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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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1220 Margot Scheithauer<br />

Whether an odorous impression is considered pleasant or unpleasant is largely determined<br />

by the functional groups <strong>of</strong> the chemical compounds (Table 18.1.7).<br />

Table 18.1.7. Scent qualities <strong>of</strong> various chemical compounds in relation to their<br />

functional groups 33,34<br />

Pleasant<br />

Odorous impression Functional group Substance group<br />

Unpleasant<br />

-OH<br />

-OR<br />

-CHO<br />

-COR<br />

-COOR<br />

-CN<br />

-NO 2<br />

-SH<br />

-SR<br />

-CSR<br />

-NC<br />

-NH 2<br />

Alcohols<br />

Ethers<br />

Aldehydes<br />

Ketones<br />

Esters<br />

Cyanogen compounds<br />

Nitro compounds<br />

Merkaptanes<br />

Thioethers<br />

Thioketones<br />

Nitriles<br />

Amines<br />

This division should be, however, only regarded as a rough guideline, since the<br />

so-called “pleasant” odor, at higher concentrations, can easily be perceived as very “unpleasant”.<br />

Almost all solvents, such as, ketones, esters, glycols, alcohols, aromatic and aliphatic<br />

hydrocarbons, contribute to a more or less intensive smell.<br />

“Odor” is not a parameter <strong>of</strong> substance, but a summarized parameter <strong>of</strong> effects. Its determination<br />

is based on the fact that the sense <strong>of</strong> a smell can be used subjectively to evaluate<br />

certain substances (odorants). The concentration <strong>of</strong> those substances suspended in the air<br />

can be determined, which is called odorant concentration.<br />

Odor threshold or perception threshold 35-38<br />

The odor threshold is a concentration <strong>of</strong> the odorant in the air, given in mg/m³. The<br />

odor threshold corresponds to an odor unit (GE). It is the amount <strong>of</strong> odorous substance,<br />

which - distributed in 1 m³ <strong>of</strong> scent-neutral synthetic air - initiates in just 50% <strong>of</strong> the evaluators<br />

a perception <strong>of</strong> smell and in the other 50%, no response.<br />

The odor threshold is very specific to substance. It is determined in several measuring<br />

series and the results form a Gaussian distribution curve. Since this is ultimately a subjective<br />

evaluation, one should not be surprised to find more as well as, less, reliable data in literature.<br />

Examples are shown in Table 18.1.8.<br />

Odorant concentration<br />

The odorant concentration <strong>of</strong> a sample is the multiple <strong>of</strong> the odor threshold and it is determined<br />

in odor units (GE) per 1 m³ neutral air.<br />

Odor intensity 39<br />

Since the odor threshold alone is an insufficient evaluation criterion for an odorant, the<br />

increase in response with increasing odorant concentration may additionally be taken into<br />

account as a scale <strong>of</strong> reference.<br />

The increase in response is mainly material and/or mixture dependent for a given<br />

odorant concentration and is called odor intensity. [Schön, p. 68] 32

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