Calcium acetylide

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Reason / purpose for cross-reference:

assessment report

Qualifier:

equivalent or similar to guideline

Guideline:

other: not reported

Principles of method if other than guideline:

A literature search was performed to provide scientific evidence for the required endpoint specific information on flammability and hydrolysis. Available information was subjected to expert evaluation.

GLP compliance:

no

Remarks:

The study date refer to the literature research prepared. The studies on which the published articles are based, are much older. Therefore they needn't to be prepared based on GLP principles or the GLP compliance hasn't been stated in the report.

Radiolabelling:

no

Analytical monitoring:

no

Details on sampling:

not applicable

Buffers:

not applicable

Estimation method (if used):

not applicable

Details on test conditions:

not applicable

Number of replicates:

not applicable

Positive controls:

no

Negative controls:

no

Transformation products:

yes

No.:

#1

No.:

#2

Key result

pH:

4

Temp.:

20 °C

DT50:

< 12 h

Type:

not specified

Key result

pH:

7

Temp.:

20 °C

DT50:

< 12 h

Type:

not specified

Key result

pH:

9

Temp.:

20 °C

DT50:

< 12 h

Type:

not specified

Reaction of calcium carbide with water

The chemical reaction between calcium carbide and water yields calcium dihydroxide (Ca(OH)2, CAS 1305-62-0) and acetylene gas (C2H2, CAS 74-86-2), accompanied by evolution of heat:

CaC2 + 2 H2O → Ca(OH)2 + C2H2 (H=-130 kJ/mol) Equation 1

Different uses of calcium carbide

The production of acetylene is an important use of calcium carbide. In the early twentieth century, CaC2 was widely used as an illuminant in so called portable “carbide lamps” in mining. The body of these lamps is formed by two containers that are arranged vertically stocked. The water drips from the upper container into the lower that is filled with calcium carbide. The formed acetylene gas is ignited and burns with a bright, broad light.

Besides, the generation of acetylene gas is used for welding applications and organic synthesis (e.g. chloroethylenes and vinyl acetate monomers). The production of the fertilizer calcium cyanide (CaCN2) and the desulfurization and desoxidation of iron and steel are based on calcium carbide.

The reaction behaviour of CaC2 with water is also used for the determination of water content in substance samples by the carbide method according to DIN 18121-2.

DIN standard 53 922

By means of the test methods according to DIN 53 922 the reaction rate and the yield of C2H2 from calcium carbide can be determined. DIN 53 922 specifies technical CaC2, including requirements on grain size, dust content, gas yield (acetylene gas) and PH3-content. As a condition for the determination of the acetylene gas yield, CaC2 has to decompose in contact with water.

The gas yield is determined by gasification of a CaC2 sample with an excess of water that is saturated with acetylene gas and is at a temperature of Twater = 15-25 °C. The ratio of calcium carbide to water is 1:8. The weighed calcium carbide sample is passed through gas saturated water in a gas tight testing apparatus and the volume of formed gas is noted. The measured gas yield at testing conditions is afterwards converted into the gas yield at p = 1013 mbar and T = 15 °C (saturated with water vapour). In order to meet the DIN standard requirements, the gas yield at p = 1013 mbar and T = 15°C is converted to receive the of an average gas yield, that is L acetylene gas per kg calcium carbide.

Hydrolysis

Calcium carbide decomposes in water with formation of acetylene and calcium hydroxide according to Equation 1 (see above). The reaction of calcium carbide, that is of technical grade, with water proceeds almost instantaneously. The thereby generated acetylene is wholly generated within a very few minutes.

For the endpoint hydrolysis no concrete information in the form of test results according to OECD Test no. 111 was found in publicly accessible literature databases. This is why an approximate calculation on the reactivity of calcium carbide with water based on available information was performed. The aim was to obtain a numerical value and to make a statement regarding the hydrolysis half-life of calcium carbide.

In this context the numerical results for the reactivity of technical CaC2 with water were used. The referring test was conducted for a patent application. In this test 2 g of powdery calcium carbide sample were placed in 30 mL of water and the volume of the acetylene released per second was measured. For calcium carbide with a technical grade of 90 % purity a flow rate of V ̇(C2H2 ) = 70 mL/s for the generated acetylene gas was measured.

The volume of acetylene gas released from reaction of 2 g calcium carbide can be calculated regarding the stoichiometry of Equation 1. With these two values the approximate time t_R needed for calcium carbide to be decomposed by the reaction with water can be determined.

The following calculations include the determination of the volume of released acetylene gas (A), and the reaction time t_R (B) (see below).

Calculation:

A) Calculation of the volume of released acetylene gas V(C2H2)

A.1) Parameter of interest: Molarity n(CaC2)

Given Values:

- Weighed Mass m(CaC2) = 2 g

- Corrected Mass (with 90 % purity): m(CaC2,cor) = 1.8 g

- Molecular Mass: M(CaC2) = 64.1 g/mol

n(CaC2) = m(CaC2)/M(CaC2) = 1.8 g/64.1 g/mol = 0.028 mol Equation 2

According to the stoichiometric ratio (see Equation 1) the following applies:

n(CaC2) = n(C2H2) = 0.028 mol Equation 3

A.2) Parameter of interest: Volume of released acetylene gas V(C2H2)

It is supposed that the water reactivity test was performed under normal conditions (normal-pressure p = 101.3 kPa, normal-temperature T = 298 K) and the effect of the heat development due to the reaction is ignored. At normal conditions the value of the molar volume of an ideal gas is 24.46 L/mol.

As 1 mol C2H2 occupies a volume of Vm = 24.46 L, the volume of n(C2H2) = 0.028 mol is:

V(C2H2) = 0.68 L = 680 mL.

B) Calculation of the reaction time t_R

The fraction of the volume of acetylene and the measured flow rate indicates the reaction time.

t_R = V(C2H2)/V ̇(C2H2) = (690 mL)/(70 mL/s) = 9.71 s Equation 4

The calculated reaction time is 9.71 s. This result supports the statement that calcium carbide and water react almost instantaneously. Therefore, the hydrolysis is assumed to proceed very fast and a short half-life can be expected. Calcium carbide is hydrolytically unstable and has a predicted half-life less than 12 hours.

The test instructions for OECD test no. 111 include testing the hydrolysis of the substance at different pH values (pH 4, 7 and 9). According to textbooks (e.g. Mortimer, 2001) the presence of acids on carbides (CaC2) leads to the formation of acetylene gas. As pure water has a pH of 7 it is assumed that the reaction between water and calcium carbide also occurs at pH 4 where acidity is increased compared to pH 7.

The reaction of calcium carbide and water can be classified as double decomposition. When in contact with water/moisture, calcium carbide decomposes to acetylene gas and calcium oxide (CaO) in the first step of the double decomposition (see Equation 5).

CaC2 + H2O → CaO + C2H2 Equation 5

As CaO is hygroscopic, it immediately reacts with liquid or gaseous water to form calcium hydroxide. Equation 6 shows the second step of the double decomposition.

CaO + H2O → Ca(OH)2 Equation 6

As these two reactions occur simultaneously, it is more usual to represent the decomposition of CaC2 by the combined equation shown in Equation 1 (see above).

It can be assumed, that the conversion of the first calcium carbide molecule takes place in an acidic respectively neutral range by the formation of CaO (see Equation 5). As CaO continues reacting with the water present in the reaction mixture, Ca(OH)2 is formed progressively (Equation 6) and a pH shift from an acidic/neutral to an alkaline milieu occurs.

The fact that the reaction of CaC2 and water is not stopped in the presence of Ca(OH)2 provides evidence that the hydrolysis in an alkaline milieu corresponds the hydrolysis in an acidic/neutral milieu.

Validity criteria fulfilled:

not applicable

Conclusions:

Calcium carbide instantly decomposes hydrolytically upon contact with water, yielding calcium hydroxide and acetylene gas. The hydrolysis half-life at pH values 4, 7 and 9 is significantly less than 12 hours.

Executive summary:

A literature search was performed to provide scientific evidence for the required endpoint specific information on flammability and hydrolysis. Available information was subjected to expert evaluation. The findings constitute well-established textbook knowledge:

Calcium carbide instantly decomposes hydrolytically upon contact with water, yielding calcium hydroxide and acetylene gas. The hydrolysis half-life at pH values 4, 7 and 9 is significantly less than 12 hours. In fact, the hydrolytical reaction is completed within a few seconds, thus half-life can be assigned a value of less than one minute.

Description of key information

In contact with water calcium carbide instantly decomposes hydrolytically,

yielding acetylene gas and calcium hydroxide.

Key value for chemical safety assessment

Half-life for hydrolysis:

1 min

at the temperature of:

20 °C

Additional information

As no standard test data according to OECD method 111 are available to cover this REACH information requirement, a search for publically available information has been conducted to indentify relevant information on hydrolysis of calcium carbide. This information was subsequently subjected to expert review.

It could be demonstrated, based on well-established textbook knowledge, that calcium carbide instantly decomposes hydrolytically, yielding acetylene gas and calcium hydroxide. The hydrolysis half-life at pH values 4, 7 and 9 is less than the REACH criterion of 12 hours. In practice, the reaction is completed in a few seconds, and the real-world half-life is hence less than one minute.