Why is potassium phosphate KH2PO4 in this reaction?

This is a continuation of this question because the first thing that came in my mind is that why potassium phosphate in this reaction is $\ce{KH2PO4}$ and not $\ce{K3PO4}$?

In the wikipedia article of phosphate ion ($\ce{PO4^3-}$), it is written that:

Aqueous phosphate exists in four forms. In strongly basic conditions, the phosphate ion ($\ce{PO4^3-}$) predominates, whereas in weakly basic conditions, the hydrogen phosphate ion ($\ce{HPO4^2-}$) is prevalent. In weakly acidic conditions, the dihydrogen phosphate ion ($\ce{H2PO4−}$) is most common. In strongly acidic conditions, trihydrogen phosphate ($\ce{H3PO4}$) is the main form.

So, $\ce{FeCl2}$ must somehow be acidic in aqueous solution so as to protonate potassium phosphate $\ce{K3PO4}$ to $\ce{KH2PO4}$. Is this statement true? This question throws some light on acidity of chromium chloride in aqueous solution but is it applicable to iron chloride as well?

In the previous question, the homework question was write the equation of reaction of Iron(II) chloride and potassium phosphate. So, students can assume the reaction to be $\ce{FeCl2 + K3PO4}$ to simply yield iron(III) phosphate or iron(II) phosphate and potassium chloride but why make the reaction complex by assuming potassium phosphate $\ce{KH2PO4}$.

Iron hydrogen phosphate are new in this world and has been recently synthesised and as not as stable as iron phosphates. So, can we just assume that reaction of Iron(II) chloride and potassium phosphate to yield iron (II or III) phosphate and potassium chloride?

• Is there a reason students will assume that the Fe(II) ions are going to be oxidized to Fe(III) by phosphate? – Efram Goldberg Dec 13 '16 at 18:40

I think that’s just another mistake the OP of the other question made. Potassium phosphate is $\ce{K3PO4}$ and any student using any hydrogenphosphate instead would lose marks if I were marking the exam.

If $\ce{KH2PO4}$ is meant to be added, use the correct term: potassium dihydrogenphosphate.

This nomenclature can be found in the current version of IUPAC’s Red Book, sections IR-8.2 and IR-8.4 and the anions in question are explicitly named in table IR-8.1 therein.

• Potassium phosphate does not refer to K<sub>3</sub>PO<sub>4</sub> but refers to 3 different compounds whose actual IUPAC names are tripotassium phosphate, potassium hydrogen phosphate, and potassium dihydrogen phosphate. The rules for IUPAC naming were made to avoid this sort of confusion. The term "potassium phosphate" should never have been used. – Efram Goldberg Dec 14 '16 at 17:22
• Also never in my years of synthetic work have I ever come across tripotassium phosphate in a lab before. – Efram Goldberg Dec 14 '16 at 17:23
• @EframGoldberg No, that is not the case. Similar to sodium carbonate, which is $\ce{Na2CO3}$ and nothing else, potassium phosphate is $\ce{K3PO4}$ and nothing else. There is no need to add a tri-label as only one phosphate is possible (only one common oxidation state is known for potassium). – Jan Dec 14 '16 at 23:04
• Irregardless of your opinion, the common name potassium phosphate has been used to describe all three compounds even requiring a wiki landing page for the term further specifying the three salts by IUPAC name. The three salts are commonly called potassium phosphate monobasic, dibasic or tribasic and are sometimes shortened in use. This and the fact that tripotassium phosphate is hardly encountered, are two important reasons to use IUPAC naming. – Efram Goldberg Dec 15 '16 at 5:25
• The analogy to sodium carbonate is not valid due to the common names being very different ie sodium carbonate and sodium bicarbonate, no one calls them sodium carbonate monobasic or dibasic. If both compounds could be referred to by the common name sodium carbonate, then IUPAC naming should be used as well, but that is not the case. – Efram Goldberg Dec 15 '16 at 5:32

H3PO4 is an acid usually present as 85% solution in water, while KH2PO4 is a solid and therefore easier to handle, weigh etc. K3PO4 is not a commonly used reagent in chemistry.

H3PO4 is a polyprotic acid with pKa values as follows, 2.1, 7.2, 12.7. It will only be mostly PO4-3 at pH>12.7. Using the definition of pKa, a value of 7.2 for pKa2 and assume pH = 7, and your calculations will explain why the ferrous chloride does not need to make the solution acidic to have H2PO4-1 exist as one of the major species.

I am still not sure exactly what you are asking.

• If you’re not sure what is being asked, you should comment to ask for clarification. – Jan Dec 14 '16 at 0:24
• @Jan I answered 3/4 questions that I could make out which were posted above, but the post is not asking a single question. There is a question about why a reactant was chosen, a question about speciation and pH, a question about Lewis acidity of metals, a question about oxidation reduction and general requests for clarifications. Perhaps the post can be edited to present only one clear question? – Efram Goldberg Dec 14 '16 at 17:13
• Just to reiterate: Yes, that should have been a comment on the question (ideally before answering). – Jan Dec 14 '16 at 23:19