1. Testing procedures
Take about 250 mL of circulating water and filter it by medium-speed filter paper, as shown in table 6 , take adequate amount of the filtrate and put it in 50 mL volumetric flask,  add 2.0 mL sulphuric acid solution (1 + 3), use water to adjust the solution volume in the volumetric flask to about 25 mL, and shake the flask well, then put it in the boiling water bath for 15 min, take it out for  cooling by flowing water to room temperature. Add 1 drop of phenolphthalein solution (1% of ethanol solution) to the volumetric flask by dropper, then drop  sodium hydroxide solution (120 g/L) to the solution of micro red, then add sulfuric acid solution (1 + 35) to red that can just disappear. Add 2.0 mL ammonium molybdate solution (26 g/L), 3.0 mL ascorbic acid solution (20 g/L), dilute it with water to the scale, shake it well, place it  at room temperature for 10 min.At 710nm of spectrophotometer, use  10 mm cuvette to test the  absorbancy by means of the blank zero adjustment  without the test solution.
Water Sampling Volume with Different Phosphorus Content.                    

2. The calculation is as follows:
Total  content (x)of inorganic phosphate (PO43 -)  in the samples expressed in mg/L is calculated according to the following formula:
                     m
        x=                       mg/L
                      V
Where
m - PO43 amount expressed in ug obtained from the working curve,
V- volume of the testing solution shifted, mL
C: Permissible difference
The difference between the two parallel testing results shall comply with the provisions of the table 1.
Take the arithmetic mean as the testing results.
Table 1:The difference between the two parallel testing results shall comply with the provisions of the following table 1.