Polyphosphoric acids, ammonium salts

Endpoint summary

Description of key information

No specific data on the ADME of ammonium polyphosphate is available.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

10

Absorption rate - inhalation (%):

100

Additional information

In the absence of specific data regarding the ADME of ammonium polyphosphate its physicochemical properties and relevant toxicity data (where available) were assessed for insights into likely ADME characteristics. Ammonium polyphosphate has a MW >500 and a water solubility of >10000 mg/L; Therefore absorption via the dermal route is unlikely. However due to the high water solubility the substance will readily dissolve in biological fluids and therefore inhalation and oral routes is anticipated. A default value of 100% absorption is considered appropriate for oral and inhalation exposure routes. Based on MW, a default of 10% for dermal absorption is recommended. There are no substance-specific data regarding distribution and metabolism but the body is likely to handle any absorbed material in the same way that it normally deals with ammonium and phosphate compounds. Following ingestion, any absorbed ammonium polyphosphate will be absorbed as ammonium and phosphate ions. Ammonium ions absorbed from the gastrointestinal tract travels via the hepatic portal vein directly to the liver, where in healthy individuals, most of it is converted to urea and glutamine.

Phosphate may be present as triphosphate, diphosphate, orthophosphate and small quantities of polyphosphate. The triphosphoric acid can undergo ionisation with loss of H+ from each of the 5 –OH groups (one on the inner P, two on each terminal P) and therefore can occur in the -1, -2 -3, -4 or -5 state. The degree of ionisation is dependent upon the associated cations and the ambient pH (if in solution). The diphosphate/pyrophosphate ion is the simplest form of a condensed phosphate group. A condensed phosphate anion has one or several P-O-P bonds. As the group contains only two phosphate groups, both of the phosphorus ions are classified as “terminal phosphorus”. The diphosphate can undergo ionisation with loss of H+ from each of the two –OH groups on each P and therefore can occur in the -1, -2 -3 or -4 state. The degree of ionisation is dependent upon the associated cations and the ambient pH (if in solution).

The polymer chains do undergo hydrolysis (increased in acidic conditions) to form shorter polymers and ultimately orthophosphoric acid (P2O5). Biotic hydrolysis is fast through ubiquitous alkaline phosphatase activity in micro- and macroorganisms.

It is anticipated that phosphate will be primarily absorbed in the orthophosphate form.

 

Discussion

 

ABSORPTION

Oral

No data were found specifically regarding the oral absorption of ammonium polyphosphate. Given the acute oral toxicity classification, it is assumed that ammonium polyphosphate is systemically available. As such, oral absorption is anticipated to be 100%.

 

Dermal

According to ECHA guidance, a molecule with a MW above 500 may be too large for dermal absorption (ECHA, 2012). For a compound to penetrate the stratum corneum, it must be sufficiently water soluble i.e. above 1 mg/L (ECHA, 2012). Although, the substance is soluble it is likely to be absorbed in the highly ionised form which reduces drastically the potential to penetrate the lipid rich environment of the striatum corneum. Therefore dermal uptake of ammonium polyphosphate will probably be minimal. As a result of binding to skin components, the uptake of ammonium ions will be slowed.

Based on these factors, a value of 10% dermal absorption is proposed.

 

Inhalation

A default value of 100% inhalation absorption is usually applied. In general, very hydrophilic substances might be absorbed through aqueous pores (MW <200) or be retained in the mucous and transported out of the respiratory tract, and subsequently swallowed (ECHA, 2012).As ammonium polyphosphate consists of a mixture of ammonium phosphates with different molecular weights absorption via the respiratory tract is likely.

In a recent study report, the mean mass median aerodynamic diameter of ammonium polyphosphate was 2.48 µm (Griffiths, 2015). Thus, the particles are considered to be respirable (ECHA, 2012).

 

DISTRIBUTION and METABOLISM

No data are available regarding the distribution and metabolism for ammonium polyphosphate. For the larger molecules, tissue distribution is expected to be limited. However, since the substance is highly soluble and consists of low molecular weight molecules as well as high molecular weight molecules the substance may be able to diffuse through aqueous channels and pores (ECHA, 2012). Presumably, any absorbed material will be handled in the same way as other absorbed ammonium and phosphate compounds. Following ingestion, any absorbed ammonium polyphosphate will be absorbed as ammonium and phosphate ions. Ammonium ions absorbed from the gastrointestinal tract travels via the hepatic portal vein directly to the liver, where in healthy individuals, most of it is converted to urea and glutamine.

 

 

EXCRETION

It is anticipated that excretion will mainly occur via the urine. Due to the presence of the ammonium ion, excretion via the bile is considered to be low.

 

References

 

ECHA (2012). Guidance on information requirements and chemical safety assessment. Chapter R.7c: Endpoint specific guidance. November 2012 (version 1.1).