An experimental study of formation of the mercury mixed halides HgClBr and HgBrI  and of their purity

Rabia Ahmad*, Jamshed Ali  and  Qamer Faisal

¹Department of Chemistry, Faculty of Natural Sciences, Jamia Millia Islamia, (Central University), Jamia Nagar, New Delhi 110025, India.

DOI : http://dx.doi.org/10.13005/OJC/310206

ABSTRACT:

Claims to have produced the mixed halides of mercury are very old.  However, their stability or even their very existence was seriously questioned by Ammlung and Brill several decades back, on the basis of their study, in several solvents, of what was thought to be HgBrI.   The mixed halide HgClI was already known to be unstable.  On the basis of these facts, which were also lent some theoretical support, it was strongly conjectured that the mixed halides of mercury and similar elements, were expected to be unstable.  However, the matter does not seem to have received the attention it deserved. It was in this light that this study was taken up.  What has been thought to be HgClBr has been produced by several methods and HgBrI by one or rather two methods.  The product has been subjected to X-ray diffraction, FTIR and Raman studies.  Studies confined to the solid product are being reported here and only those results are being presented for which all the three techniques could be employed.   These studies show that a new product is indeed formed in most of these cases, but the product is not pure in any of these cases, although the impurity seems to be quite small in most of these cases.  This calls for having a thorough look at not only the mixed halides of the elements, but of all compounds claimed to be  like:

KEYWORDS:

Mixed halides; impure product; d-values; FTIR Studies

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Introduction

However, rather surprisingly, almost no notice of Ammlung and Brill’s work seems to have been taken for quite a long time.  Workers continued to claim formation of the di-halides of not only mercury but also of cadmium and zinc^5,6. Claims have been made of determination of structures of mixed mercury halide crystals⁷. Nobody seems to have thoroughly investigated the extent of the correctness of these claims.

We seem to be the first workers to have thoroughly investigated what seems to be HgClBr and to some extent HgBrI also.  The main reason was that Ammlung and Brill had confined their study only to what was thought to be HgBrI dissolved in some solvents.  We have extended our study to not only what would be regarded as HgBrI but much more extensively to what would be regarded as HgClBr.  Besides considering many more solvents, we have additionally studied the product in the solid state. This work is concerned only with the product mixed halide in the solid state.

We briefly mention the different ways in which what is regarded as HgClBr has been prepared, using the methods suggested in the literature as well as some of our own. Earlier some X-ray studies of the products formed by these methods have been taken up by some authors⁸.

By heating an equmolar mixture of HgCl₂ and HgBr₂ in an oven at 80°C, for 48 hours (The same reaction has also been carried out at 100°C and 150°C for 10 hours in each case).

HgCl₂  + HgBr₂  →   HgClBr

HgCl₂   +  HgBr₂  → 2HgClBr

In the method due to R.P. Rastogi, bromine gas is passed over solid mercurous chloride until a constant weight is obtained⁷ 

Hg₂Cl₂   + Br₂  →  2HgClBr

An equimolar mixture of HgBr₂ and HgI₂ is taken and then to grind the two halides together at room temperature.  This is called Type-II mixture.

Extension of this approach for concluding if a new product has been formed in a reaction is direct and straightforward.  This is discussed here.

We have taken the diffractograms of HgCl₂, HgBr₂ and of their Type-I mixture.  These are given in Figs. 1, 2 and 3 respectively.  We prepare the equimolar mixture in another way also.  We take suitable equimolar amounts, mix these and grind these in a mortar.  We call it the Type-II mixture.  The diffractogram is given in Fig. 4.   The d-values for the above cases are given in Tables 1-4, respectively.

Figure1: HgCl2   Click here to View figure

 

Figure2: HgBr2  Click here to View figure

 

Figure3: Equimolar mixture of HgCl2 and HgBr2 at room temperature (Type I)  Click here to View figure

 

Figure4: Equimolar mixture of HgCl2 and HgBr2 at room temperature (Type II)  Click here to View figure

 

By comparing the three most prominent peaks of HgCl₂ and the single most prominent peak of HgBr₂ (all the other peaks of HgBr₂ being very insignificant), with the corresponding peaks in the Type I mixture we can find out the maximum permissible deviation in the d-values, of a prominent peak, when the compound be present in a mixture.

By the above comparison, we may ascribe the maximum deviation to the d-values of 2.6263 (a₁)  in the Type I mixture, if we identify it with  the d-value of 2.7022(a₁)  in HgCl₂ with an intensity of 57.7%.   This would be about 0.08°A.  However, if we identify the peak at  d=2.8121 (a₁), in the Type-I mixture, with the d value of 2.7022 (a₁)   in HgCl₂, the permissible deviation would be about 0.1°A.   To be on the safe side, we take the latter to be the permissible deviation for HgCl₂ in a mixture.  Any difference in the d-value within 0.1°A from those for the HgCl₂ peaks would be taken to signal presence of HgCl₂ but d-values outside the maximum upper limit would generally mean absence of HgCl₂ and presence of some other compound.  We see that for all the other prominent peaks of HgCl₂ and HgBr₂, the expected deviation to be much less than 0.1°A.   However, the deviation of the d-values may be larger if the number of compounds in the mixture is large.  We  may finally add that  d-values should always be seen in conjunction with the relative intensity.

In the above light, we examine our products. The presence of the most prominent peak at d = 3.1239, in the Type II mixture, is a clear indication of the formation of a new product.  However, presence of a fairly strong peak at d =6.1972 may be taken as a clear indication that HgBr₂ is also present in the Type II mixture along with the new product.  By comparing its peak intensity counts of 213 with the corresponding counts of 666 in the Type I mixture, we may conclude that the amount of the HgBr₂ along with the new product, is about 16%.

Relative intensity consideration is often helpful in assigning a peak to a reactant.

If the density of a compound AB₂ is denoted by  and its molecular weight by , it can be easily seen that the density D of a well-packed equimolar mixture of the compounds AB₂ and AC₂ is given by

Taking the densities of HgCl₂ and HgBr₂ obtained from the standard literature as also their molecular weights ¹², we get the density D of their equi-molar mixture.  The densities of HgCl₂ and HgBr₂ are given in Table-5, along with the density of HgClBr, as prepared by Method-I.  The density regarding HgClBr as an equimolar mixture of HgCl₂ and HgBr₂ is given inside the bracket.  The value of the density for what is called HgClBr is that measured by one of the authors (R.Ahmad).  The melting points are taken from ref.12 and are given in Table 5.

Table 5

Compound	Experimental Density (gm/ c.c.)	Melting point (oC)
HgCl2 HgBr2 HgClBr	5.44 6.11         5.3 (5.8)*	276 236 205

We may examine the products of the reactions using the criteria arrived at above.  In this study, we have included only those products for which the X-ray d-values, the FTIR absorption wave numbers as well as the wave numbers of the Raman lines are available.  The results are given below (the wave numbers are in cm^-1 ).