Classification of mixtures

Although chemists pay a great deal of attention to pure substances, this form of matter almost never occurs around us. In the natural world, matter usually occurs as mixtures, such as air, seawater, soil, and organisms. 

A homogeneous mixture is also called a solution. Although we usually think of solutions as liquid, they can exist in all three physical states. For example, air is a gaseous solution of mostly oxygen and nitrogen molecules, and wax is a solid solution of several fatty substances. Solutions in water, called aqueous solutions, are especially important in chemistry and comprise a major portion of the environment and of all organisms. 

Anything that has mass and volume Exists in three physical states solid, liquid, gas 

Two or more elements or compounds in variable proportions Components retain their properties 

Heterogeneous mixtures have visible boundaries between the components. Homogeneous mixtures have no visible boundaries because mixing occurs at the molecular level. A solution is a homogeneous mixture and can occur in any physical state. Mixtures (not compounds) can have variable proportions, can be separated physically, and retain their components properties. 

---

Briesch, E. M. 2000. NEC Group Classification of Mixtures. Paper 5d, AlChE 34th Annual Loss Prevention Symposium, March 6-8, 2000, Atlanta, GA. 

FPN No. 3) The classification of mixtures of gases or vapors according to their maximum experimental safe gaps and minimum igniting currents is described in lEC publication 79-12 (1978). 

The classification of mixtures will be dealt with in detail in Vol. in. 

D5.2 Impact sensitivity test for the classification of mixtures of less sensitive oxidizers and red phosphorus... 

The classification criteria for substances and mixtures are presented in Parts 2 and 3 of this document, each of which is for a specific hazard class or a group of closely related hazard classes. The recommended process of classification of mixtures is based on the following sequence ... 

The cut-off values for classification of mixtures are normally specified by concentrations expressed as % of the component substance. In some cases, for example acute toxicity (human health), the cut-off values are expressed as acute toxicity values (ATE). The classification of a mixture is determined by additivity calculation based on acute toxicity values (see Chapter 3.1) and concentrations of component substances. Similarly acute aquatic toxicity classification may be calculated on the basis of acute aquatic toxicity values (see Chapter 4.10) and where appropriate, corrosion/irritation by adding up concentrations of individual substances (see Chapters 3.2 and 3.3). Component substances are taken into consideration for application of the formula when the concentration is equal to or greater than 1 %. Some competent authorities (CA) may use this cut-off as a basis of obligation to compile an SDS. 

Figure 3.1.1 Tiered approach to classification of mixtures for acute toxicity...

Classification of mixtures for acute toxicity can be carried out for each route of exposure, but is only needed for one route of exposure as long as this route is followed (estimated or tested) for all ingredients. If the acute toxicity is determined for more than one route of exposure, the more severe hazard category will be used for classification. All available information should be considered and all relevant routes of exposure should be identified for hazard communication. 

Classification of mixtures where acute toxicity test data are available for the complete mixture... 

Classification of mixtures based on ingredients of the mixture (Additivity formula)... 

Classification of mixtures when data are not available for the complete mixture Bridging principles... 

In general, the approach to classification of mixtures as irritant or corrosive to skin when data are available on the components, but not on the mixture as a whole, is based on the theory of additivity, such that each corrosive or irritant component contributes to the overall irritant or corrosive properties of the mixture in proportion to its potency and concentration. A weighting factor of 10 is used for corrosive components when they are present at a concentration below the concentration limit for classification with Category 1, but are at a concentration that will contribute to the classification of the mixture as an irritant. The mixture is classified as corrosive or irritant when the sum of the concentrations of such components exceeds a cut-off value/concentration limit. 

Classification of mixtures on the basis of information/data on ingredients... 

Classification of mixtures will be based on the available test data of the individual ingredients of the mixture using cut-off values/concentration limits for those ingredients. The classification may be modified on a case-by case basis based on the available test data for the mixture as a whole. In such cases, the test results for the mixture as a whole must be shown to be conclusive taking into account dose and other factors such as duration, observations and analysis (e.g. statistical analysis, test sensitivity) of carcinogenicity test systems. Adequate documentation supporting the classification should be retained and made available for review upon request. 

---