Preparation of Normal Ammonium Tungstate

Preparation of Normal Ammonium Tungstate. Weigh 0.5 g of tungstic acid in a test tube, put the latter into a cooling mixture (solid carbon dioxide with acetone), and pour in 1-2 ml of liquid ammonia. Stir the contents of the test tube with a cooled glass rod and let the ammonia evaporate. Examine the precipitated crystals under a microscope. What is the composition of the product  

Preparation of Sodium Paratungstate, a. Pour 20 ml of a 50% sodium hydroxide solution into a 100-ml porcelain beaker provided with a mechanical mixer, put the beaker on a water bath, and introduce the calculated amount of a freshly precipitated wet suspension of tungstic acid in small portions while vigorously stirring. At the end of the reaction, the pH of the solution should be 6-6.5. Filter the solution, evaporate it a little, and let it stand for 24 hours. Separate the formed crystals by filtration and dry them in the air. 

Preparation of Sodium Metatungstate. Place a porous clay cylinder playing the role of a diaphragm and anode vessel into a thick-walled glass beaker (Fig. 129). Pour 300 ml of a 0.1 N sodium hydroxide solution into the glass beaker, and a solution containing 

5 g of sodium tungstate in 100 ml of water into the inner cylinder. Lower a glass mixer into the anode compartment. A platinum plate is the anode. Use a nickel electrode in the form of a band encircling the diaphragm as the cathode. 

Conduct electrolysis until the pH of the solution in the anode compartment becomes equal to 3 take samples every hour. Check the completeness of converting the tungstate into the metatungstate by means of the Scheele reaction. To do this, add 1 ml of concentrated hydrochloric acid dropwise to 5 ml of a sample. The absence 

Write the equations of the reactions occurring in the anode and cathode compartments, and also of the Scheele reaction. Transfer the anolyte and catholyte into different vessels. After the anolyte cools, check the pH of the solution again. 

Evaporate the anol3d e in a water bath up to one-fourth of its initial volume. Put the solution for crystallization into a desiccator over an alkali. Perform crystallization as completely as possible, but so that the mother liquor remains. Separate the crystals from the mother liquor by decantation, dry them in the air with filter paper, and weigh them. Calculate the yield in per cent. 

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