In acidic medium, active phosphate reacts with ammonium molybdate to form phosphomolybdenum yellow, which is reduced to phosphomolybdenum blue by ascorbic acid. The absorbance is determined at the wavelength of 882nm.

tool

Spectrophotometer.

reagent

Sulfuric acid (6 mol/L) 300 ml of sulfuric acid was slowly added to 600 ml of water while stirring.

Ammonium molybdate solution (140g/L) Dissolve 28g of ammonium molybdate [((NH4) 6mo7o24 4h2o]] in 200mL of water. When the solution becomes turbid, it should be reconstituted.

Antimony potassium tartrate solution (30g/L) 6g of antimony potassium tartrate was dissolved in 200mL of water and stored in a polyethylene bottle. When the solution becomes turbid, it should be reconstituted.

Add 45mL ammonium molybdate solution into 200mLH2SO4, add 5mL antimony potassium tartrate solution, and mix well. Store in a brown glass bottle. When the solution becomes turbid, it should be reconstituted.

Ascorbic acid solution (100g/L) Weigh 20g of ascorbic acid, dissolve it in 200mL of water, and put it in a brown reagent bottle or polyethylene bottle. Stored in the dark at 4℃, it can be stable for 65438 0 months.

Phosphate standard stock solution ρ(P)=0.300mg/mL, and weigh 1.3 18g potassium dihydrogen phosphate (KH2PO4, excellent, pure,115438+0). It can be kept in a cool place for half a year.

Phosphate standard solution ρ(P)=3.00μg/mL Measure 1.00mL of phosphate standard reserve solution into a 100mL volumetric flask, add water to the graduation line, mix well, and add two drops of chloroform (CHCl3). This solution is effective within a week.

calibration curve

Measure 0 ml, 0.50 ml, 1.00 ml, 2.00 ml, 3.00 ml and 4.00 ml of phosphate standard solution in a 50 ml measuring cylinder with a stopper, add water to the 50 ml graduation line and mix well. The concentrations were 0 mg/L, 0.030 mg/L, 0.060 mg/L, 0. 120 mg/L, 0. 180 mg/L and 0.240 mg/L in turn.

Add 1.0mL mixed solution and 1.0mL ascorbic acid solution respectively, and mix well. After 5 minutes of color development, it was injected into a 5cm cuvette, and the absorbance Ai was measured at the wavelength of 882nm with distilled water as reference. Where the zero concentration is the standard blank absorbance A0.

Draw a calibration curve with absorbance (Ai-A0) as the ordinate and corresponding phosphate concentration (mg/L) as the abscissa.

Analytical method

Measure 50 ml of water sample filtered by 0.45μm microporous membrane in a graduated cylinder with plug, and measure the absorbance Aw according to the above steps of drawing calibration curve.

At the same time, measure 50 ml of water, and measure and analyze the blank absorbance Ab according to the same steps.

According to the value of (Aw-Ab), the phosphate concentration (mg/L) of water sample was found on the calibration curve.

Matters needing attention

1) water samples should be filtered immediately after collection and determined immediately. If it cannot be determined immediately, it should be kept in the refrigerator, but it should also be determined within 48 hours.

2) The microporous membrane for filtering water samples should be soaked in 0.5mol/LHCl and washed with water before use.

3) When the sulfide content is higher than 2 mg/L, it will interfere with the determination. At this time, the water sample was acidified with H2SO4, and nitrogen was introduced for 65438±05min to drive away H2S, which could eliminate the interference.

4) The color of phosphomolybdenum blue is stable within 4h.