In geological systems, the element P is released as“phosphate” during rock weathering. It formsthe singly, doubly and triply protonated weak acid,phosphoric acid, in solution.
The chemical form of dissolved phosphate in waters isalways partitioned between various “species” ofphosphoric acid:H3PO4,H2PO4- ,HPO42-, PO43-,according to the equilibrium constants below:
H3PO4 H2PO4-+ H+K= 10^-2.1
H2PO4- HPO42-+ H+K= 10^-7.4
HPO42- PO43-+ H+ K=10^-12.4
(1.1) In a local alpine lake, geochemical measurementsshow that the total concentration ofphosphate species is 10-4 moles/liter(also known as 100 micromolar). Determine the pH atwhich[H2PO4-]and[HPO42-]are each approximately half of the total P insolution. Show how you knowthis using an equation.
(1.2) Now make a full “speciationdiagram” (similar to the one forH2S orH2CO3 done inclass) for H3PO4.Use this diagram to show the dominant form of phosphate as afunction of pH (i.e. how it changes fromH3PO4 toH2PO4- toHPO42- etc. as pH increases. Your y-axiswill be log [P]T and your x-axis is pH,and remember that [P]T was given as10-4 moles/liter in part(1.1). Show the pH and concentration ofthe two most abundant phosphate species at every“cross-over” point (where a“cross-over” point on these diagrams is wherewe see a shift from one dominant form of phosphate to another). Youcan make this diagram by hand; it is also ok to use exceletc.
(1.3) The lake pH is measured to be8. Estimate how muchPO43-should be dissolved in the water?(Hint: you’ll need to do a calculation, but first figureout what the most abundant form of phosphorous is at pH 8 and makea good guess about its concentration to use). Your calculationshould agree with your diagram.
(1.4) The concentration of Ca2+in the lake is 10-5M (10micromoles/liter). Would you expect the mineral hydroxyapatite(Ca5(PO4)3OH)to be supersaturated and precipitate from the lake, or ishydroxyapatite undersaturated? You need to know that theKsp of the hydroxyapatite(Ca5(PO4)3OH)is 10-60 and remember the pH of the lakeis 8 (so you can calculate the [OH-]concentration remembering that the K for water is10-14).

Question

In geological systems, the element P is released as“phosphate” during rock weathering. It formsthe singly, doubly and triply protonated weak acid,phosphoric acid, in solution.

The chemical form of dissolved phosphate in waters isalways partitioned between various “species” ofphosphoric acid:H3PO4,H2PO4- ,HPO42-, PO43-,according to the equilibrium constants below:

H3PO4<--> H2PO4-+ H+K= 10^-2.1

H2PO4-<--> HPO42-+ H+K= 10^-7.4

HPO42-<--> PO43-+ H+ K=10^-12.4

(1.1) In a local alpine lake, geochemical measurementsshow that the total concentration ofphosphate species is 10-4 moles/liter(also known as 100 micromolar). Determine the pH atwhich[H2PO4-]and[HPO42-]are each approximately half of the total P insolution. Show how you knowthis using an equation.

(1.2) Now make a full “speciationdiagram” (similar to the one forH2S orH2CO3 done inclass) for H3PO4.Use this diagram to show the dominant form of phosphate as afunction of pH (i.e. how it changes fromH3PO4 toH2PO4- toHPO42- etc. as pH increases. Your y-axiswill be log [P]T and your x-axis is pH,and remember that [P]T was given as10-4 moles/liter in part(1.1). Show the pH and concentration ofthe two most abundant phosphate species at every“cross-over” point (where a“cross-over” point on these diagrams is wherewe see a shift from one dominant form of phosphate to another). Youcan make this diagram by hand; it is also ok to use exceletc.

(1.3) The lake pH is measured to be8. Estimate how muchPO43-should be dissolved in the water?(Hint: you’ll need to do a calculation, but first figureout what the most abundant form of phosphorous is at pH 8 and makea good guess about its concentration to use). Your calculationshould agree with your diagram.

(1.4) The concentration of Ca2+in the lake is 10-5M (10micromoles/liter). Would you expect the mineral hydroxyapatite(Ca5(PO4)3OH)to be supersaturated and precipitate from the lake, or ishydroxyapatite undersaturated? You need to know that theKsp of the hydroxyapatite(Ca5(PO4)3OH)is 10-60 and remember the pH of the lakeis 8 (so you can calculate the [OH-]concentration remembering that the K for water is10-14).

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