Quote: Originally posted by Magpie

Let's start over.

Hi Magpie,

1. Provide a concise statement of the objective of this etching process.

==Purely artistic, making copper art with drawings and/or poetic writings.

2. Provide the recipe for your etch solution, in quantitative terms.

==The original stoichemetry calculations served no purpose. The beginning solution (2 months ago) was Cu+2HCl+Oxygen via an airpump. This was based on the fact that 1 mole of Copper chloride to 2 moles of Concentrated hydrochloric acid was needed. Cu + 2HCl = CuCl (insoluble) then this reacts further: CuCl2 + 2HCl = H2CuCl3 (soluble and oxidizes the copper surface and etches it.

This whole process is self regenerating. I have only had to keep the oxygen levels sufficient, and top up with Concentrated HCl when the Sodium hydroxide test revealed that there was no more free acid available. The etch solution is exactly 2 litres and has remained that way.

3. Provide a step-by-step procedure of your etching process, again as quantitatively as possible.

==The etching solution is two months old, and as said needs nothing done to it other than what is said. I simply add the copper piece to etch and check after the first two hours and every hour after that.

4. State again what help you want us to provide.

==I want to be able to measure the free HCl levels (the concentration in the bath) and have more control in keeping the levels to between 1 and 3 moles.

To be able to measure specific gravity, (copper content in the bath) between 1.2 and 1.4 g/cm3.

The Cu(1+) ion concentration I can easily do via colour observation.

If I have failed to be accurate enough or if there is anything Iyou think I have missed please let me know. Thankyou.

CHRIS25 - 11-7-2012 at 10:43

Quote: Originally posted by Magpie

If I have failed to be accurate enough or if there is anything Iyou think I have missed please let me know. Thankyou.

Magpie - 11-7-2012 at 14:42

This does not seem to be a difficult request. I'll give you my recommendations. I kindly ask others to correct them if I they appear to be in error or are incomplete:

If I understand you correctly you just want to do the following:

1. Determine the pH of your etch solution and correct it to between 1 and 3 if it is too high, using hydrochloric acid. Stated an equivalent way, you want your hydrogen ion concentration between 0.1 and 0.001 molar.

2. Determine the specific gravity of the etch solution to determine if it is within the required range of 1.2 to 1.4.

The second one is easy: buy a hydrometer that covers the range of interest.

The first request is a little more tricky:

a. A calibrated pH meter would make it easy but they can be expensive and need calibration standards, periodic calibration, etc.

b. Use pH paper. This works well when you have clear solutions that are not too acid or not too basic. Your solution can be quite acidic and therefore the paper may not be sensitive enough for your needs. Also your solution may be highly colored which can give erroneous or confusing results.

c. Titrate to determine the hydrogen ion molarity, [H+]. You want this molarity to be within the range of 0.001 - 0.1. This actually is a very large spread in molarity, ie, a 100-fold.

If you suspect that the hydrogen ion molarity is high, then:

Take a 10 mL sample of your etch solution and dilute it with 50 mL of deionized or distilled water. Add 3 drops of a phenolphthalein indicator. Then titrate it with 0.1M NaOH using a burette. At the endpoint the color will change from clear to pink, abruptly. Record the mLs of the 0.1M NaOH used as V.

Calculate the hydrogen ion molarity, [H+], as follows:

[H+] = 0.01V

If you suspect that the hydrogen ion molarity is low, then do the same only titrate with 0.001M NaOH. Record the mLs of 0.001M NaOH as V.

Then [H+] = 0.0001V

I'm hoping that blue or green coloration of your etch solution does not interfere with the color change of the titration. If it does, a more suitable indicator might be available.

[Edited on 11-7-2012 by Magpie]

[Edited on 11-7-2012 by Magpie]

[Edited on 11-7-2012 by Magpie]

[Edited on 11-7-2012 by Magpie]

CHRIS25 - 11-7-2012 at 16:15

Hi Magpie, very helpful thankyou. Two questions, if you don't mind.

1. You said, and I believe that this is better way of deternining the HCl concentration,

=====If you suspect that the hydrogen ion molarity is high, then:

Take a 10 mL sample of your etch solution and dilute it with 50 mL of deionized or distilled water. Add 3 drops of a phenolphthalein indicator. Then titrate it with 0.1M NaOH using a burette. At the endpoint the color will change from clear to pink, abruptly. Record the mLs of the 0.1M NaOH used as V.

Calculate the hydrogen ion molarity, [H+], as follows:

[H+] = tbd

If you suspect that the hydrogen ion molarity is low, then do the same only titrate with 0.001M NaOH. Record the mLs of 0.001M NaOH as V.

Then [H+] = tbd======

First, what on earth is 'tbd'? please. Volume = t x b x d?

=======(It's changed to [H+] = 0.01V) funny, this was there earlier on but then it changed and now it is back to this, was there a glitch in the software just now? Anyway this I do understand ===============

Second, curious about the use of Phenolpthalein seeing that Bromphenol blue is colourless up to PH 3 and Methyl Violet changes colours very distinctly between ph 0 and ph 2? I would have thought that since the phenol indicator stays clear until a ph of 8.3, (another chart showed ph 6), that this was too high an indicator to be using, I am confused over this one. Since I know that my HCl concentration will not ever reach even a ph of 4. (When my Copper chloride solution no longer had any free HCl the basic litmus paper that I have still turns red at PH of 1, and I came to realise that this was because there is a lot of HCl tied up in the Cu2Cl complex, ok I understand that this is more about H ions and plusses and minus ions but please don't go there it is too much beyond my grasp at this stage), suffice it to say that there is HCl present in solution but it is not free and therefore no etching will take place, but it is still a highly acidic solution.

I will want to follow your lead on the titration as you have mentioned but just a little understanding about why phenolpthalein is used - that's all.

Very grateful for your guidance so far, thankyou.

[Edited on 12-7-2012 by CHRIS25]

Magpie - 11-7-2012 at 18:11

Sorry for the confusion about the "tbd". That stands for "to be determined." I was in the process of correcting those equations and used tbd as a placeholder. They've been corrected now as you've discovered.

An indicator can show you whether you are above or below a certain pH or pH band. That is the way in which you are thinking about them.

The way I specified the use of phenolphthalein is as an indication that neutrality has been reached during a titration. This allows you to calculate the hydrogen ion molarity of the etch solution, which I showed.

I chose phenolpthalein as it is cheap, readily available, and has an abrupt and dramatic color change from clear to pink at neutrality.

FYI:
===========================================
pH and hydrogen ion molarity, or [H+], are mathematically introconvertible, ie,

pH= the negative log (to the base 10) of [H+].

eg, if the molarity of the hydrogen ion concentration is 10^-7, then the pH = 7. If [H+] = 10^-3, then the pH = 3.
============================================

CHRIS25 - 12-7-2012 at 02:16

Hallo Magpie,

=====The way I specified the use of phenolphthalein is as an indication that neutrality has been reached during a titration=====

Ok, I just need to clear this up because maybe I am thinking a little bit off-mark here. For titration in this particular purpose, I understand it has to reach neutrality. Because as I understand various other LOW Ph indicators, their purpose is to clearly define divisions at the lower end of the Ph scale exactly where I am needing to ascertain, therefore their neutrality point will require Less NaOH? But Phenolpthalein won't change until Higher up the scale, requiring MORE NaOH before a HIGHER Ph is reached and therefore giving a different reading? I know I am thinking in terms of Logical brainwaves and am demonstrating my chemistry ignorance. It's just that if you use a lower ph indicator then endpoints will be reached with LESS NaOH giving different results? This is what I am not quite understanding at his moment. (Then I can go and buy some along with a burette).

[Edited on 12-7-2012 by CHRIS25]

woelen - 12-7-2012 at 03:00

I think it is best to take one step back and have a look at your problem. If I understand correctly you want to assure that your etching solution does good etching consistently.

In order to achieve that goal you have to assure that
1) There is enough acid in the solution
2) There is enough copper(II) in solution

It is not copper(I) which does etching, but copper(II), combined with acid. Simply look at the color of the solution. If it is brown and dark, then you have a lot of copper(I) in it. This can be resolved by allowing the copper(I) to be oxidized by oxygen from the air. Leave it overnight in contact with air. This oxidation process consumes acid. Just pour in so much concentrated acid, that the solution becomes clear and green again, after allowing it to stand in contact with air for a night or so. No rocket science needed here, just use your eyes and your experience. Over time, you'll get a feeling for the process and you'll know when it is time to replenish acid.

CHRIS25 - 12-7-2012 at 08:56

Quote: Originally posted by woelen

Just pour in so much concentrated acid, that the solution becomes clear and green again, after allowing it to stand in contact with air for a night or so. No rocket science needed here, just use your eyes and your experience. Over time, you'll get a feeling for the process and you'll know when it is time to replenish acid.

Yes But Woelen if you add water, or H2O2 you get the exact same colour of the solution - clear and blue green. The situation is that I want complete control, guess work over the last few months has produced a number of small but irritating inaccuracies in my etching, this can only be resolved through analysis of solution's contents to within a reasonable level that at least 75% of the guess work is taken out. Also I love the chemistry, I do other things without problems (except for that darn ferric nitrate...ugh), but I like to learn chemistry and if I were just a metal worker I would take your reasonable advice, I am sure some people do what you say, but yeh, I like to understand chemistry and take some of the guess work out of this particular situation.

Thankyou though for your input.

Magpie - 12-7-2012 at 09:07

Chris your understanding is basically correct.

Woelen understands the etching process better than I do and I would recommend using his easier, cheaper method if it will work for you.

CHRIS25 - 12-7-2012 at 14:14

Hi Magpie, thanks a lot for all your helpful insights, I will give your methods a go because the least that can happen is that I learn...woelen has always been helpful and I hope he is not put off from helping me agian because of what I said, I have been doing things that way which led me to try and improve by aiming for just a little more accuracy and control than has been just recently.

Just a quick question please....Copper oxychloride by the way is the same as copper1 chloride (CuCl with the 9 water molecules)? The insoluble chemical that gets converted to copper2 chloride which does the actual etch? (forgive the amateur way of expressing things at the moment, I am presently reading about ions in my chemistry book, but for now this is how I can grasp things) Only asking because I have a kilo of this stuff that I bought off the fly (intuitively).

thankyou

[Edited on 12-7-2012 by CHRIS25]

Magpie - 12-7-2012 at 15:07

I think the actual etch chemistry is best handled by woelen. Anything I say would be just a guess.

AJKOER - 12-7-2012 at 15:35

Here is a prior thread that attempts to discuss some of the chemistry:

http://www.sciencemadness.org/talk/viewthread.php?tid=19655

I believe my comments contained therein remain valid.

[Edited on 12-7-2012 by AJKOER]

CHRIS25 - 12-7-2012 at 16:03

Quote: Originally posted by AJKOER

Here is a prior thread that attempts to discuss some of the chemistry:

http://www.sciencemadness.org/talk/viewthread.php?tid=19655

I believe my comments contained therein remain valid.

[Edited on 12-7-2012 by AJKOER]

Thankyou, digestive juices at work.

woelen - 13-7-2012 at 01:44

@AJKOER, your post makes things more complicated than necessary. I'm certain that addition of H2O2 will lead to some formation of HOCl, but this is not important for CHRIS25's questions. Please do not add more variables to the equation than necessary.

@CHRIS25: I understand your desire to have a better understanding and more control of the process. You're eager to learn the chemistry and this goes beyond the typical technical application. There's nothing wrong with this.
A good theoretical understanding of certain processes indeed is very nice and I also like to have such understanding myself. Unfortunately I have found that theoretical understanding cannot always be verified in practical setups, due to lack of equipment and lack of specialty chemicals. This is especially true in home-setups, where resources are limited.

One thing I can add. No need to add H2O2, just let oxygen from air do its work. Only add acid to keep the etching solution going.

The etching agent in the solution is the complex ion CuCl4(2-). This is a copper(II)-species. This complex has an olive green color and it reacts with metallic copper as follows:

CuCl4(2-) + Cu ---> 2 CuCl2(-)

The copper(II) species reacts with copper to a copper(I) species. The presence of chloride is essential for this. The copper(I) species only remains in solution because of the presence of chloride ion.

The copper(I) species cannot oxidize any more copper. So, if your copper(II) is depleted, then the solution does not work any more as an etching solution. The good thing is that the copper(I) complex is very easily oxidized by oxygen from the air. This oxidation process, however, also consumes acid and chloride ions:

4CuCl2(-) + O2 + 4H(+) + 8Cl(-) ---> 4CuCl4(2-) + 2H2O

The acid and chloride ions are added to the solution by adding conc. HCl. If no acid is added, then the copper(I) complex also is oxidized by oxygen from the air, but in that case, an insoluble oxychloride is formed instead of the soluble CuCl4(2-) complex.

The dark brown/black color is due to formation of a complicated mixed oxidation state complex of copper(I) and copper(II). The CuCl4(2-) and CuCl2(-) complexes react with each other, forming a very dark brown complex of uncertain composition (some websites are talking about Cl-Cu-(mu-Cl)-Cu-Cl with copper in formal oxidation state +1.5), but I'm not 100% sure about this.

Use of malachite green or some other pH-indicator which changes color somewhere around pH equal to 2 is not useful. You want a much more acidic solution. You want definitely more than 1 M HCl in solution, which means that pH will be below 0. pH is not a good measuring scale for this kind of processes, because you need high concentrations of acid and pH always will need to be somewhere close to -0.5. No, you should not use pH as indicator. A better option may be to prepare a 1 M solution of NaOH and add this dropwise to your etching solution while stirring very well and measuring how much of this solution is needed before the solution becomes turbid and remains turbid after some stirring. This does not give a very accurate measure of the concentration of acid, but good enough for your purpose. If your solution becomes less than say 2 M HCl, then add a little conc. HCl again, such that the solution becomes e.g. 4 M HCl. No need to retitrate with NaOH, just use 30% HCl from a hardware store, which has a known concentration and compute how much of this is needed to add to your solution.

Magpie - 13-7-2012 at 08:28

Quote: Originally posted by woelen

A better option may be to prepare a 1 M solution of NaOH and add this dropwise to your etching solution while stirring very well and measuring how much of this solution is needed before the solution becomes turbid and remains turbid after some stirring.

Woelen do you mean for Chris to pull a small known sample volume, say 10 mL, of his etch solution and then titrate that with the 1M NaOH? Then using the results of that titration he can add the required amount of HCl to his bulk etchant solution?

CHRIS25 - 14-7-2012 at 04:00

Woelen, thankyou for the breakdown and the advice about the Ph indicators, I can understand what you are saying much more clearly when you talk about the ions and so on. It's sinking in now.

As for your titrate method above I actually do that, but it only tells me if there is enough acid around, not how much or anything.

I found a formula in a chemistry book but after working that out was stupified by the end result. Here it is:

I began with 50ml warm water. Added 1 ml of copper chloride etchant and then added a 1 ml of a 1mol NaOH via syringe. The initial colour changes were expected, a bit of blue copper hydroxide formed underneath the spout and then quickly re-dissolving back into the solution indicating the excess of copper 2+ ions and H+ ions from the free acid. (hope I am getting that bit right). Now I continue to add the NaOH until no more re-dissolving of the copper hydroxide takes place/ ie, the solution does not return to clear but begins its milky look. This happened after 2.5ml of NaOH was added. There was a precipitate. Now am I to assume that this means that there is approximately a Free HCl concentration of 1.5 mole in my copper chloride etchant or 2.5 mol?

went through a whole complicated mathematical procedure from my chemistry book that was demonstrating titrations with unknown concentrations of Hydrochloric acid and was left with 28 moles! Do I need to say anymore? Agghhh

woelen - 14-7-2012 at 08:40

Quote: Originally posted by Magpie

Quote: Originally posted by woelen

Yes, that is what I meant. My excuse if I was unclear.

@CHRIS25: You have appr. 2.5 mol/l of free HCl in your solution. This is quite strongly acidic, appr. 8% of HCl by weight in your solution.

[Edited on 14-7-12 by woelen]

CHRIS25 - 14-7-2012 at 10:44

Quote: Originally posted by woelen

Quote: Originally posted by Magpie

Quote: Originally posted by woelen

@CHRIS25: You have appr. 2.5 mol/l of free HCl in your solution. This is quite strongly acidic, appr. 8% of HCl by weight in your solution.

Hi Woelen, Yes, the only reference that I have ever found on the web states a 2.5 to 3.0 mole operating at 1.25 density, that last bit to ascertain. But I think that based on feelings my 2.5 mol concentration seems to be adequate, it takes 5 to 7 hours for what I am etching, it's BIG. Once I had to etch for 8 hours.

Woelen, I appreciate your help and you have helped enormously with your steering. I just wished that someone had mentioed that my NaOH titration was revealing acid concentration, I actually did not know that. Thankyou. I may even post some work photos on this board for those who have contributed to my difficult chemical learning. I don't think that would be wrong because after all, it is all Pure and Unadulterated chemistry, no added preservatives or colours, no paints, it's all chemistry in action albeit artistic.

watson.fawkes - 15-7-2012 at 07:38

1) Perhaps this was too obvious for me to bother stating before, but titrations on concentrated salt solutions can also be done by diluting the solution before titration. Working with a more dilute solution can make it easier to see the endpoint over one with such a high concentration of ions. Using a ratio of 1 part test solution to 9 parts distilled water gives a 10% solution, which makes the arithmetic more obvious in the last step when you back-calculation from the titration results.

I should point out that the dilution will almost certainly affect the chlorine complex structure. Dilute too much and you may get precipitates, so don't do that. But if what you want to measure at the end is pH, the [H⁺] concentration isn't affected by the copper-chlorine coordination structure, so you'll still get a useful result.

2) I would highly recommend getting a hydrometer for measuring the specific gravity of a solution. The specific gravity gives a decent estimate of the copper concentration of the solution, and even though you don't get a molar concentration of copper out of it, you can get some process consistency out of it.

To use one, all you do is to put some solution into a (graduated) cylinder, put in the hydrometer, and read the specific gravity off the scale. For not many dollars, you get a quick quantitative measurement. In fact, you may want to buy two. Buy the first in the range 1.0 to 1.4 SG. This may be enough, if the scale on the hydrometer is accurate enough for you. At the very least you get a decent ballpark number to know whether you're close to having an etchant that's what you want. Buy the second, if you need it, with a narrower range of measurement that contains your process target. Use this one for fine-tuning.

3) When you use a copper etchant, you increase the copper concentration in the spent etchant over the original etchant. In order to recycle the etchant back to the same initial concentration, there's some necessary dilution required. Some of that water comes with the extra HCl; some may need to be just water. The upshot of doing both (etch + recycle) is that the total volume of etchant increases each time you use it. If you're not expecting this volume increase, you'll almost certainly be having trouble.

I might point out that you don't need to recycle the entirety of a spent etchant, if all you want for the next batch is the volume that fits in your ordinary etch tank.

CHRIS25 - 15-7-2012 at 07:58

Hallo WatsonFawkes, yes, thankyou. I actually did some exprimeting with this last night by comparing three test tube titrations. One was just 3 ml CUCl, the next was 30 ml water and then CUCl added and the last was 50ml water and CUCl added and then the NAOH was added to each. I did get slightly different readings and concluded that 30 ml water seemed to be best. So your suggestion is quite handy. Thanks for the rest of your tips, much appreciated.