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Question: What would the "cage-like" arrangement of Formazine molecules look like in a Formazine particle while it is still suspended in water? Would it be totally random, or have some kind of imperfect but recognizable regularity?

Formazine is used as an easily defined and prepared calibration or reference standard for turbidity measurements.

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above x2: Formazine from PubChem.

From Wikipedia:

Formazine (formazin) is a heterocyclic polymer produced by reacting hexamethylenetetramine with hydrazine sulfate.

The hexamethylenetetramine tetrahedral cage-like structure, similar to adamantane, serve as molecular building block to form a tridimensional polymeric network.

From Formazine:

The readings shown by turbidimeters are not scaled in the measured light intensity, but in concentration of a reference suspension. Because the accuracy of this calibration solution determines the reliability of subsequent turbidity measurements, it is of crucial importance.

The internationally established turbidity standard for calibration is formazine. This standard can be reproduced at any time with the recipe taken from standard ISO 7027 (Water quality - Determination of turbidity).

It is extremely important to observe the prescribed preparation temperature, because it affects the particle size of the formazine particles perceptibly. The following Figure illustrates this. Errors caused by temperature variations are on the order of 1..2% per °C. Consequently, Sigrist keeps the preparation temperature constant to within ± 1°C.

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above: "Particle size distributions of formazine at various preparation temperatures" from here.

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above: Enlarged from TurbidityStandards.

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above: Slide #10 from What is Turbidity? John Daly, ISA NorCal President, South Fork Instruments, Inc. Click for full size.

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First, note that

  • formazine structure attached from PubChem is a different compound;
  • note that there are also, related, formazan $\ce{HN=N-CH=N-NH2}$ and “formaldazine$\ce{CH2=N-N=CH2}$ (dimethylidenehydrazine);
  • formazin (polymer) does not necessarily has to have cage-like structure, as this characterization belongs to hexamethylenetetramine in the quoted citation.

Hexamethylenetetramine (urotropine, $\ce{(CH2)6N4}$) can be prepared from formaldehyde and ammonia. In water it also somewhat decomposes to its precursors:

Fig.1 - urotropine to/from formaldehyde and ammonia

Formazin (also called formalazine) structure might be still unknown, as a related publication from 1976 states:[1]

… This substance, an insoluble condensation product of uncertain composition, is prepared by mixing solutions of hexamethylenetetramine and hydrazine sulfate. …

Sometimes empirical-like formula $\ce{(C2H4N2)_n}$ is claimed.[2]

Sometimes, claimed structure like following one is presented:[3,4]

Fig.2 - claimed formazin structure w/ prep.

It was (with several other structures, less nice or likely), hypothesized (well, rather more complicated, with some ether linkages $\ce{-CH2-O-CH2\bond{-}}$ and $\ce{-NH\bond{-}}$ groups present), based on spectroscopic studies.[5,6]


References:

  1. Rice, E. W. The preparation of formazin standards for nephelometry. Analytica Chimica Acta 1976, 87 (1), 251–253.
  2. Ziegler, A. Issues Related to Use of Turbidity Measurements as a Surrogate for Suspended Sediment. Turbidity and Other Sediment Surrogates Workshop; Reno, NV, USA, 2002.
  3. Kaur, N.; Kishore, D. An Insight into Hexamethylenetetramine: A Versatile Reagent in Organic Synthesis. J. Iran. Chem. Soc. 2013, 10 (6), 1193–1228.
  4. Sadar, M. J. Stabilized Formazin Composition. US Patent 5,777,011, July 1998.
  5. Mashima, M. The Infrared Absorption Spectra of the Condensation Products of Formaldehyde with Hydrazine. Bull. Chem. Soc. Jpn. 1966, 39 (3), 504–506.
  6. Bondybey, V. E.; Nibler, J. W. Infrared and Raman Spectra of Formaldazine. Spectrochimica Acta Part A: Molecular Spectroscopy 1973, 29 (4), 645–658.
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  • $\begingroup$ Thank you for taking the time to put together such a thoroughly sourced answer and clarification! It's been a while now, so I'm going to take some time and have a look at your references and think about this a bit. $\endgroup$ – uhoh Jan 7 at 0:44
  • $\begingroup$ I was able to track most of these down and I have a better understanding of the situation now, thanks! $\endgroup$ – uhoh Jul 23 at 0:01

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