Calcium wolframate

Endpoint summary

Key value for chemical safety assessment

Genetic toxicity in vitro

Description of key information

No genetic toxicity study with calcium wolframate is available, thus the genetic toxicity will be addressed with existing data on the individual moieties calcium and wolframate.

Calcium wolframate is not expected to be genotoxic, since the two moieties calcium and wolframate have not shown gene mutation potential in bacteria.

Additional information

Read-across - calcium wolframate:

Calcium wolframate is the salt of wolframic acid. It is composed of calcium cations and wolframate anions so that calcium wolframate is electrically neutral (i.e. without a net charge). Any dissolution ofcalcium wolframatewill release the wolframate anion that is pH-dependent in equilibrium with undissociated wolframic acid (as based on the pKa of wolframic acid).

Thus, wolframate and wolframic acid, coexist in aqueous solution in adynamic pH-dependant equilibrium. Under neutral and basic conditions, the wolframate ion predominates whereas under more acidic conditions, the hydrogen wolframate anion and finally (below a pH of ca. 5) wolframic acid is more prevalent.

 

                                  H₂WO₄ HWO₄^-+ H⁺ WO₄^2-+ 2H⁺

The pKa values for wolframic acid is estimated to be 3.50 and 4.60 based on handbook data (Hollemann Wiberg, Lehrbuch der Anorganischen Chemie, 101.Auflage). The mean pKa value for calcium cations is estimated to be 12.6 based on handbook data (Lide, D.R., 2008).A Hägg-graph representing the equilibrium ofwolframic acid/ wolframateis provided below.

 

Due to its electronegativity E₀(Ca/Ca²⁺) = -2,84V calcium is present in the environment only in the divalent cationic form. At high solution pH, i.e. above 12, calcium hydroxide complexes are formed.

Other calcium species, potentially relevant for the environmental of human health hazard assessment, are not present at environmentally or physiologically relevant redox conditions and solution pH.

According to the Hägg-graph and the Pourbaix- diagram wolframate is the dominant species under ENV conditions (Seiler et al., 2005). Anthropogenic use of W utilizes W metal or WC, which are thermodynamically unstable and when introduced into environmental systems begins to alter to a more stable form (WO₄^2-) (Andersson and Bergstrom 2000).

Wolfram metal is (due to its negative potential (-0.09 to -1.074, pH0 -pH14)) transformed to wolframate which represents the most stable oxidation state of Wolfram(see Pourbaix Diagram below).

W + H₂O/1.5 O₂ --> 2 H⁺+ WO₄^2-

Hence, under physiological conditions wolframic acid, hydrogen wolframate and wolframate co-exist in a pH-dependent manner, irrespective of their origin.

Genetic toxicity in vitro - wolfram metal:

The substance should not be considered to have a mutagenic potential based on a bacterial reverse mutation assay (OECD 471). The substance does not require classification according to Regulation (EC) No 1272/2008 and subsequent adaptations.

Genetic toxicity in vitro - calcium ion:

Ribeiroet al.(2004) evaluated the genotoxic potential of calcium hydroxide using mouse lymphoma cells and human fibroblasts cells in vitro by the comet assay. Mouse lymphoma L5178Y cells were cultivated in suspension and human fibroblasts were gained from biopsy taken from human skin and cultured in DMEM medium supplemented with 20 % fetal calf serum. Calcium hydroxide was solved in distilled water and both cell lines treated with calcium hydroxide in three different concentrations (20, 40, 80 µg/mL). MMS served as positive control (10µg/mL). Cells were incubated for 1 hour and at the end of treatment period washed and resuspended with fresh medium. The comet assay was conducted according to the protocol published by Tice et al (2000). Two parameters were estimated: tail moment and tail intensity from 50 cells per treatment. Cell viability test for both cell lines was performed using trypan blue staining and treated cells were compared to untreated cells. At least 100 cells per group were counted. They reported that calcium hydroxide had no detectable genotoxic effect on both cell lines in three independent experiments. Compared to that the positive control induced a significant increase in tail moment and tail intensity. No significant differences in cellular viability in any of the treatment groups compared to untreated cells could be observed. 

A further study also conducted by Ribeiroet al.(2005) evaluated the genotoxic potential of calcium hydroxide in Chinese hamster ovary (CHO) cells under the same conditions as already described above. CHO K-1 cells were cultured in Ham´s F-10 medium and seeded into 96-well microtiter plates. Ten microliters of the calcium hydroxide solution (100 µg/mL) were added to the CHO cells. MMS was used as positive control (1 µg/mL). Cells were incubated for 3 hours, washed and resuspended in fresh medium. The comet assay was conducted according to the protocol published by Tice et al (2000). Two parameters were estimated: tail moment and tail intensity from 50 cells per treatment group. The trypan blue test was also conducted to evaluate the cell viability. Data of three independent experiments showed that calcium hydroxide had no genotoxic or cytotoxic effect on CHO K-1 cells.

The Food and Drug Administration (FDA) evaluated the mutagenic potential of calcium phosphate and reported the results in their FDA report in 1975 (Report number RB-245-509). The FDA performed the bacterial reverse mutation test with the Salmonella typhimurium strains TA-1535, TA-1537 and TA-1538. All strains were tested in phosphate buffer for one hour at 37°C with a calcium phosphate concentration up to 0.75 % (50 % survival level) with and without metabolic activation. S9 was obtained from different animals and different tissues. Liver, lung and testes of male rats, mice and monkeys were used for the S9 preparation. Strain specific controls were included. All plates were incubated for four days and then scored. Each compound was done in duplicate. At a concentration of 0.75 % calcium phosphate was not mutagenic for any of the bacterial indicator strains with or without activation.

Justification for classification or non-classification

Calcium wolframate is not expected to be genotoxic, since the two moieties calcium and wolframate/wolfram metal have not shown gene mutation potential in bacteria. Thus, calcium wolframate is not to be classified according to regulation (EC) 1272/2008 as genetic toxicant.