GB 8538-2022National food safety standard - Methods for examination of natural mineral drinking water (English PDF)

2.1 Principle

It is formulated with potassium chloroplatinate and cobalt chloride to make a standard color with the same hue as natural water yellow for visual colorimetric determination of water samples. Regulation

The color of 1mg/LPt [existing in the form of (PtCl6)2-] is used as a chromaticity unit, which is called 1 degree. Even slight turbidity can interfere with the measurement, so turbid water samples need to be centrifuged to make them clear.

2.2 Reagents and materials

Unless otherwise specified, the reagents used in this method are of analytical grade, and the water is grade 3 water specified in GB/T 6682.

2.2.1 Potassium chloroplatinate (K2PtCl6).

2.2.2 Cobalt chloride (CoCl2.6H2O).

2.2.3 Platinum-cobalt standard solution. accurately weigh 1.246g potassium chloroplatinate (K2PtCl6) and 1.000g dry cobalt chloride (CoCl2.6H2O), dissolve in 100mL water, add 100mL hydrochloric acid (rho20= 1.19g/mL), dilute to 1000mL with water. The color of this standard solution is 500 degrees.

2.3 Instruments and equipment

2.3.1 Colorless colorimetric tube with stopper. 50mL.

2.3.2 Centrifuge.

2.3.3 Analytical balance. the sensed amount is 0.1 mg.

2.4 Analysis steps

2.4.1 Sample processing

Pipette 50mL of transparent water sample into a colorimetric tube. If the color of the water sample is too high, take less water sample, add water to dilute the color, and multiply the result by

dilution factor.

2.4.2 Determination

Take another 11 colorimetric tubes, add platinum-cobalt standard solution (2.2.3) 0mL, 0.50mL, 1.00mL, 1.50mL, 2.00mL,

2.50mL, 3.00mL, 3.50mL, 4.00mL, 4.50mL and 5.00mL, add water to the mark, shake well, that is, the chromaticity is 0 degrees,

Standard series of 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees, 35 degrees, 40 degrees, 45 degrees and 50 degrees.

Compare the water sample with the platinum-cobalt standard color series, if the color tone of the water sample is inconsistent with the standard series, it is a different color, which can be described in words.

2.5 Presentation of analysis results

The chromaticity in the sample is calculated according to formula (1).

Chroma = V1x500V(1)

In the formula. chromaticity---unit is degree;

V1 --- equivalent to the amount of platinum-cobalt standard solution, in milliliters (mL);

V—volume of water sample, in milliliters (mL).

3.1 Taste analysis steps

Take a small amount of water sample and put it in your mouth (this water sample should be harmless to human body), do not swallow it, taste the taste of the water, describe and record it with appropriate words.

3.2 Odor analysis steps

Measure 100mL water sample, put it in a 250mL conical flask, after shaking, smell the smell of water from the bottle mouth, describe and record with appropriate words.

5.1 Principle

The intensity of the scattered light from the standard suspension of formazin was compared with the intensity of the scattered light from the water sample under the same conditions. The intensity of scattered light

The larger the value, the higher the turbidity.

5.2 Reagents and materials

Unless otherwise specified, the reagents used in this method are of analytical grade, and the water is grade 3 water specified in GB/T 6682.

5.2.1 Hydrazine sulfate solution (10g/L). Accurately weigh 1.000g of hydrazine sulfate [(NH2)2.H2SO4], dissolve in water, and dilute to a 100mL volumetric flask.

Warning---The solution is carcinogenic, avoid inhalation, ingestion and skin contact!

5.2.2 Hexamethylenetetramine solution (100g/L). accurately weigh 10.00g of hexamethylenetetramine [(CH2)6N4], dissolve in water, and dilute to

in a 100mL volumetric flask.

5.2.3 Formazin standard suspension. respectively pipette 5.00mL hydrazine sulfate solution and 5.00mL hexamethylenetetramine solution into a 100mL volumetric flask, mix well, and place at 25 degrees C+/-3 degrees C for 24h Then, add water to the mark and mix well. The turbidity of this standard suspension is 400 NTU. Book

Standard solution can be used for one month.

5.2.4 Formazin standard working solution. Dilute the formazin standard suspension 10 times with water. The turbidity after dilution is 40NTU, and it should be diluted appropriately when needed.

5.3 Instruments and equipment

Scattering Turbidimeter.

5.4 Analysis steps

Operate according to the instruction manual of the instrument. When the turbidity exceeds 40NTU, it can be diluted with water and measured.

5.5 Presentation of analysis results

The result is calculated by multiplying the turbidity reading displayed by the instrument by the dilution factor.

Note 1.When the room temperature rises, the saturated potassium chloride solution in the calomel electrode may change from a saturated state to an unsaturated state, so a certain amount of chloride should be maintained in the electrode.

Potassium crystals.

Note 2.For solutions with pH > 9, a high alkali glass electrode should be used to measure the pH.

7.1.1 Principle

Total Dissolved Solids is the total amount of dissolved inorganic mineral components in water. The water sample was filtered through a 0.45 µm filter membrane to remove the suspended solids, and a certain volume of the filtrate was evaporated to dryness and dried at 105 °C to constant weight.

Half (bicarbonate decomposes when drying loses carbon dioxide and converts to carbonate) is the total dissolved solids.

7.1.2 Instruments and equipment

7.1.2.1 Porcelain evaporating dish.

7.1.2.2 Oven. temperature control accuracy +/-1°C.

7.1.2.3 Water bath.

7.1.2.4 Dryer.

7.1.2.5 Analytical balance. 0.1 mg of sense.

7.1.3 Analysis steps

Put the cleaned porcelain evaporating dish into an oven to dry at 105 degrees C for 1 h, then take it out and cool it to room temperature in a desiccator, and weigh it. Repeat dry

Dry, cool and weigh until constant weight (the difference between two consecutive weighings is less than 0.0005g). Draw an appropriate amount (so that the measured soluble solids are 2.5mg to.200mg) of clear water samples (water samples containing suspended solids should be filtered through a 0.45µm filter) into a constant weight porcelain evaporating dish, and evaporate to dryness in a boiling water bath.

Put the porcelain evaporating dish into the oven, dry at 105 degrees C for 1 h, then take it out and cool it to room temperature in the desiccator, and weigh. Repeated drying, cooling

But, weigh until constant weight.

7.1.4 Presentation of analysis results

The content of total dissolved solids in the sample is calculated according to formula (2).

7.1.5 Precision

Under the conditions of repeatability, the absolute difference between the two independent determination results obtained shall not exceed 10% of the arithmetic mean.

7.2 180°C drying-gravimetric method

7.2.1 Principle

When the water sample has permanent hardness, the calcium and magnesium ions that make up the permanent hardness will form sulfate and chloride when evaporated to dryness. Dry at 105°C.

During the determination by the method, the crystal water contained in the sulfate of calcium and magnesium cannot be completely removed, which will make the result high; the chloride of calcium and magnesium has a strong

The hygroscopicity will also affect the measurement accuracy. Add an appropriate amount of sodium carbonate to the water sample in advance, so that calcium and magnesium ions form carbon after evaporation to dryness

acid, and drying at 180 °C will eliminate the above effects.

7.2.2 Reagents and materials

Sodium carbonate (Na2CO3).

7.2.3 Instruments and equipment

Same as 7.1.2.

7.2.4 Analysis steps

Weigh 0.2g ~ 0.4g of sodium carbonate (Na2CO3) in a cleaned porcelain evaporating dish, put it in an oven to dry at 180 degrees C for 2h. Take out and let dry

Cool to room temperature in a desiccator and weigh. Repeat drying, cooling and weighing until constant weight (the difference between two consecutive weighings is less than 0.0005g). Take an appropriate amount of clear water sample into a constant-weight porcelain evaporating dish and evaporate to dryness on a water bath.

The porcelain evaporating dish was dried in an oven at 180 °C for 2 h, then taken out and cooled to room temperature in a desiccator, and weighed. Repeat drying, cooling,

Weigh until constant weight.

7.2.5 Presentation of analysis results

Same as 7.1.4.

7.2.6 Precision

Under the conditions of repeatability, the absolute difference between the two independent determination results obtained shall not exceed 10% of the arithmetic mean.

8.1 Principle

When there is chromium black T indicator in the water sample, it forms purple chelates with calcium and magnesium ions, and the instability constants of these chelates are greater than

Instability constants for calcium and magnesium chelates of diaminetetraacetate. When pH=10, disodium EDTA forms first with calcium ions, and then with magnesium ions.

At the end point of titration, the solution showed the blue color of chrome black T indicator.

Since the reaction between calcium ions and chrome black T indicator can not show obvious color change when the titration reaches the equivalence point, so when the water sample

When the magnesium content is very small, it is necessary to add a known amount of magnesium salt to make the color of the equivalence point clear. When calculating the result, subtract the added magnesium salt

amount, or add a small amount of complexing EDTA magnesium salt to the buffer solution to ensure a clear endpoint.