Perhaps you refer to immunohistochemical investigation rather than histochemical techniques for localization of CA. Anyhow, the fixative of choice for IHC is always 4% PFA.

Dear Amina CHEBOUB, not trying to teach you, but: just some comment on your interesting request  and possible help from outside/RG-community:

i) to compare the written terms:

"10%Formalin" as opposed to 4%PFA (Re Giacomo Zaccone):

For such special task and delicate enzymic activity (we know you had fixed WHICH ?specimens?  with 10% formalin; HOW LONG? and which post-treatment after fixation, LOCALIZATION of C.A. in WHICH specimens / parts of the general specimen, e. g. paraffin embedded, cryo-specs,doing cryo-sections...etc.)

you should use the best fixative one could use:

my question: did you use "10% Formalin" [=*chemically correct: 4% formaldehyde solution] as a solution FILTERED and diluted correctly from e. g. 37% Formaldehyde solution, at least 'Histology grade' (stabilized by approx. 10% Methanol and with a layer of carbonate for maintaining pH)? I guess, Giacomo is right in saying to use at least 4% PFA (cp.to * above) because 4% PFA (paraformaldehyde) is the same concentration as 10%Formalin BUT always made fresh from powder (thereforeno addition of any stabilizer compoments which finally may [be able to] disturb the localization of the enzyme).

So in order to get helpful advices you should also mention the specimen or anatomical localization your target for examining/displaying the enzymatic activity is as well as the you would like to examine by the technique.

Assuming (from your request text) it will be a histochemical approach:

As a starting point to collect more information on ==> (!ancillary!) HC-techniques please search also for "Hanssons's medium" [=cobalt-phosphate histochemical method]   and see: 

Original Paper: OKAMURA Hiro-oki, Naonori Sugai, Kazunori Suzuki, Iwao Ohtani: Enzyme-histochemical localization of carbonic anhydrase in the inner ear of the guinea pig and several improvements of the technique. Histochemistry and Cell Biology, October 1996, Volume 106, Issue 4, pp 425-430 [Note: "correct and proper fixation is important, electron microscopic approach) Paper/Article unfortunately only available by PPV (Pay Per View) @http://link.springer.com/article/10.1007%2FBF02473302  or  @http://link.springer.com/article/10.1007%2FBF02473302#page-1

or:  Wistrand PJ, Schenholm M, Lönnerholm G.,

Carbonic anhydrase isoenzymes CA I and CA II in the human eye.  in 

Invest Ophthalmol Vis Sci. 1986 Mar;27(3):419-28.  @http://www.ncbi.nlm.nih.gov/pubmed/3081459

Further sources and references/articles perhaps doing a Google-search for keywords/search phrase: <  Carbonic Anhydrase AND Histochemi* Techn  >

But I guess that there are more modern localization techniques like IHC and others available....(i have not searched for)

Best wishes and good luck, WM

Dear Wolgang,

Incidentally you give me  the reminder of my loved Wolgang Amade who is besides my scientific work, the great passion of my life(membership of  AMI coupled with Salzburg Mozarteum, and also I'm writing a book on Mozart.

By going back to  fixation of carbonic anydrase you are right.Also oldest papers(Parkkila et al.1990) suggested paraformaldehyde fixed tissues to study the IHC of cabonic anhydrase I,II,V, besides Carnoy.Also in my oldest studies on amphibian skin epidermal flask cells with Uri Katz, I also adopted this kind of fixation.

Best,

Giacomo

Dear Giacomo, unfortunately I lost all the message-text I already had written as a reply to your kind words and the historic aspect of histochemical C.A. localisation. I am not sure about a recovery of the former, now lost text out of my brain but I will try later on a second time. Meanwhile: take care, thank you and my best wishes and greetings to you and Messina, Wolfgang

Dear Wolgang,

There is no haste.And take care too.Thank you for your kind reply and greetings to you and Salzburg.You are fortunate to live in that town.

Best,

Giacomo

Dear sirs ,

thank you both for your answers and so precise advice.