EXAMPLE 1 50 mg. of dithizone and 1 g. of thiourea are dissolved in 50 ml. of alcohol and mixed with a solution of 8 g. of sodium carbonate and 2 g. of gelatin in 50 ml. of water. A filter paper (Schleicher & Schuell No. 1450 CV, 2316, or 1451) is impregnated with the resultant solution and dried at 80 C. Test papers are cut into strips of a width of 6 mm. and sealed on a synthetic resin film tape 75 mm. wide. After cutting the film tape at right angles to the tape direction into 6 mm. wide strips, test strips are obtained having a length of 75 mm. and a Width of 6 mm. carrying a test paper section of a size of 6 x 6 mm.
The test strip exhibits an orange'red coloring after immersion into a solution containing mg./l. of zinc ions, corresponding to the appropriate value on the associated standardized color scale.