INCINERATION.

=28. Determination of Ash.=—The principle to be kept in view in the

preparation of the ash of agricultural products is to conduct the

incineration at as low a temperature as possible to secure a complete

combustion. The danger of too high a temperature is two-fold. In the

first place some of the mineral constituents constantly present in the

ash, notably, some of the salts of potassium and sodium are volatile

at high temperatures and thus escape detection. In the second place,

some parts of the ash are rather easily fusible and in the melted state

occlude particles of unburned organic matter, and thus protect them

from complete oxidation. Both of these dangers are avoided, and an ash

practically free of carbon obtained, by conducting the combustion at

the lowest possible temperature capable of securing the oxidation of

the carbonaceous matter.

=29. Products Of Combustion.=—The most important product of combustion,

from the present point of view, is the mineral residue obtained. The

organic matter of the sample undergoes decomposition in various ways,

depending chiefly on its nature. Complex volatile compounds are formed

first largely of an acid nature. The residual carbon is oxidized to

carbon dioxid and the hydrogen to water. The relative proportions

of these bodies formed, in any given case, depend on the conditions

of combustion. With a low temperature and a slow supply of oxygen,

the proportion of volatile organic compounds is increased. At a high

temperature, and in a surplus of oxygen, the proportions of water and

carbon dioxid are greater. At the present time, however, our attention

is to be directed exclusively to the mineral residue; the organic

products of combustion belonging to the domain of organic chemistry. As

has already been intimated, the ash of agricultural samples consists

of the mineral matters derived from the tissues, together with any

accidental mineral impurities which may be present, some unburned

carbon, and the sulfur, phosphorus, chlorin, nitrogen, etc., existing

previously in combination with the mineral bases. The organic sulfur

and phosphorus may also undergo complete or partial oxidation during

incineration and be found in the ash. Unless special precautions be

taken, however, a portion of the organic sulfur and phosphorus may

escape as volatile compounds during the combustion.[15] The organic

nitrogen is probably completely lost, at most, only traces of it being

oxidized during the combustion in such a way as to combine with a

mineral base. The rare mineral elements that are taken up by plants

will also be found in the ash. Here the analyst would look for copper,

boron, zinc, manganese, and the other elements which, when existing

in the soil, are apt to be found in the tissues of the plants, not,

perhaps, as organic or essential compounds, but as concomitants of the

other mineral foods absorbed by growing vegetation. This fact is often

of importance in toxicological and hygienic examinations of foods.

For instance, traces of copper or of boron in the ash of a prescribed

food would not be evidence of the use of copper or borax salts as

preservatives unless it could be shown that the soil on which the food

in question was grown was free of these bodies.

This fact manifestly applies only to those cases where mere traces

of these rare bodies are in question. The presence of considerable

quantities of them, enough to be inimical to health, could only be

attributed to artificial means.

=30. Purpose and Conduct of Incineration.=—In burning a sample of an

agricultural product the analyst may desire to secure either a large

sample of ash for analytical purposes as already described or to

determine the actual percentage of ash. The first purpose is secured

in many ways. In the preparation of ash for manurial purposes, for

instance, little care is exercised either to prevent volatilization of

mineral matters or to avoid the occurrence of a considerable quantity

of carbon in the sample. With this operation we have, at present,

nothing whatever to do. In preparing a sample of ash for chemical

analysis it is important, where a sufficient quantity of the sample

can be obtained, to use as large a quantity of it as convenient.

While it is true that very good results may be secured on very small

samples, it is always advisable to have a good supply of the material

at hand. Since the materials burned have only from one to three per

cent of ash, a kilogram of them will supply only from ten to thirty

grams. To supply all needful quantities of material and replace the

losses due to accident, whenever possible at least twenty grams of the

ash should be prepared. The combustion can be carried on in platinum

dishes with all bodies free of metallic oxids capable of injuring the

platinum. Otherwise porcelain or clay dishes may be employed. As a

rule the combustion is best conducted in a muffle at a low red heat.

With substances very rich in fusible ash, as for instance the cereals,

it is advisable to first char them, extract the greater part of the

ash with water, and afterwards burn the residual carbon. The aqueous

extract can then be added to the residue of combustion and evaporated

to dryness at the temperature of boiling water. During the combustion

the contents of the dish should not be disturbed until the carbon is as

completely burned out as possible. The naturally porous condition in

which the mass is left during the burning is best suited to the entire

oxidation of the carbon. At the end however, it may become necessary to

bring the superficial particles of unburned carbon into direct contact

with the bottom of the dish by stirring its contents. In most instances

very good results may be obtained by burning the ash in an open dish

without the aid of a muffle. In this case a lamp should be used with

diffuse flame covering as evenly as possible the bottom of the dish and

thus securing a uniform temperature. The carbon, when once in active

combustion, will as a rule be consumed, and an ash reasonably pure be

obtained.

The second purpose held in view by the analyst is to determine the

actual content of ash in a sample. For this purpose only a small

quantity of the material should be used, generally from two to ten

grams. The combustion should be conducted in flat-bottomed, shallow

dishes, and at a low temperature. In many cases the residue, after

determining the moisture, can be at once subjected to incineration,

and thus an important saving of time be secured. A muffle, with gentle

draft, will be found most useful for securing a white ash. The term,

white ash, is sometimes a deceptive one. In samples containing iron or

manganese, the ash may be practically free of carbon and yet be highly

colored. The point at which the combustion is to be considered as

finished therefore should be at the time the carbon has disappeared

rather than when no coloration exists. In general the methods of

incineration are the same for all substances, but some cases may arise

in which special processes must be employed. Some analysts prefer to

saturate the substance before incineration with sulfuric acid, securing

thus a sulfated ash. This is practiced especially with molasses. In

such cases the ash obtained is free of carbon dioxid and roughly the

difference in weight is compensated for by deducting one-tenth of

the weight of the ash when comparison is to be made with ordinary

carbonated ash. Naturally this process could not be used when sulfuric

acid is to be determined in the product.

[Illustration: FIGURE 17. COURTOUNE MUFFLE.]

=31. German Ash Method.=—The method pursued at the Halle Station for

securing the percentage of ash in a sample is as follows:[16] Five

grams of the air-dried sample are incinerated in a platinum dish and

the ash ignited until it has assumed a white, or at least a bright

gray tint. As soon as combustible gases are emitted at the beginning

of the incineration they are ignited and allowed to burn as long as

possible. It is advisable to hasten the oxidation by stirring the mass

with a piece of platinum wire. If the ash should become agglomerated,

as sometimes happens with rich food materials, it must be separated by

attrition. The ash, when cooled on a desiccator, is to be weighed. When

great exactness is required, it is advised, as set forth in a former

paragraph, to first carbonize the mass and then extract the soluble

ash with hot water before completing the oxidation. When the latter is

complete and the dish cooled the aqueous extract is added, evaporated

to dryness and the incineration completed.

=32. Courtonne’s Muffle.=—The ordinary arrangement of a muffle, as in

assaying, may be conveniently used in incineration. A special muffle

arrangement has been prepared by Courtonne which not only permits of

the burning of a large number of samples at once, but also effects a

considerable saving in gas. The muffle as shown in Fig. 17, is made in

two stages, and the floor projects in front of the furnace, forming a

convenient hearth. The incineration is commenced on the upper stage,

where the temperature is low, and finished on the lower one at a higher

heat. The furnace is so arranged as to permit the flame of the burning

gas to entirely surround the muffle. The draft and temperature within

the muffle are controlled by the fire-clay door shown resting on the

table.