1454 Journal of Applied Microscopy 



//. Oxalic Acid added to solution containing Zinc causes the separation of Zitic 

 Oxalate. 



Z11SO4 + H2C2O4 == ZnCoO^ . 2H2O + H2SO4. 



Method. — The reagent is applied to the test drop, as in previous tests, with 

 oxalic acid, i. e., employ a concentrated solution and cause it to flow into the 

 test drop. 



Small double spherulites, pseudo-octahedra, either singly or united in twos, 

 and thin rhombs result. (Fig. 69.) 



{\ ^ The great majority of the crystals separating have 



^ ^^ ^^ their angles rounded. It is tare that a preparation is 

 O ^P '^ B,-^ obtained yielding clear-cut crystals. 

 ^ ,,=po=^ -) Remarks. — The solution to be tested should be neu- 



^ vis^ /^A (-i-^i Qj- Qi^]y slightly acid. 



"^ Crystals of zinc oxalate, when examined with a low 



NX)w--o.oVvyx«^ power, often bear a striking resemblance to the oxalates 

 Fig. 69. of calcium and strontium ; for this reason the alkaline 



earths should be first removed. 



M ignesium interferes. Under certain conditions a double oxalate of zinc 

 and magnesium separates in the form of hexagonal plates. 



Ammonium salts should be removed before adding the oxalic acid. 



In the presence of cadmium this test for zinc is unreliable. 



Recrystallization of the zinc oxalate from a solution of ammonium hydroxide 

 sometimes yields good results, and will aid in reaching a decision as to what 

 element has been precipitated by the oxalic acid. In recrystallizing proceed as 

 follows after adding the oxalic acid : Carefully separate the solution from the 

 precipitate ; add to the latter a large drop of ammonium hydroxide ; warm 

 gently ; cool and examine. Zinc oxalate separates from such solutions in the 

 form of tufts and aggregates of very fine needles. Occasionally masses of 

 radiating, curving needles are seen. In most preparations the crystals separat- 

 ing resemble the tufts formed by calcium sulphate. These crystals are not 

 obtained if cadmium or magnesium is present. 



If lead, copper, cobalt or nickel should be present, it is necessary to first 

 effe' t a separation before testing for zinc with oxalic acid. 



Unless present in very small amount, iron interferes. 



Oxalates of Group I also yield precipitates consisting of normal and double 

 oxalates, but these are of little value as tests for zinc. 



Exercises for Practice. 



See suggestions under Cadmium. 



///. Zinc forms, loith P/imary Sodium Carbonate, a Double Carbonate of Zinc 

 and Sodium of low solubility. 



8 ZnSO^ + 22 HNaC03= [3 (NajCOg) . 8 (ZnCO.j) • 8H2O] -^ 8 Na2S04 



+ 11 CO2 + 11 H2O. 



Method. — Prepare a saturated solution of the reagent. Place a large drop 



