GB 7173-1987 Determination of total nitrogen in soil (semi-micro Kelvin method)
Some standard content:
National Standard of the People's Republic of China
Method for the determination of soil total nitrogen
(Semi- micro Kjeldahl method)
Method for the determination of soil total nitrogen
(Semi- micro Kjeldahl method)This standard is applicable to the determination of soil total nitrogen content. 1 Principle of determination
UDC 631.423
GB 7173—87
When the sample is digested with concentrated sulfuric acid in the presence of an accelerator, various nitrogen-containing organic compounds undergo a complex high-temperature decomposition reaction and are converted into ammonium nitrogen. The ammonia distilled out after alkalization is absorbed with boric acid and titrated with an acid standard solution to determine the soil total nitrogen content (excluding all nitrate nitrogen).
For the determination of total nitrogen, including nitrate and nitrite nitrogen, before the sample is digested, potassium permanganate must be used to oxidize the nitrite nitrogen in the sample into nitrate nitrogen, and then reduced iron powder must be used to reduce all nitrate nitrogen and convert it into ammonium nitrogen. 2 Instruments and equipment
2.1 Soil sample crusher 3
2.2 Agate mortar,
Soil sieve: aperture 1.0mm (18); 0.25mm (60 mm), 2.4
Analysis day: sensitivity 0.0001g 3
Hard Kelvin flask: volume 50ml, 100ml; Semi-micro nitrogen distillation device:
2.7. Semi-micro burette: volume 10ml, 25ml, 2.8
Conical flask: volume 150ml,
Electric furnace: 300W variable temperature electric furnace.
3 Reagents
3.1 Sulfuric acid (GB625-77): chemically pure; 3.2 Sulfuric acid (GB625-77) or hydrochloric acid (GB622-77): analytically pure, 0.005mol/L sulfuric acid or 0.01mol/L hydrochloric acid standard solution;
3.3 Sodium hydroxide (GB629--81): industrial or chemically pure, 10mol/L sodium hydroxide solution; 3.4 Boric acid-indicator mixture
3.4.1 Boric acid (GB628-78): analytically pure, 2% solution (W/V); 3.4.2 Mixed indicator: 0.5g bromocresol green (HG3-1220-79) and 0.1g methyl red (HG3-958-76) are added to an agate mortar with a small amount of 95% ethanol, and ground until the indicator is completely dissolved, then add 95% ethanol to 100ml. Before use, add 20ml of mixed indicator to each liter of boric acid solution and adjust to red purple with dilute alkali (pH value is about 4.5). This solution should not be left for too long. If the pH value changes during use, it needs to be adjusted with dilute acid or dilute alkali at any time. 3.5 Accelerator: 100g potassium sulfate (HG3-920-76, chemically pure), 10g copper sulfate pentahydrate (GB665-78, issued by the National Bureau of Chemical Standards on January 3, 1987, implemented on August 1, 1987, pure), 1g selenium powder (HG3--926-76) are ground into powder with a hand mortar and must be fully mixed. 3.6 Potassium permanganate solution: 25g potassium permanganate (GB643-77) dissolved in 500ml deionized water, stored in a brown bottle; 3.7 1:1 sulfuric acid;
3.8 Reduced iron powder: grind to pass through a 0.15mm (100mm) sieve; 3.9 Octanol.
4 Preparation of soil samples
Spread the soil sample that passes through a 1mm (18mm) sieve into a thin layer on kraft paper and divide it into several small squares. Use a small spoon to take an equal amount of soil sample (the total amount must not be less than 20g) from each square and grind it in an agate mortar so that it all passes through a 0.25mm sieve. Mix well and set aside.
5 Determination steps
5.1 Weigh 1.0×××g of terroir sample (passing a 0.25mm sieve) (containing about 1mg of nitrogen), and determine the moisture content of the soil sample at the same time. 5.2 Sample digestion point
5.2.1 Digestion excluding nitrate and nitrate nitrogen: Place the soil sample at the bottom of a dry Kelvin flask, add a small amount of deionized water (about 0.5-1 ml) to moisten the soil sample, then add 2g of accelerator and 5ml of concentrated sulfuric acid and shake well. Tilt the Kelvin flask on a 300W variable temperature electric stove and heat it with a low fire. When the reaction in the flask is moderate (about 10-15 minutes), increase the fire to keep the digested soil liquid slightly boiling. The heated area should not exceed the liquid surface in the flask to prevent the flask wall temperature from being too high and causing the ammonium salt to decompose due to heat, resulting in nitrogen loss. The digestion temperature should be such that sulfuric acid vapor condenses and refluxes at the upper 1/3 of the bottleneck. After the digestion liquid and the upper particles have all turned gray with a slight green color, continue digestion for another 1 hour. After digestion is completed, cool and wait for distillation. While digesting the soil sample, make two blank\ determinations. Except for not adding soil sample, other operations are the same as when determining the soil sample. 5.2.2 Digestion of nitrate and nitrate nitrogen: Place the soil sample at the bottom of a dry 50ml Kjergler flask, add 1ml potassium permanganate solution, shake the Kjergler flask, slowly add 2ml 1:1 sulfuric acid, rotate the Kjergler flask continuously, then place for 5 minutes, and then add 1 drop of octanol. Place 0.5g (±0.01g) of reduced iron powder at the bottom of the Kjergler flask through a long-necked funnel, cover the flask mouth with a small funnel, rotate the Kjergler flask to allow the iron powder to contact the acid, and when the violent reaction stops (about 5 minutes), place the Kjergler flask on an electric furnace and slowly heat for 45 minutes (the soil liquid in the flask should be kept slightly boiling so as not to cause a large amount of water loss). Stop the fire, wait for the Kjergler flask to cool, add 2g of accelerator and 5ml of concentrated sulfuric acid through the long-necked flask, and shake well. Follow the steps in 5.2.1 and digest until the soil liquid turns yellow-green, and then continue to digest for 1 hour. After digestion is completed, cool and wait for distillation. While digesting the soil sample, make two blank determinations. 5.3 Ammonia distillation
5.3.1 Before distillation, check whether the distillation device is vented and clean the pipes with water distillate. 5.3.2 After the digestion liquid is cooled, transfer the digestion liquid quantitatively into the distiller with a small amount of deionized water, and wash the Kelvin flask with water 4 to 5 times (the total water cut should not exceed 30 to 35 ml). In a 150ml flask, add 5ml of 12% boric acid-indicator mixture and place it at the end of the condenser, with the tube mouth placed 1:3 to 4cm above the boric acid liquid surface. Then slowly add 20ml of 10mol/L sodium hydroxide solution into the distillation chamber and pass steam distillation. When the volume of the distillate is about 50ml, the distillation is completed. Wash the end of the condenser with a small amount of water adjusted to pH 4.5. 5.3.3 Titrate the distillate with 0.005mol/L sulfuric acid (or 0.01mol/L hydrochloric acid) standard solution from blue-green to just turning red-purple. Record the volume of the acid standard solution used (ml). The volume of the acid standard solution used for blank determination shall not exceed 0.4ml in general. 6 Calculation of determination results
6.1 Calculation formula
Total nitrogen in soil (%) =
(V-Vo)×Ch×0.014×100
The volume of the acid standard solution used in titrating the test solution, ml; ---The volume of the acid standard solution used in titrating the blank, ml; 157
GB 7173—87
Concentration of the acid standard solution, mol/L,
0.014--Millimolar mass of nitrogen aliquots;
m--Sample mass, g.
6.2 The determination results shall be expressed as the arithmetic mean with three decimal places reserved. 6.3 Difference of determination results: when soil nitrogen content is greater than 0.1%, it shall not exceed 0.005%; when nitrogen content is 0.1-0.06%, it shall not exceed 0.004%; when nitrogen content is less than 0.06%, it shall not exceed 0.003%. Additional remarks:
This standard is proposed by the Ministry of Agriculture, Animal Husbandry and Fisheries of the People's Republic of China. This standard was drafted by Beijing Agricultural University. The main drafters of this standard are Zhou Feide and Shao Zeyao. 158
1 Principle
Determination of fluoride in soil Ion selective electrode method When the fluoride electrode contacts the fluoride-containing test solution, the electromotive force of the battery changes with the change of fluoride ion activity in the solution (according to the Nernst equation). When the total ionic strength of the solution is a constant, the electromotive force of the battery is linearly related to the logarithm of the fluoride ion concentration in the solution. After the sample is melted with sodium hydroxide at high temperature, it is leached with hot water, and an appropriate amount of hydrochloric acid is added to convert the interfering cations into insoluble hydroxides, which are then removed by clarification. Then adjust the pH of the solution to near neutrality, and directly use the fluorine electrode method to determine in the presence of a total ionic strength buffer solution.
2 Interference and elimination
Common A13+ and Fe3+ have serious interference in the determination,Most cations such as Ca2+, Mg2+, Si4+, Zr+, Th4+, Ce4+, Sc3+ and H+ also have certain interference, and the degree of interference depends on the type and concentration of these ions, the concentration of fluoride and the pH of the solution. Since the electrode's selectivity for F- is only nearly 10 times greater than that for OH-, it is obvious that OH- is an important interfering ion for the fluoride electrode. Other common anions and cations do not interfere with the determination. In order to reduce the error caused by the difference between the concentration and activity of each ion in the analytical solution, it is essential to add a total ionic strength buffer. High concentrations of sodium citrate and titanium iron reagent are usually used as complexing agents. The fluorine concentration of the determination system is generally 10-4, 10=5mol/lL. When a 1mol/L sodium citrate solution is added and the pH of the solution is maintained between 6.0 and 7.0, the fluorine electrode can be measured within the most ideal range. The addition of sodium citrate can mask 3000μgA13+ or Fe3+. 3 Scope of application
The minimum detection limit of this method is 25 ug fluoride, which is applicable to the determination of fluoride in general soil, rock and sediment. 4 Instruments
4.1 Fluoride ion selective electrode and saturated calomel electrode. 4.2 Ion activity meter pH meter (accuracy ±0.1mV). 4.3 Magnetic stirrer.
4.4 Polyethylene cup (100mL).
4.5 Nickel crucible (50mL).
4.6 Muffle furnace.
5 Reagents
5.1 Fluoride standard stock solution: Accurately weigh 0.2210g of standard sodium fluoride (NaF, dried at 105-110℃ for 2h), dissolve it in water, transfer it to a 1000ml volumetric flask, dilute it to the mark with water, and shake it well. Store in a polyethylene bottle. This solution contains 100μg/mL fluoride. 5.2 Fluorine standard working solution: Use a non-divided pipette to draw 10.00 mL of fluorine standard stock solution, inject it into a 100 mL volumetric flask, dilute it to the mark with water, and shake it well. This solution contains 10.0 μg/mL fluorine. 5.3 Total ionic strength buffer (TISAB)
5.3.11 mol/L sodium citrate (TISAB1): Weigh 294 g of sodium citrate dihydrate, dissolve it in a 1000 mL beaker, add about 900 mL of water, adjust the pH to 6.0~7.0 with hydrochloric acid, transfer it into a 1000 mL volumetric flask, dilute it to the mark, and shake it well. 5.3.2 1mol/l. Hexamethylenetetramine-1mol/l.-potassium nitrate-0.15mol/L titanium iron reagent (TISABI): weigh 142g hexamethylenetetramine [(CH2)NJ and 85g potassium nitrate (KNO:), 49.9g titanium iron reagent (C.HNa2OSz·H,O), dissolve in water, adjust pH to 6.0~7.0, transfer to a 1000mL volumetric flask, dilute to the mark, and shake well. 159
5.4 Acid and base solution:
5.4.1 (1+1) hydrochloric acid solution.
5.4.2 Sodium hydroxide.
5.5 Bromocresol purple indicator: weigh 0.10g bromocresol purple, dissolve in 9.25mL 0.2mol/L sodium hydroxide, dilute with water to 250 ml..
6 Operation steps
6.1 Sample pretreatment: accurately weigh about 0.1g of sample into a 50mL nickel crucible, add 0.8g sodium hydroxide, put it into a muffle furnace and heat it. When it rises from low temperature to 550C, continue to keep warm for 20min. Take it out and cool it. Use about 50mL of freshly boiled water to soak it several times until the frit is completely dissolved. Move it into a 100mL beaker, slowly add 5-8mL hydrochloric acid, stir constantly, and heat it on an electric furnace until it is almost boiling. After cooling, transfer all the precipitate into a 100mL volumetric flask, dilute it to the mark with water, and shake it well. Let it clarify and take the supernatant for testing. Prepare a reagent blank solution without adding sample according to the same operation steps. 6.2 Preparation of standard series: Accurately pipette 0, 0.501.00, 2.00, 5.00, 10.00, 20.00mL of standard operating solution containing 10.0μg/mL of fluorine, respectively, and inject into 50mL volumetric flasks, add 10mL of reagent blank solution, 1-2 drops of bromocresol purple indicator, and then add (1+1) hydrochloric acid drop by drop while shaking until the solution changes from blue to yellow, add 15mL of total ionic strength buffer, dilute with water to the mark, and shake. Pour the test solution into a plastic cup, put a stir bar, insert the fluorine electrode and calomel electrode, start from the blank solution and measure from low to high concentration, read the millivolt (mV), use semi-logarithmic coordinate paper, use equidistant coordinates to represent millivolts, and logarithmic coordinates to represent fluorine content (ug) to draw a standard curve.
6.3 Determination of samples: Accurately pipette 10.00mL of the supernatant of the sample solution, inject it into a 50mL volumetric flask, add 1 to 2 drops of bromocresol purple indicator, add (1+1) hydrochloric acid until the solution just changes color, immediately add 15mL of total ionic strength buffer, and dilute with water to the mark. According to the requirements of the product manual, transfer the test solution and the standard series into 100mL polyethylene beakers, put a stirring rod in, and place it on an electromagnetic stirrer. After immersing the electrode, start the stirrer, measure the potential of the solution, balance it for 3 minutes under stirring, and read the potential value. After the measurement is completed, rinse the electrode with detergent to the blank potential value, and absorb the water to determine the next sample test solution. According to the measured potential value, find the corresponding fluorine content from the standard curve. 7 Result calculation
Fluorine content is calculated according to formula (1):
Fluoride (mg/kg) = m = m. . V入公
Where: m-
The fluorine content is obtained from the calibration curve, ug: The fluorine content of the reagent blank is obtained from the calibration curve, g, the volume of the sample, mL;
The volume of the sample solution is absorbed during the determination, mL; the mass of the sample is weighed, g. bzxz.net
8Precision and accuracy
The results of the determination of fluorine in soil standard samples by this method are shown in Table 1. Table 1#
Precision and accuracy of soil fluorine by electrode methodLaboratory number
Soil standard number
Guaranteed value
Overall mean
Indoor relative standard deviation
Inter-laboratory relative standard deviation
Relative error
9Explanation
9.1When melting sodium hydroxide, the initial temperature should not be too high, and the temperature should be gradually increased and slowly heated to the working temperature. The working temperature is generally 500-600℃. Temperatures above 650°C can cause serious damage to the nickel crucible. To prevent the contents from bubbling and overflowing, a crucible with a larger volume should be used. 9.2 When adding hydrochloric acid, the contents may splash due to the intense reaction, so the operation should be extremely careful and cautious. The correct operation sequence is to first leaching with hot water, and after the molten block is completely dissolved, transfer all of it to a 100mL beaker, then slowly add hydrochloric acid and stir continuously. 9.3 There are a large number of interfering ions such as A13+ and Fe3+ in the soil with complex composition. Before the sample is measured, the pH must be adjusted to 9~~10, and about 5~8mL of hydrochloric acid must be added to precipitate it and remove it after clarification. However, the solution should be avoided to be neutral or weakly acidic to prevent the loss of fluorine. 9.4 When the fluoride ion concentration of the test solution is in the range of 10-4~10~5mo1/L, the pH of the solution should be controlled at about 6.0~~7.0 for measurement.
9.5 Using a blank solution similar to the test solution (i.e. the test solution in the zero tube of the calibration curve) as a special reagent for washing the electrode will greatly shorten the electrode's equilibrium time. When measuring, the concentration should be low first and then high to eliminate the "memory effect" of the electrode. Generally, the more dilute the fluoride ion concentration in the solution, the longer the equilibrium time. When the fluoride ion concentration is 10-5mol/L, the equilibrium time is 3min, and when it is 10-3~10-^mol/L, equilibrium is reached almost within 1min. Stirring is necessary during measurement, but the stirring speed should not be too fast. The method stipulates that the reading should be taken after 3min of equilibrium under stirring.
9.6 The standard solution and the solution to be tested should be measured at the same temperature, and the conditions of the determination system should be kept consistent as much as possible to avoid large drifts in the measured potential due to changes in conditions. A measurement error of 1mV causes a relative error of about 4% in activity measurement for monovalent ions. 9.7If the sample components are very complex, the substandard addition method can be used to reduce the influence of the matrix. However, it should be noted that the amount of standard solution added to the unknown sample should not cause a significant change in the ion concentration of the solution system. The added volume should be about 1% of the sample solution, and the potential change AE should be between 30 and 40 mV. 161
1 Principle
Determination of chloride in seven inlays Silver nitrate titration method Since the concentration of silver ions required to generate silver chloride is much smaller than that required to generate silver chromate, the principle of graded precipitation is used to titrate chloride ions with silver nitrate, and potassium chromate is used as an indicator. Silver ions first react with chloride ions to generate a white precipitate of silver chloride. When the chloride ions in the test solution are completely precipitated by silver ions (equivalence point), the excess silver nitrate can react with potassium chromate to generate a brick-red precipitate, which means the titration end point is reached. The reaction is as follows: NaCl + AgNO:- + NaNO: + AgCl +
When the solution is dropped to the equivalence point, the excess silver nitrate reacts with the indicator potassium chromate to produce a brick-red silver chromate precipitate. K,CrO, + 2AgNO. → 2KNO; + Ag?CrO
(brick-red precipitate)
The chloride ion content can be calculated from the amount of standard silver nitrate consumed. 2 Reagents
2.15% potassium chromate indicator: Dissolve 5g of potassium chromate (K2CrO) in a small amount of water, add saturated silver nitrate solution until a red precipitate is formed, filter and dilute to 100mL.
2.20.03mol/L. Silver nitrate standard solution: Accurately weigh 5.097g of silver nitrate dried at 105C and dissolve in distilled water, transfer to a volumetric flask, add water to make up to 1L, shake well, and store in a dark bottle. If necessary, use 0.04mol/lL sodium chloride standard solution for calibration. 2.3 0.04mol/L sodium chloride standard solution: Accurately weigh 2.338g of sodium chloride dried at 105℃. Dissolve it in water and then add water to make up to 1L, shake it.
3 Operation steps
3.1 Weigh 100g (accurate to 0.1g) of air-dried soil sample that has passed through a No. 18 sieve (1mm mesh), and put it into a 1000mL large-mouth plastic bottle. Add 500mL of carbon dioxide-free distilled water.
3.2 Plug the plastic bottle with a rubber stopper and oscillate it on an oscillator for 3 minutes. 3.3 After oscillation, immediately evacuate and filter. If the soil sample is not too sticky or the alkalinity is not high, filter it with a flat porcelain funnel until it is filtered clear. For samples with heavy soil and high alkalinity, evacuate and filter it with a Pasteur filter tube, and store the clear liquid in a 500mL triangular flask, plug it with a rubber stopper for later use. If the solution of potassium and sodium ions is not to be determined temporarily, it should be stored in a small plastic bottle of about 50mL. 3.4 Take 25mL of the solution to be tested and add sodium bicarbonate (about 0.2-0.5g) to make the pH of the solution neutral or slightly alkaline. 3.5 Add 5 drops of potassium chromate indicator to the solution and titrate with standard silver nitrate until the solution turns light red. Record the number of milliliters. 4 Calculation of results
The chloride content is calculated according to formula (1):
CI- (%) =
X0.0355×100
W is the sample volume equivalent to the number of milliliters of the solution to be tested; 0.0355 is the number of grams of chloride ions per 1mmol; 100 is converted to mmol or percentage per 100 grams of soil. 5.1 Potassium chromate can only be used as an indicator in a neutral or slightly alkaline solution, and the measurement solution should be between pH 6.5 and 10.5. This is because in an acidic solution, the chromate ion (CrO.-2) of the indicator reacts with the hydrogen ion as follows: 2H++2Cr0,-2HCrO4 Cr2O,-+H2O, thereby reducing the concentration of chromate ions and affecting the formation of silver chromate precipitation. However, in a strong alkaline solution, silver ions react with hydroxide ions to form silver oxide precipitation, which will affect the analysis results. The reaction is as follows: 2Ag++20H--2Ag0H =Ag20 ★ +H,05.2 If the color of the test solution affects the determination of the titration end point, it can be decolorized with activated carbon acidified with dilute nitric acid (the filtrate should be neutralized with sodium hydroxide at the end); or the test solution can be evaporated to dryness, the color of the organic matter can be removed with hydrogen peroxide, and then dissolved with distilled water for determination. 5.3 When the chloride ion content is too high, the endpoint will be affected by the generation of too much white silver chloride precipitate. In this case, the amount of the test solution can be reduced. 5.4 The presence of a large amount of sulfate will interfere with the determination. When the amount of sulfate ions in the test solution is below 32 mg, there is generally no interference with this determination.
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