An Experimental Study of Formation of the Mercury Mixed

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:


INTRODUCTION
Mercury has been known for a long time and its compounds studied 1 .Claims to have produced mixed halides of mercury go fairly long back into time 2 .However, Ammlung and Brill and others have thoroughly studied the problem and these authors have seriously challenged existence of the mixed halides 3,4 .Ammlung and Brill carried out a study of the Raman lines of what was claimed to be HgBrI, dissolved in some solvents.They found strong Raman lines of the constituent di-halides of mercury 3 .The mixed halide HgClI had already been found to be highly unstable.Some theoretical support to this idea was also indicated.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 crystals 7 .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 authors 8 .
We have produced what would be regarded as HgClBr by several methods and have also produced what would be regarded as HgBrI by two methods.All these methods are given below.

Preparation of HgClBr Method I
By heating an equmolar mixture of HgCl 2 and HgBr 2 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).

Method II
In the method of preparation, a saturated solution of HgCl 2 and HgBr 2 in water mixed in 1:1 ratio, was kept in a desiccator over CaCl 2 until solid crystals separate out HgCl 2 + HgBr 2  2HgClBr HgClBr produced by this method is in nanoform 9 .Several methods are available for grain size measurements e.g. 10 .However, we have used the Debye-Scherer formula 11 .

Method III
In the method due to R.P. Rastogi, bromine gas is passed over solid mercurous chloride until a constant weight is obtained 7 Hg 2 Cl 2 + Br 2  2HgClBr

Preparation of HgBrI Method I
HgBrI was prepared by mixing equimolar solutions of HgBr 2 and HgI 2 in acetone.Crystals of HgBrI separated after evaporation HgBr 2 + HgI 2  2HgBrI

Method II
An equimolar mixture of HgBr 2 and HgI 2 is taken and then to grind the two halides together at room temperature.This is called Type-II mixture.

Type I and Type II mixture and use of the X-ray d values for identification of a new product
X-ray d-values are extensively used for identifying the compounds present in a mixture.The standard practice is to locate the three most prominent peaks of the compound for its identification.The most important matter is to decide how much deviation in the d-values is permissible.
This is what we find out.
Extension of this approach for concluding if a new product has been formed in a reaction is direct and straightforward.This is discussed here.
One needs the diffractogram of the constituent reactants and additionally the diffractogram of an equimolar mixture of the reactants, when they are separately ground and then added to each other, is very helpful.This type of equimolar mixture may be called the Type-I mixture.We discuss a simple case as an illustration.
We have taken the diffractograms of HgCl 2 , HgBr 2 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.
One would expect no chemical or even physical change to have occurred in the Type I mixture.Therefore, it may be taken to give the shifts in the d-values that would be expected to occur in a mixture of these compounds.Greater shifts from those noted here would be regarded as belonging to a new compound.A totally new peak would be an unmistakable signal for the formation of a new compound.Occurrence of peaks of the reactants within the permissible range, as determined from the Type I mixture, would be regarded as a signal that these represent presence of some unused reactant.However, in some cases, these might well belong to a new compound.
With this background in mind we study the data of the mercury halides and their mixtures mentioned above.
By comparing the three most prominent peaks of HgCl 2 and the single most prominent peak of HgBr 2 (all the other peaks of HgBr 2 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 (a 1 ) in the Type I mixture, if we identify it with the dvalue of 2.7022( 1 ) in HgCl 2 with an intensity of 57.7%.This would be about 0.08°A.However, if we identify the peak at d=2.8121 ( 1 ), in the Type-I mixture, with the d value of 2.7022 ( 1 ) in HgCl 2 , 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 HgCl 2 in a mixture.Any difference in the d-value within 0.1°A from those for the HgCl 2 peaks would be taken to signal presence of HgCl 2 but d-values outside the maximum upper limit would generally mean absence of HgCl 2 and presence of some other compound.We see that for all the other prominent peaks of HgCl 2 and HgBr 2 , 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  = 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 HgBr 2 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 HgBr 2 along with the new product, is about 16%.

Summary of the analysis of the X-ray diffractograms
Occurrence of a new prominent peak which is not found in the diffractogram of either of the reactants, may be taken as a strong evidence for the formation of a new product.
If a prominent peak considered as belonging to a reactant is found to have suffered a displacement which is more than what may be regard as permissible.(say, a maximum value of 0.1° A in the d-values, for our system), that may also be taken as belonging to a new product.
Relative intensity consideration is often helpful in assigning a peak to a reactant.

Density Considerations
If the density of a compound AB 2 is denoted by d AB 2  and its molecular weight by M AB 2 , it can be easily seen that the density D of a well-packed equimolar mixture of the compounds AB 2 and AC 2 is given by Taking the densities of HgCl 2 and HgBr 2 obtained from the standard literature as also their molecular weights 12 , we get the density D of their equi-molar mixture.The densities of HgCl 2 and HgBr 2 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 HgCl 2 and HgBr 2 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.
The results encourage us to believe that HgClBr (Method-I) is largely a new product.A small fraction of the constituent reactants cannot be ruled out.

Results of the examination of the product and the conclusions
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 ).

1.
HgCl 2 + HgBr 2 (Type II mixture ) The X-ray study indicates presence of a relatively small fraction of the reactants along with a new product.T h e FTIR and the Raman do not show any wave numbers of the reactants.

2.
HgClBr (Ist Method) : The X-ray study indicates a fairly pure product.The FTIR Heating the equimolar no wave-number of the reactants.The Raman shows the mixture, at 80°C, for 48 hours.

HgClBr (IInd Method)
The X-ray study indicates a small fraction of the reactants along with a new product.The FTIR shows the HgBr 2 wave number 835.0430.The Raman does not show any lines of the reactants.4.
HgClBr (IIIrd Method) The X-ray study indicates presence of the reactants, possibly along with a new product.The FTIR shows the HgBr 2 wave number 835.0436 and 1916.936.The Raman does not show any lines of the reactants.

5.
HgBr 2 + HgI 2 (Type II mixture) A new product has been formed.The X-ray study does not indicate presence of the reactants.The FTIR shows the HgI 2 wave number 1610.303.The Raman does not show any lines of the reactants.

HgBrI
The new product could be reasonably pure.The FTIR shows the HgI 2 wave number 1384.668.The Raman does not show any lines of the reactants.With the possible exception of the third method for producing HgClBr, in all the other cases, including those for producing HgBrI, a new product which may be regarded as a mixed halide of mercury, is formed, but in all these cases, a small quantity of the constituent di-halides is present along side the mixed halide of mercury.By Method -II of producing HgClBr nano form of HgClBr has been produced.It is clearly seen from all the three studies that the product in each cases is somewhat different.It is intriguing that probably small quantity of one of the reactants remain.This might mean that in a small number of cases a more complicated additive compound is formed.Probably this is arising not due to the instability of the compound formed but due to the formation of a complicated additive compound.Since all the three kinds of data are not available for the two other slight modifications of Method-I for producing HgClBr, these are not being discussed here.The same applies to the product obtained by the application of high pressure to a pellet of an equimolar mixture of HgCl 2 and HgBr 2 or HgBr 2 and HgI 2 .
We like to carry out the required analysis that remains in these cases and then to hopefully report these results.However, it seems likely that the mixed halides would have some fraction of the constituent di-halides.In this light one may have a critical experimental study of the mixed halides of other elements.In fact, one may examine any compounds of the form etc., in this light one should also have a look at the claims of having produced the single crystals of the mixed halides.Other methods for producing the mixed halides may still be tried out.Use of nanoreactants may make a difference.

Table 5 : Compound Experimental Melting point Density (gm/ c.c.) ( o C)
* The density on the assumption that this is an equimolar mixture of HgCl 2 and HgBr 2 .