In water, hydrogen ions are generated due to dissociation or hydrolysis of solute (production of inorganic acids, ferrous sulfate, aluminum sulfate, etc.), and the amount consumed by these hydrogen ions and the alkali standard solution to a certain pH is defined as acidity. The magnitude of the acidity value will vary depending on the pH of the endpoint indicated by the indicator used. The acidity titrated to a pH of 8.3 (using phenolphthalein as an indicator) with a sodium hydroxide solution, referred to as "phenolic acidity", also known as total acidity, including strong acids and weak acids.
classification
Acidity testing includes total acidity (titrated acidity), effective acidity (hydrogen ion activity, pH) and verification of volatile acids
(1) Total acidity
The total acidity refers to the total amount of all acidic components, including the concentration of the acid which has been dissociated to H+ before the measurement (free state), and also the concentration of the undissociated acid (bound state, acid salt). Its size can be determined by means of standard alkali solution titration, so the total acidity is also known as "titrate acidity".
(2) Effective acidity
The effective acidity refers to the concentration of H+ in the tested solution. It should be the activity of H+ in the solution. It reflects the concentration of the partially dissociated acid. It is usually expressed by the pH value. The size can be measured by the acidity meter. That is, the pH meter is used for measurement.
(3) volatile acids
Volatile acid refers to the volatile organic acids in foods, such as formic acid, acetic acid (acetic acid), butyric acid and other low-chain fatty acids. The size can be separated by distillation and titrated by standard alkali solution. Volatile acids do not include lactic acid, succinic acid, sorbic acid, and CO2, SO2, and the like.
The volatile acid consists of two parts, free and bound, the former being more volatile during distillation and the latter being more difficult.
In cow's milk, the following two acidities are included:
Appearance acidity:
Also known as intrinsic acidity (potential acidity), it refers to the acidity of the freshly squeezed milk itself, which is caused by phosphoric acid, casein, albumin, citric acid and CO2. Appearance acidity accounts for about 0.15%-0.18% (calculated as lactic acid) in fresh milk.
Real acidity:
Also called fermentation acidity, it refers to the part of the acidity that lactose fermentation produces lactic acid and increases under the action of lactic acid bacteria. If the acid content in the milk exceeds 0.15%-0.20%, it indicates that lactic acid is present. Therefore, it is customary to refer to the milk containing 0.2% or less of sour milk as fresh milk. If it is 0.3%, it will have a sour taste, and 0.6% will solidify.
There are two main methods for specifically indicating the degree of bovine lactate:
(1) Use To to indicate the acidity of the milk, that is, the number of mL of 0.1000 mol/L sodium hydroxide solution consumed by titrating 100 mL of the milk sample; or the number of mL of 0.1000 mol/L sodium hydroxide used to titrate 10 mL of the cow's milk multiplied by 10, that is the acidity of the milk.
The acidity of fresh milk is generally 16-18 To.
(2) It is expressed as a percentage of lactic acid, and is the same as the total acidity calculation method, and lactic acid is used to indicate the degree of bovine lactate.
test methods
(1) Determination of total acidity
Principle: The organic weak acid in food, tartaric acid, malic acid, citric acid, oxalic acid, acetic acid, etc. have an ionization constant greater than 10-8, which can be directly titrated with a strong base standard solution. Using phenolphthalein as an indicator, when titrated to the end point (the solution is light red, 30 s does not fade), the total acid content in the sample can be calculated based on the concentration and volume of the standard alkali solution consumed.
Calculation formula:
Total acidity =
Where: C-standard NaOH solution concentration, mol / L;
V-titration consumption standard NaOH solution volume, mL;
M-sample mass or volume, g or mL;
V0 - total volume of sample diluent, mL;
V1-drip timing sample volume, mL;
The K-conversion factor, ie 1 mmol NaOH, corresponds to the number of grams of the main acid.
Since the food contains a variety of organic acids, the total acidity measurement results are usually expressed as the most abundant acid in the sample.
(2) Determination of effective acidity (pH)
There are many methods for measuring pH, such as potentiometry, colorimetry, and chemical methods. The acidity (ie, pH meter) is commonly used to determine the activity of H+ (approximating the concentration).
(3) Determination of volatile acids
There are direct and indirect methods for determining volatile acids.
The direct method is to separate the volatile acid by steam distillation or solvent extraction, and then titrate with a standard alkali solution. This method is convenient to operate and is more commonly used, and is suitable for samples with high volatile acid content.
The indirect method is to remove the volatile acid by evaporation, and titrate the non-volatile acid with a standard alkali solution, and finally subtract the non-volatile acid from the total acidity to obtain the volatile acid content. It is suitable for the case where the volatile acid content in the sample is small or the distillate is lost or contaminated during the distillation operation.
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