HCl is the worst dopant you can choose (I have explained already several times). HCl creates side reactions by forming some covalent bond somewhere (unknown where) in the PAni backbone. Moreover, HCl doped PAni is not thermally stable.

The best dopant is para-toluene sulfonic acid.

The PAni colour changes from undoped "EB" (blue) to doped "ES" (green).

The "black" you observed may come from green PAni which may have become dispersed (but not in nano-particles, already flocculated) in the HCl solution because you may not have removed the NMP well enough from your Petri dish, so the EB film maybe was not really dry. NMP does not get easily removed, takes time and heat.

Dear Elmira

HCl is good.

but you can use some polymeric materials that are water soluble. These materials can reduce the pH by release the protons. 

Kind regards

Reza

HCl is the worst dopant you can choose (I have explained already several times). HCl creates side reactions by forming some covalent bond somewhere (unknown where) in the PAni backbone. Moreover, HCl doped PAni is not thermally stable.

The best dopant is para-toluene sulfonic acid.

The PAni colour changes from undoped "EB" (blue) to doped "ES" (green).

The "black" you observed may come from green PAni which may have become dispersed (but not in nano-particles, already flocculated) in the HCl solution because you may not have removed the NMP well enough from your Petri dish, so the EB film maybe was not really dry. NMP does not get easily removed, takes time and heat.

thanks all

Polyaniline solution is best doped with  DBSA since it does not affect the  solubility. It is a bit costly but you can cast nice films. CSA or PTSA  do not give smell free films. The conductivity of the PANI doped with dbsa is also quite goo. 

The color of PANI is dependent on the doping level and in which of the four chemical states are formed. Emarldine PANI is green which is conducting. Over reaction with HCl in fact does not give good conducting material and it may also degrade.  Aniline black is not conducting. There has to be optimum doping level for best conductivity.

Subramanian, please look at my RG site and look for my various publications which show that PAni is totalle and principally insoluble. There are no "polyaniline solutions", every system (whether it is EB or doped with DBSA and you can cast nice films) are colloidal dispersions. This is a fundamental difference, like plus / minus, black / white, equilibrium / non-equilibrium.

Thermodynamically looked at, solutions are equilibrium systems, colloidal dispersions are non-equilibrium systems, I have published also about this topic and developed a non-equilibrium thermodynamical theory desciring dispersions.

Also, I have experimentally and theoretically proven that what you (and many other people) call (conductive polymer or Polyaniline "solutions") are dispersions.

I don't agree with Wessling point. Emeraldine base is soluble in NMP and these are not dispersions. Of-course, polyaniline doped with dopants are not soluble completely and remain in dispersion state. Like if PANI is doped with CSA, then you can obtain quite good film in m-cresol. With DBSA, you obtain polymer in chloroform. With PTSA, we don't get polyaniline soluble in NMP/DMSO. But as such, PTSA is a good dopant for PANI.

Sundeep, if you don't agree, please show facts, not beliefs. Please look at this publication

https://www.researchgate.net/publication/202290125_Conductive_Polymer__Solvent_Systems_Solutions_or_Dispersions

and show me proofs how you found EB is soluble. There, I have given all the facts and evidence which show why these systems are dispersions, and I have given a thermodynamic explanation for why they can't be soluble.

Moreover, I have given very clear criteria how to distinguish between solutions and dispersions. This is not an easy task if (like in case of EB where the particle size is only 5 nm), but it can be done if you follow the suggestions given in my publication.

for understanding, why EB has smaller particle size in dispersions than the doped forms (which have primary particle size of 10 nm, and in dispersions depending on the systems between 30 and 70 nm), you may want to read this publication:

https://www.researchgate.net/publication/49593603_New_Insight_into_Organic_Metal_Polyaniline_Morphology_and_Structure

Article Conductive Polymer / Solvent Systems: Solutions or Dispersions?

Article New Insight into Organic Metal Polyaniline Morphology and Structure