Dear Jan Sher, the preparation of the reagent is described in detail in the article cited below (also see attached). The paper is available on RG. A search on RG for the term "preparation of Folin-Ciocalteu reagent" also revealed numerous entries, several of which can be downloaded as full texts.
Article Colorimetric determination of capsaicin in capsicum fruits w...
Dear all, complete answer is given by Prof. Frank T. Edelmann. I would like to add two things, first the test in which the reagent is to be used should be specified, though it is most known for phenol and protien quantification. Second, I attached a document which gives recent guidance and progress in using this reagent. My Regards
Folin Ciocalteu reagent- Dilute commercially available Folin-Ciocalteu reagent (2N) with an equal volume of distilled water. Transfer it in a brown bottle and store in a refrigerator. It should be goldern in colour. Do not use it if it turns olive green.
Reference: THE AYURVEDIC PHARMACOPOEIA
OF INDIA PART - II (FORMULATIONS), VOLUME - II, First Edition, Appendices 1 to 5