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Diphenylcarbazide is used in test strips for heavy metals detection in drinking water but I can't find much info on this reagent, apart from it's MSDS and a preparation for detecting Chromium VI ions, in which diphenylcarbazide 250 mg is dissolved in 50ml acetone and few drops added to test sol which is then mixed with sulfuric acid. Also, "Vogel's qualitative inorganic analysis" references this reagent but it is very vague explanation.

If you can elaborate on how the reagent works to detect mercury, and possibly how it is stored and synthesized, that would be really appreciated.

Also, how are the test strips for heavy metals in drinking water made from this compound, I'm curious?

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The Diphenylcarbazide Test for the presence of mercury was used as early as 1920s. However, since the test is very delicate (the test can detect $\pu{250 \mu g/L}$ mercury in solution), it has been used predominantly in neutral solutions, until it have been adapted to Quantitative Analysis Scheme by Scott in 1929 (Ref.1) to ditect in acidic medium:

The mercury is precipitated in Group I is dissolved in aqua regia, that of Group II is dissoved in hydrochloric acid and sodium chlorate. In either case, the solution is evaporate to a volume of about $\pu{1 cc.}$, placed in a test tube, and diluted with $5$ to $\pu{6 cc.}$ of water. Four to eight drops of a saturated alcoholic solution of diphenylcarbazide is added and a large excess of solid sodium carbonate is gradually dropped into the solution. When the mercury is present, the form produced on neutralization assumes a blue tinge, and after the addition of an excess of carbonate the entire solution turns blue.

The author stated at the end that neither sodium acetate nor sodium bicarbonate can be successfully substituted for the sodium carbonate, indicating the sensitivity of the test for $\mathrm{pH}$. It is worth noting that the test outlined here (vide supra) has been successfully used in Quantitative Analysis Courses in University of Georgia, Athens, GA for two years before publication.

The major drawback of the test is the deterioration of stock solution of diphenylcarbazide in different solvents. Thus, it needs to be freshly prepared. To avoid this problem, a solution has been found in the publication by Urone by 1955 (Ref.2). Accordingly, non-aqueous ethyl acetate and acetone were the better solvents, diphenylcarbazide solutions of which are stable for months. Diphenylcarbazide solutions of Methyl ethyl ketone, methyl cellosolve (2-methoxyethanol), and 2-propanol are usable for 1-2 weeks. Aqueous solutions and solvents tending to be basic such as methanol and ethanol, and those containing traces of water and basic impurities, do not make good solvents for stock solutions of the colorimetric reagent (Ref.2).

Notes: (1) All solutions tested (diphenylcarbazide solutions in seven different solvents) in Ref.2 are $0.25\%$ in diphenylcarbazide. (2) Also, you don't have to synthesize 1,5-diphenylcarbazide; it is commercially available in Millipore-Sigma. (3) I don't think anybody can answer your question about "how are the test strips for heavy metals in drinking water made from this compound," because it should be a corporate secret, similar to $\ce{KI}$ papers for oxidant check.

References:

  1. Alfred W. Scott, “Adaptation of the Diphenylcarbazide Test for Mercury to the Scheme of Qualitative Analysis,” J. Am. Chem. Soc. 1929, 51(11), 3351-3352 (https://doi.org/10.1021/ja01386a503).
  2. P. F. Urone, “Stability of Colorimetric Reagent for Chromium, s-Diphenylcarbazide, in Various Solvents,” Anal. Chem. 1955, 27(8), 1354-1355 (https://doi.org/10.1021/ac60104a048).
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In the beginning of the $20th$ century, the following procedure was developed to prove the presence of mercury in solution. One drop of the solution to be tested is deposited on a filter paper which had been dipped into a freshly prepared 1% alcoholic solution of diphenylcarbazide. Mercury salts produce a purple spot, even in a very diluted solution. Chromates and molybdates produce the same reaction. See more detailed reports :

W. Böttiger, Bestimmung kleiner Mengen Quechsilbersalz in stärker Verdünnung. Z. Elektrochem. 22, 69 (1916)

A. Stock, E. Rohland, Kolorimetrische Bestimmung sehr kleiner Quecksilbermengen, Z. Angew. Chem. 39. 791 (1926)

A. Stock, W. Zimmermann, Zur Bestimmung kleinster Quecksilbermengen, Z. Angew. Chem. 41, 546 (1928)

A. W. Scott, Adaptation of the Diphenylcarbazide Test for Mercury to the Scheme of Qualitative Analysis, J. Am. Ch. Soc. 51, 3351, (1929)

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Synthesis routes From molbase[dot]com :

Phenylhydrazine + urea -> 1,5-Diphenylcarbazide ~96% yield Pasha; Madhusudana Reddy Synthetic Communications, 2009 , vol. 39, # 16 p. 2928 - 2934

Phenylhydrazine + urethane -> 1,5-Diphenylcarbazide + 1-Phenylsemicarbazide + Ammonia Journal of the Chemical Society, vol. 53, p. 551

Phenylhydrazine + phosgene + Et2O + Toluene -> 1,5-Diphenylcarbazide Justus Liebigs Annalen der Chemie, vol. 263, p. 277

Phenylhydrazine + 1-Methyl-1-phenylure -> 1,5-Diphenylcarbazide Gazzetta Chimica Italiana, , vol. 68, p. 516,520

Phenylhydrazine + 1,1-Diphenylurea -> 1,5-Diphenylcarbazide Gazzetta Chimica Italiana, , vol. 68, p. 516,520

Phenylhydrazine + 1-Phenylsemicarbazide -> 1,5-Diphenylcarbazide Journal of the Chemical Society, , vol. 53, p. 556

1-benzhydrylidene-5-phenyl carbonohydrazide -> 1,5-Diphenylcarbazide + 1,3-bis(benzhydrylideneamino)urea Journal of the Chemical Society, , p. 2376 Journal of the Chemical Society, , p. 2114 Anm.

Phenylhydrazine + urethane -> 1,5-Diphenylcarbazide + 1-Phenylsemicarbazide Journal of the Chemical Society, , vol. 53, p. 556 Chemische Berichte, , vol. 23, p. 2837

Phenylhydrazine + urethane -> 1,5-Diphenylcarbazide Fresenius' Zeitschrift fuer Analytische Chemie, , vol. 72, p. 119

Phenylhydrazine + ? -> 1,5-Diphenylcarbazide
Zhurnal Organicheskoi Khimii, , vol. 4, p. 837 - 844,814 - 820

Phenylhydrazine + 1,3-diphenylurea -> 1,5-Diphenylcarbazide Gazzetta Chimica Italiana, , vol. 68, p. 516,520

Phenylhydrazine + phosgene -> 1,5-Diphenylcarbazide Justus Liebigs Annalen der Chemie, , vol. 263, p. 278

? -> 1,5-Diphenylcarbazide Chemische Berichte, , vol. 44, p. 3744,3748

1,5-diphenyl-3-nitroformazan -> 1,5-Diphenylcarbazide Chemische Berichte, , vol. 44, p. 3744,3748

Phenylhydrazine + diphenyl carbonate -> 1,5-Diphenylcarbazide Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, vol. 129, p. 1256

Phenylhydrazine + Guaiacol carbonate -> 1,5-Diphenylcarbazide Justus Liebigs Annalen der Chemie, , vol. 446, p. 263 Journal of the American Chemical Society, , vol. 52, p. 1132 Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, , vol. 129, p. 1256 Bulletin de la Societe Chimique de France, , vol. <3> 23, p. 52

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