A range of qualitative and quantitative tests have been developed to detect phosphate ions (PO₄^3-) in solution. Such tests find use in industrial processes, scientific research, and environmental water monitoring.

Quantitative method

A quantitative method to determine the amount of

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

present in samples, such as

boiler feedwater Boiler feedwater is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the m ...

, is as follows. A measured amount of boiler water is poured into a mixing tube and

ammonium heptamolybdate Ammonium heptamolybdate is the inorganic compound whose chemical formula is (NH4)6Mo7O24, normally encountered as the tetrahydrate. A dihydrate is also known. It is a colorless solid, often referred to as ammonium paramolybdate or simply as ammoni ...

reagent is added. The tube is then stoppered and vigorously shaken. The next step is to add dilute

stannous chloride Tin(II) chloride, also known as stannous chloride, is a white crystalline solid with the formula . It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent (in acid ...

reagent, which has been freshly prepared from concentrated stannous chloride reagent and distilled water, to the mixture in the tube. This will produce a blue colour (due to the formation of

molybdenum blue Molybdenum blue is a term applied to: *reduced heteropolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI), and a hetero atom such as phosphorus or silicon *reduced isopolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI) ...

) and the depth of the blue colour indicates the amount of phosphate in the boiler water. The

absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...

of the blue solution can be measured with a colorimeter and the concentration of phosphate in the original solution can be calculated. Alternatively, a direct (but approximate) reading of phosphate concentration can be obtained by using a

Lovibond comparator The Lovibond comparator is an example of a colorimeter made in Britain by The Tintometer Ltd. It was invented in the 19th century by Joseph Williams Lovibond anupdated versionsare still available. Description The device is used to determine t ...

. This method for phosphate determination is known as Denigés' method. {{Cite web , url=http://homepages.ius.edu/DSPURLOC/c121/week5.htm , title=Qualitative Analysis of Some Common Ions Class Notes , access-date=2013-06-18 , archive-url=https://web.archive.org/web/20130626012027/http://homepages.ius.edu/DSPURLOC/c121/week5.htm , archive-date=2013-06-26 , url-status=dead

Qualitative method

A simple qualitative method to determine the presence of

ions in a sample is as follows. A small amount of the sample is acidified with concentrated

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

, to which a little

ammonium molybdate Ammonium molybdate can refer to: *Ammonium orthomolybdate, (NH4)2MoO4 * Ammonium heptamolybdate, (NH4)6Mo7O24, usually encountered as the tetrahydrate *Ammonium phosphomolybdate Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic ac ...

is added. The presence of phosphate ions is indicated by the formation of a bright yellow precipitate layer of

ammonium phosphomolybdate Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula (NH4)3PMo12O40. The salt contains the phosphomolybdate anion, a well known heteropolymetalate of the Keggin structural class. Synthesis Ammonium ph ...

. The appearance of the precipitate can be facilitated by gentle heating. This test is also used to detect

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, bu ...

, a yellow precipitate being formed.

References

Chemical tests

Quantitative method

Qualitative method

See also

References