Re Density of CrO4 and CrO7

I think you mean a mixture of yellow chromate(VI), CrO4--, and orange-red dichromate(VI), Cr2O7-- anions, usually as the sodium or potassium salts. Chromate for analytical purposes and for organic synthesis is usually sold as the sodium salt, while dichromate is usually sold as the potassium salt, because these have the least molecules of water of crystallization. (Other commercially available chromates are the slightly soluble zinc and lead(II) chromates, sold as intensely red paint pigments which also inhibit rusting of mild steel for which they are used as primers, and the soluble green copper(II) chromate which (usually combined with arsenate) is used as a softwood timber preservative.

When I was doing chemistry at university over 40 years ago, one of the laboratory experiments was precisely on this problem, namely the use of UV-visible spectrometry to determine the (relative) concentrations of chromate and dichromate in a range of aqueous mixtures. The method was called "Job's method of continuous variations", after someone of that name who developed it for his PhD thesis. It could probably be also applied to aqueous mixtures of green manganate(VI) and purple permanganate(VII) anions, of blue perchromate and yellow chromate anions, of green ferrous(II) and yellow ferric(III) cations, and of hexa-coordinate complexes of cations such as cobalt(II) and (III) in which different isomers (resulting from different positions of the ligands) have different colors (particularly deep-blue and magenta in the case of cobalt). Such mixtures often result from chemical reactions, especially where there are equilibria dependent on factors such as
 concentration, pH, and temperature. 


Here are some PPT and PDF files on the method:

http://www.uaa.alaska.edu/chemistry/labs/upload/Lab3_106L.pptx
http://web.centre.edu/shiba/che332/Lab/Jobs_method.pdf‎
http://cscdic.pbworks.com/f/LakeheadU+4.pdf‎
http://www.sci..nu.ac.th/chemistry/elearning/experiment/Experiment%202.pdf‎
http://www.scribd.com/doc/36702927/01-Job-s-Method-of-Continuous-Variation
http://www.ncbi.nlm.nih.gov/pubmed/1200365
http://cartwright.chem.ox.ac.uk/tlab/experiments/202.pdf‎
http://facta.junis.ni.ac.rs/phat/pcat2010/pcat2010-02.pdf

Search this string on Google or Yahoo: 
"job's method" "continuous variation"  filetype:PDF
for more results.


Here is a 43-minute video clip on it:

http://www.youtube.com/watch?v=Wn6PS-oTSyM‎



John W.





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>From: "[email protected]" <[email protected]>
>To: [email protected] 
>Sent: Friday, 13 December 2013 10:31 PM
>Subject: [The Chemistry Cluster] Density of CrO4 and CrO7
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>  How can we calculate Density of CrO4 and CrO7 in a Solution of both ions with UV-Vis method , can anyone help ? TNX
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