inorganic chemistry - What is the alternative to chromyl chloride test?

Chromyl chloride test is by far the best test to determine the presence of chloride ions in a salt because no analogous compounds are formed with fluorides, bromides, iodides and cyanides, so this test is therefore specific for chlorides. This test is actually considered selective for $\ce{Cl-}$.

But this test possess certain drawbacks. This test does not give positive results to covalent chloride like $\ce{HgCl2, SbCl3, SnCl2 }$ etc. owing to their slight ionisation and only partial conversion occurs when added to chromyl chloride solution. Thus, this test is only positive to ionic chlorides. There are also other tests like conc. sulfuric acid test, $\ce{MnO2}$ test or lead acetate test which will almost determine the presence of chloride ions though it possess some drawbacks (see comment section).

For a particular cation or anion, a selective test is chosen in such a way that it will give positive result to any salt, no matter how complex it is and has the capability to differentiate from other analogous salts. So, why Chromyl chloride is still considered as the selective test for $\ce{Cl-}$ despite its drawbacks?

Answer

Why chromyl chloride test is used as confirmatory test for $\ce{Cl-}$:

According to this Wikipedia page:

The chromyl chloride test entails heating a sample suspected of containing chloride with potassium dichromate and concentrated sulfuric acid. If chloride is present, chromyl chloride is formed and red fumes of $\ce{CrO2Cl2}$ are evident. If there is no chloride present, no red fumes are produced. No analogous compounds are formed with fluorides, bromides, iodides and cyanides, so this test is therefore specific for chlorides. ^[1]

Most chemistry lab manuals mention that only red vapour is not the confirmation for presence of chloride ion. Rather, if the red vapour dissolves in a solution of $\ce{NaOH}$ and turns the solution yellow (due to $\ce{Na2CrO4}$), and that solution gives yellow precipitate with lead acetate solution, then only, presence of chloride is confirmed.

However, this paper mentions that chromyl fluoride is produced on heating calcium fluoride with chromate and sulfuric acid. The compound chromyl fluoride is a red-vapour at normal temperatures, and is also produced by reaction of dichromates and $\ce{HF}$.^[2].

Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis^[3] mentions that for chromyl chloride test:

Fluorides give rise to volatile chromyl fluoride, $\ce{CrO2F2}$, which is decomposed by water, and hence should be absent or removed.

These suggest that there is a possibility of production of chromyl fluoride by reaction of potassium dichromate, concentrated sulphuric acid and a fluoride sample. As chromyl fluoride is red vapour, this would result in a false positive. Moreover, chromyl fluoride is also hydrolysed in water, ^[4] $$\ce{CrO2F2 + H2O->CrO3 + 2HF}$$ and thus it should also turn $\ce{NaOH}$ solution yellow, and produce yellow precipitate of $\ce{PbCrO4}$ with lead acetate. So, there is no way they can be distinguished without performing some other test for chloride or fluoride.

It is not possible to know whether chromyl fluoride is actually produced in large quantities like chromyl chloride without performing the actual experiment. But, I think there is a chance of error with fluoride sample.

Also, bromide salts produce red vapours of bromine when heated with $\ce{K2Cr2O7}$ and conc. $\ce{H2SO4}$(this is from my own experience). I don't know why the Wikipedia page specifically mentions that no red fumes are produced in absence of chlorine, and no analogous compound is formed with fluoride. It appears to me that the information is questionable.

Alternative to chromyl chloride test:

A feasible (and in my opinion, easier) alternative is this procedure—

1. Prepare the soda extract or sodium carbonate extract of the salt in question. This would produce $\ce{NaCl}$, even from covalent chlorides. For insoluble chlorides, it has to be done in melt.


2. Make a water solution of the extract, and filter it.



3. Add $\ce{AgNO3}$ soln. to this solution. A white precipitate of $\ce{AgCl}$ would form.


4. To confirm that it is indeed $\ce{AgCl}$ and not $\ce{AgBr}$ or $\ce{AgI}$, add sodium arsenite($\ce{Na3AsO3}$) solution, and shake. Yellow precipitate of $\ce{Ag3AsO3}$ is formed^[3]. This does not occur in case of $\ce{AgBr}$ or $\ce{AgI}$. [I am not sure about $\ce{AgCN}$ though.]

Another alternative is—

1. 
   

   Add $\ce{AgNO3}$ solution to the sodium carbonate extract.

   
   


2. 
   

   $\ce{AgCl}$ is white and soluble in $\ce{NH4OH}$. $\ce{AgBr}$ is pale yellow and partially soluble in $\ce{NH4OH}$. $\ce{AgI}$ is yellow and insoluble in $\ce{NH4OH}$. To improve colour detection, use a grooved tile made from black granite, and perform spot test^[5].

   
   
   


3. For detecting cyanide, add iron(II) sulphate to the soda extract and then add a mineral acid. If a precipitate of prussian blue does not appear, then there is no cyanide.

References:

[1] https://en.wikipedia.org/wiki/Chromyl_chloride

[2] Pure Chromyl Fluoride. Alfred Engelbrecht, Aristid V. Grosse. J. Am. Chem. Soc., 1952, 74 (21), pp 5262–5264 |http://pubs.acs.org/doi/abs/10.1021/ja01141a007?journalCode=jacsat

[3] Vogel's Textbook of Macro and Semimicro Qualitative Inorganic Analysis, 5th edition, Longman Group Limited, 1979.

[4] https://en.wikipedia.org/wiki/Chromyl_fluoride

[5] Greener Qualitative Analysis for Anions in a Mixture. Indu Tucker Sidhwani, Sushmita Chowdhury, Veena Tucker, Shweta Bansal, Elipsha Sharma and Parul Johar. DU Journal of Undergraduate Research and Innovation. Volume 2, Issue 2 pp 70-79, 2016| http://journals.du.ac.in/ugresearch/DU%20e-Journal%20-%20UG%20Research_files/DUJ4%20Vol%202%20Issue1/J%201.8.pdf