How To Measure Phosphate In Water

Phosphorus is a common element in water. Generally, the content of phosphorus in natural water is relatively small. It mainly exists in various industrial and domestic wastewater in the form of phosphate. While the phosphorus in this water comes from the chemical fertilizer, smelting, detergent, and so on.

Although phosphorus is necessary for plants’ growth, too much phosphorus in the water will cause water eutrophication and the soaring of algae and other plants, thus poor the water quality. Therefore, determining phosphorus content in water can also further evaluate the pollution of water quality. There is soluble and insoluble phosphate in water.

Related:

How To Measure Phosphorus In Water

How To Measure NPK In Soil

Measurement methods of phosphate

There are three common phosphate measurement methods: titration, spectrophotometry, and rapid test package.

1) Spectrophotometry

The spectrophotometer uses a light source that can produce multiple wavelengths and generates a light source with a specific wavelength through a series of spectroscopic devices. After the light passes through the tested sample, part of the light is absorbed; thus, we can calculate the absorbance value of the sample to convert it into the phosphorus concentration of the sample.

There are molybdenum antimony anti spectrophotometry and malachite green spectrophotometry. So what is the difference between them?

1) Molybdenum antimony resistance Spectrophotometry

Molybdenum antimony anti spectrophotometry is widely used to determine phosphate in water. Compared with other methods, it has a significant advantage in a wide linear range of 0.01 ~ 0.6mg/l, which is twice that of malachite green spectrophotometry. 

There is a little advantage in the standard recovery rate at 98.5% ~ 105.0%, while that of malachite green spectrophotometry is 98.8% ~ 101.67%.

Pros：

Simple operation and convenience.

Cons：

• It has a limited time of observation and recording due to the short stability time of the complex.
• Very strict operating conditions and the measurement process.
• High detection limit and low sensitivity.
• There are some defects in the determination of trace phosphorus in water samples.

2) Malachite green spectrophotometry

At present, malachite green spectrophotometry is second only to molybdenum antimony anti-spectrophotometry. 

Since its linear range of determination is only about half that of molybdenum antimony anti spectrophotometry,there is a small gap with molybdenum antimony anti spectrophotometry in the standard recovery rate.

Compared with other methods, it has the advantages:

high sensitivity and accuracy;

much simpler operation than molybdenum antimony anti spectrophotometry;

and the stability time of ionic complex is much longer than molybdenum antimony anti spectrophotometry;

So, a beginner can also determine it.

Comparison:

1) The determination range of molybdenum antimony anti spectrophotometry is 0.01 ~ 0.6mg/l, and the determination range of malachite green spectrophotometry is 0 ~ 0.3mg/l. It is suitable for the determination of trace phosphorus in water samples. The accuracy is similar within the linear range of determination, and there is no significant difference in the determination results.

2) Molybdenum antimony anti spectrophotometry and malachite green spectrophotometry can directly determine the phosphate content in drinking water, surface water, groundwater, and industrial wastewater. Both determination results are affected by the color and turbidity of water samples. The two methods have high sensitivity, simple operation, and are suitable for determining phosphorus in the water.

3) The stability time of the malachite green spectrophotometric complex is longer (up to at least 8h) than that of the molybdenum antimony anti spectrophotometric complex.

4) The recovery of molybdenum antimony anti spectrophotometry is 98.5% ~ 105%, 

The average recovery is 99.63%; 

The recovery of malachite green spectrophotometry is 98.8% ~ 101.67%; The average recovery is 99.98%, a little higher than that of molybdenum antimony anti spectrophotometry.

2) Rapid test package method

The rapid test package method also uses the molybdenum blue method, which does not need instruments. It only requires a small tube and reagent to test the phosphorus content quickly. In other words, it is equipped with a color card to compare the concentration range with the naked eye.

Pros:

• Cheap and fast
• lightweight and convenient.

Cons:

• it is not accurate enough
• Only estimate values.

3) Titration

Th (IV) salt and phosphate can quantitatively produce precipitation, while precipitation and EDTA can produce a stable complex with Th (IV) at pH 2 ~ 3.

Pros：

It can accurately test the phosphorus content.

Cons:

• Cumbersome operation.
• time-consuming

How to measure phosphate in water?

The principle：

Under acidic conditions, orthophosphate reacts with Ammonium Molybdenum antimony and potassium antimony tartrate to produce phosphomolybdenum heteropoly acid, which is immediately reduced by reducing agent ascorbic acid to become a blue complex (phosphomolybdenum blue). We can measure the absorbance at the wavelength of 700nm and quantified by the standard curve method.

reagent

1) Sulfuric acid, 1 + 1 sulfuric acid solution.

2) 10% ascorbic acid solution

3) Molybdate solution

4) Orthophosphate stock solution (50.0 μ G / ml in P)

5) Orthophosphate solution (2.0) μ G / ml in P)

Steps

1. Drawing of the standard curve

1) Draw 0, 1.00, 2.00, 6.00, 10.00, and 20.00ml orthophosphate solution respectively into 50ml plugged colorimetric tube; dilute with water to the mark, add 1ml 10% ascorbic acid solution to the colorimetric tube and mix well for the 30s. Add 2ml molybdate solution, fully mix and place for 15min.

2) At the wavelength of 700nm, measure the absorbance with a cuvette with an optical path of 10mm and distilled water as a reference. After calibration, we can measure the absorbance (UG ~ m) by subtracting the measured absorbance of the blank absorbance tube.

Determination of water samples

Draw three appropriate water samples (5ml) respectively, add them into a 50ml color comparison tube with a stopper, dilute them with water to the marked line, and the measurement according to the drawing calibration curve. At the same time, do the blank experiment, subtract the absorbance of the blank tube from the measured absorbance to get the corrected absorbance, and calculate the concentration of orthophosphate in the sample with the help of the calibration curve.

Calculation:(P,mg/L=m/v)

m – orthophosphate amount (UG) checked from the standard curve.

V – water sample volume (ML).