The key points are these:

1. Ultraviolet light converts simple phosphates into polyphosphates.

2. There was plenty of UV light available at the surface of the early Ocean.

3. Polyphosphate causes the polymerization of amino acids into peptides, and ribonucleic acid monomers onto polymers.

4. Phosphate is reacts with soluble calcium to form insoluble calcium phosphate which would render the phosphate useless for purposes of synthesis.

5. There are numerous simple organic compounds that bind to calcium and prevent the formation of calcium phosphate. Two of them are citric acid and lactose. There are many more. The presence of these materials in the Ocean would have kept phosphate free and available for polymerization by UV.

Here are some references:

http://www.nature.com/nature/journal/v352/n6335/abs/352516a0.html

http://en.wikipedia.org/wiki/Origin_of_life#Polyphosphates
Polyphosphates

The problem with most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates.[69][70] Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4−3. Several mechanisms for such polymerization have been suggested. Polyphosphates cause polymerization of amino acids into peptides. They are also logical precursors in the synthesis of such key biochemical compounds as ATP. A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early earth.[71

http://www.asa3.org/ASA/Education/origins/cheme-bw.htm
Such phase-separated vesicles would not yet be living entities, but they would provide good cradles within which life could emerge. With a free-energy source producing a proton gradient, for example, polyphosphate could be made (Morowitz 1992; Deamer and Harang 1990). The formation of polyphosphate is important because such compounds have a unique mix of thermodynamic instability and kinetic stability (Westheimer 1987) and will participate in phosphorylation of amino acids and nucleotides that were accumulated by the cell also through the use of the electrochemical potential of the proton gradient. The phosphorylation of monomers in the protocell would help drive thermodynamically their polymerization to polypeptides and polynucleotides. At some point polyphosphate was replaced by ATP.

David F Mayer