Phosphorus

Hydrolysis

Description of key information

Hydrolysis results in mixtures of phosphorus oxyacids and phosphine and / or traces of hydrogen. The hydrolysis starts with an immediate dissolving of less than 5% soluble impurities and a rate of continuous dissolution which is usually in the beginning about 3mg/g*d. The corresponding half-life of all available long term measurements varies from 0.5 to 0.8years, Exceptional long half-life of 4.4 or 8 years are caused by an extremely particle size or a reduction of the pH value during the dissolving at high concentrations.
The ratio between the phosphine formation and the dissolved oxyacid is 1:3. 
This corresponds in the worst case of a short half life of 0.5 years with a phosphine formation rate of about 0,1mg phosphine /day* g phosphorus. 
Since these disproportionating and hydrolysis reactions proceed at a very slow rate, even such a critical dissolving products like traces of phosphine (which will also volatilise from water) will be finally converted to phosphoric acids in oxygen containing environments.
Fast hydrolysis i.e. with very fine powder, high temperature or the presence of leach might result in a phosphine/hydrogen formation leading to a concentration above the lower explosive limit (LEL phosphine PH3 : 1.6Vol-%, LEL hydrogen H2: 4Vol-%). Measurements of the dryer exhaust gas during at high temperatures indicate of the hydrogen / phosphine ratio of ca. 3 to 1. 
As result of the performed study phosphine formation rates, dependant on hydrolysis time, were observed in a Hydrolysis study according OECD 111. As a theory, this behaviour might be explained by the reduction of the reactive surface by the produced reaction by-products, which are known to be surface active. 
Summarising, under all tested conditions, formation rates below 1 µmol phosphine / h could be expected after approx. 140 h of hydrolysis.
	

Additional information

Red phosphorus is thermodynamically unstable in the presence of water and oxygen and will very slowly undergo disproportionation and oxidation reactions. For this reason the surface of red phosphorus is usually permanently coated by a layer of such oxyacids, with a thickness depending of the production and storage condition of the red phosphorus.

Suspending red phosphorus in water leads to an immediate dissolving of these oxyacids and a slow continuous dissolving by further disproportionation of the remaining pure red phosphorus into phosphorus components and traces of hydrogen according to the qualitative description below:

Px + H₂O + O₂---> H₃PO₄+ H₃PO₃+ H₃PO₂+ H₂+ PH₃

Typical values of the hydrolysis rates measured in the pseudo-linear region in the beginning of the dissolving process vary between 0.1 to 1% per day and fit well with the data of the ecotoxicoligical studies.

Long term measurements of the half life leads to 0.48years for an unstabilized sample from the production of the applicant at 37°C and a range between 0.5 to 8years for commercial grades at 25°C.

The different results are based on the strong sensitivity of the hydrolysis reaction from experimental conditions, like the pH value and the temperature of the water, the ion content, the availability of oxygen and the accessible surface area of the red phosphorous, depending on the particle size as well as on the particle size distribution and the stirring intensity and the surface coating by stabilizing agents for commercial grades (see below).

The exceptional long half-lifeof 4.4 (Ronald et al 1985) respectively or 8 years (Walz et al 2000) are caused in one case by the particle size respectively a pH value effect. Especially at high nominal concentration the dissolved oxyacids reduces the pH value and retards the further dissolving.

In addition almost all of these measurements rely on the investigation of the dissolved amount of phosphorus in water, neglecting the losing of phosphine into the gaseous phase.

Only the long term measurements of an unstabilized sample (Gedig 2010) considered the phosphine loss into the gaseous state and estimated it to be about 32% of dissolved components in aqueous solution.

So in dependence of the experimental condition, especially from the gas flow and oxygen accessibility, all other hydrolysis rates might be up to 32% larger than the given values.

 

Commercial grades of red phosphorus might show an additional immediate dissolving of soluble impurities from the production process and might also show longer half life caused by stabilizers coating the particle surface.

Hydrolysis in the context of this study means reaction of water at different pH values and temperatures forming Phosphine PH₃and other Phosphor oxidation products. The formation of Phosphine as a function of pH and temperature was determined following a modified to OECD Guideline No. 111 for the test item Red Phosphorus, untreated (batch number: Fra 07/23) from 2010-08-27 to 2010-10-06 atDr.U.Noack-Laboratorien, Sarstedt, Germany.

The test was conducted with a test item concentration of 5 g Red Phosphorus, untreatedin 150 mL buffer solution of pH 4, 7 and 9 at temperatures of 20, 30 and 50 °C, respectively.

For each temperature, after three days of incubation (48 hours without and 19-20 hours with nitrogen flow) samples were taken and analysed immediately, separated by at least four hours (two samples at the first two days and one sample on the following two days each). Analyses of the test item were performed via “Dräger-Röhrchen”, phosphine selective detector tubes.

Theoretically a zero order reaction kinetic was expected for the PH₃formationof Red Phosphorus, untreated. The formation rate of the phosphine should be dependent on the reactive surface of the test item, which was assumed to be constant. With a zero order reaction kinetic a constant phosphine formation rate was expected.

As result of the performed study, phosphine formation rates, dependant on the hydrolysis time, were observed (see tables below). Theoretically, this behaviour might be explained by the reduction of the reactive surface by the produced reaction by-products, which are known to be surface active.

Summarising, under all tested conditions, formation rates normally below 1 µmol phosphine / h could be expected after approx.140 h of hydrolysis.