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1.
Ashing container
The crucible is usually used as the ashing container for ash determination
.
There are many types of crucibles, such as bisque-fired porcelain crucibles, platinum crucibles, quartz crucibles and stainless steel crucibles
The size of the crucible should be selected according to the size of the sample amount and the nature of the sample (such as easy expansion, etc.
)
.
Sometimes there are too many samples, so choose a bisque evaporating dish
In Japan, a self-made aluminum foil cup is used for direct ashing
.
Features: The cup does not absorb moisture, is easy to maintain weight, is light in weight, easy to cool, and reduces errors
2.
Sampling amount
According to the type and properties of the sample, the ash content after burning is generally controlled to be 10-100mg
.
Weigh 2~3g for samples with ash content greater than 10g/100g; weigh 3~10g for samples with ash content less than 10g/100g
3.
Ashing temperature
The ashing temperature has a great influence on the result of ash determination
.
Since the composition, nature and content of inorganic ingredients in various foods are different, the ashing temperature should also be different
4.
Ashing time
Generally, the sample is burned until the ash is white or light gray, no carbon particles are present, and the weight reaches a constant weight
.
The time to reach a constant weight varies from sample to sample, and generally takes 2 to 5 hours
5.
Methods of accelerating ashing
For samples that are difficult to ash, the following methods can be used to shorten the ashing time and make the ashing complete
.
(1) After the sample of the grinding method is initially burned, take out the crucible, cool it, and slowly add a small amount of deionized water from the edge of the ashing container to fully moisten the residual ash (do not sprinkle directly on the residual ash to prevent the residual ash from flying Loss), grind with a glass rod to dissolve the water-soluble salts and expose the coated carbon particles.
Wash the sample on the glass rod into the container with water, and evaporate on the water bath to dryness, at a temperature of 120~130 Dry in an oven at ℃, and then burn to constant weight
.
(2) Adding ashing aids method, after preliminary burning, let it cool, add ashing aids (such as nitric acid or hydrogen peroxide , ethanol , ammonium carbonate, etc.
), evaporate to dryness and burn to constant weight, use the aids The oxidation effect to accelerate the ashing of carbon particles
.
It is also possible to add 10% (NH 4 ) 2 CO 3 and other bulking agents to decompose into gas and escape during burning, so that the ash becomes loose and promotes ashing
(3) Magnesium acetate ashing method For food samples such as beans and their products, meat and poultry products, egg products, aquatic products, milk and dairy products with high phosphorus content, magnesium acetate is added during pretreatment , which can be used for testing.
It is loose, oxygen is easy to enter, and can increase the melting point of ash, avoid melting of ash, increase the burning temperature, and shorten the ashing time
.
However, the generated MgO will increase the weight, so a blank test should be done
(4) Sulfuric acid ashing method For sugar products such as white sugar, soft white sugar, glucose, caramel, etc.
, there is an excess of potassium-based cations, and the residual ash after ashing is carbonate .
By adding sulfuric acid, all the cations are in the form of sulfate.
Become a certain component
.
The strong oxidizing property of sulfuric acid is used to accelerate ashing, and the result is expressed by sulfuric acid ash
.
When adding concentrated sulfuric acid, pay attention.
If a part of the residual ash solution and carbon dioxide gas is raised in a mist, add sulfuric acid while covering the ashing container with a watch glass.
After it does not foam, use a small amount of deionized water to add sulfuric acid .
The attachments on the watch glass are washed into the asher
.