200 Mr. Croft on a neta Oxalate of Chromium and Potash. 



drous oxalate of potash, minus six atoms of water. The 

 agreement of the experiment with the calculation speaks for 

 the correctness of the above formula, in which one might, 

 perhaps, otherwise not place so much confidence. 



The constitution of this salt led me to consider the theory 

 of its formation, and also that of the black salt, more particu- 

 larly as in employing the known formula} for making the black 

 salt I always obtained it mixed with other bodies. 



In forming the red salt from bichromate of potassa, 7 atoms 

 of oxalic acid are required. K O, 2 Cr 3 and 7 C 2 O s = 

 K O, C 2 3 + Cr 2 3 , 3 C 2 O s and 3 C 2 3 + 3 O, or 6 C 2 . 

 On mixing the two substances in this proportion I obtained 

 perfectly pure red salt. It is evident that seven atoms of ox- 

 alic acid, either free or in combination with potash, must be 

 used in making the black salt. None of the numbers in the 

 formulae given for preparing the black salt agree with this. 



Dr. Gregory gives 190 parts bichromate of potash, 157'5 

 parts crystallized oxalic acid, and 517 parts binoxalate of pot- 

 ash ; that is, one atom of the bichromate, two atoms oxalic 

 acid, and three of binoxalate of potash; on trying these num- 

 bers I obtained a mixture of black salt with oxalate and chro- 

 mate of potash. 



Prof. Graham proposes one part of bichromate, two of bi- 

 noxalate, and two of crystallized oxalic acid. In these pro- 

 portions a large quantity of chromate of potassa remains un- 

 decomposed, which requires, if 19 grains bichromate, 23 grains 

 binoxalate, and 16 grains crystallized oxalic acid be taken, 

 exactly 36 grains of crystallized oxalic acid to effect its perfect 

 decomposition, and making the whole quantity of oxalic acid 

 52 grains. 



According to the formula which I would propose, there are 

 required 19 grains bichromate of potash 



23 ... oxalate of potash 

 55 ... crystallized oxalic acid. 



If the salts be taken in these proportions, nothing but black 

 salt is obtained ; it is however better to evaporate the whole 

 to dryness and then re-dissolve. 



I have not been able to obtain an intermediate salt, namely, 

 2 K O, C 2 3 4- Cr 2 3 , 3 C 2 3 . This, if it exists, ought to 

 be produced from two atoms chromate of potash, and eight 

 atoms oxalic acid : I obtained, however, oxalate of potash and 

 red salt. 



A similar salt may probably exist with oxide of iron, but 

 it does not crystallize. On dissolving sesquioxide of iron in 

 quadroxalate of potash a solution is obtained, which dries to 

 a brown gummy mass without traces of crystallization. 



