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Physical & Chemical properties

Water solubility

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Description of key information

According to a reliable factual database (Gestis), silver metal is considered as practically insoluble.
Conventional water solubility testing is not appropriate for sparingly soluble metals and metal compounds (ECHA Guidance on information requirements and Chemical Safety Assessment. Chapter R.7a: Endpoint specific guidance. Version 2.1, August 2013). Therefore, Transformation / Dissolution (T/D) testing (OECD Series on testing and assessment No. 29) was conducted for silver massive and several examples of silver powder and is presented in section 5.6 of IUCLID. In addition, as requested in the Silver Substance Evaluation Final Decision, T/D testing has been performed on the nanosilver form in aquatic test media and soil: see section 5.6 of IUCLID.


Key value for chemical safety assessment

Water solubility:
0.03 µg/L

Additional information

Two sets of full T/D test results are available for non-nano silver powder. CIMM (2009) report T/D results for a silver powder with a median particle size of 1.9 µm after seven (loading rates of 1, 10 and 100 mg/L) and 28 days (loading rate of 1 mg/L) in OECD media at both pH 6 and pH 8. ECTX (2010a) reports T/D results for a silver flake with a median particle size of 2.6 µm after seven (loading rates of 1, 10 and 100 mg/L) and 28 days (loading rate of 1 mg/L) in OECD media at pH 6 only. The results of the tests performed at pH 6 on the two different materials are similar, with average dissolved silver concentrations after seven days of 1.25 and 1.8 µg/L (1 mg/L loading rate) and average dissolved silver concentrations after 28 days of 3.6 and 3.7 µg/L (1 mg/L loading rate). In the CIMM study (2009), dissolved silver concentrations were comparable at both pH values at the highest loading rate, which suggests a limit to the solubility of silver. At lower loading rates, silver showed somewhat greater dissolution at pH 8 than at pH 6.

Initial studies to determine the T/D behaviour of silver in massive form (ECTX 2010b), which were performed using an epoxy resin carrier to control the exposed surface area of silver during the test, indicated unusual dissolution/solubility behaviour. Dissolved silver concentrations increased rapidly, but then declined to a steady state concentration. Further experimentation suggested that epoxy resin could act as an adsorbent phase for silver, which would result in the decline in the dissolved silver concentrations observed. It was further hypothesised that during preparation of the test item minute particles of silver may have become embedded in the epoxy vehicle, considerably increasing the exposed surface area during the test. Given these complications further experiments were proposed using a quartz glass vehicle.

To address the limitations of the 2010 study, a further “definitive test” on massive silver was performed using a quartz glass vehicle to control the available surface area to a “surface equivalent” of 3, 9, and 27 mg l-1(ECTX 2013). This study was conducted at pH 8 only (based on the results of silver powder T/D testing) and used fluorinated ethylene propylene vessels to minimise adsorption of silver to the test system during the course of the 28 days test. The test items were also cleaned ultrasonically prior to the start of the exposure to remove residual particles of silver from the surface of the quartz glass tubes remaining from sample preparation. The average blank corrected dissolved silver concentrations in the test medium after 28 days exposure were below the analytical limit of detection of 0.02 µg l-1at loading rates of 3 and 9 mg l-1, and was 0.03 (+/- 0.07) µg l-1at a loading rate of 27 mg l-1.

In addition, as requested in the Silver Substance Evaluation Final Decision, T/D testing has been performed on a nanosilver powder. VITO NV (2017) determined the T/D behaviour of a silver powder with a median particle size of 8 nm (see section 4.5 of IUCLID) in modified Daphnia and algae media (loading of 1153 and 1230 µg Ag/l in Daphnia and algae medium, respectively) over a period of 28 days. Three silver fractions in solution were measured: total silver, conventional dissolved silver (< 0,45 µm; i.e. ionic silver + silver particles with diameter < 450 nm) and truly dissolved silver (< 1 kDa; i.e. ionic silver).

In the Daphnia medium, there is an initial loss of total silver (to 254 µg/l after 7 days). This loss is partially transient reaching a plateau level of 473 µg/l after 28 days. The same steep fall in concentration of conventional dissolved silver is seen during the first days (99 µg/l after 7 days), after which the concentration becomes stable at levels around 120 µg/l (127 µg/l after 28 days). The truly dissolved silver concentration is very low at the start of the test, then slightly increases to about the same level as the conventional dissolved silver concentration (88 µg/l after 7 days and 146 µg/l after 28 days).

In the algae medium, the T/D behaviour is very different from the one described above for Daphnia medium. The total and conventional dissolved silver concentrations in this medium decrease slightly over time but fluctuate around 1000 µg/l over the whole test period (total silver concentration is 1006 µg/l after 7 days and 957 µg/l after 28 days; conventional dissolved silver concentration is 974 µg/l after 7 days and 912 µg/l after 28 days). The truly dissolved silver concentration is very low at the start of the test, then slowly increases during the test (125 µg/l after 7 days and 214 µg/l after 28 days).