GB/T 12157-2022Determination of dissolved oxygen in water for industrial circulating cooling system and boiler (English PDF)

5.4 Test steps

5.4.1 Sampling Place the cleaned sampling bottle A and B in the cleaned sampling barrel at the same time. The sampling barrel shall be at least 15cm higher than the sampling bottle. Insert two cleaned polyethylene plastic tubes or inert material tubes into the bottom of bottle A and bottle B, respectively. Simultaneously introduce the water sample into bottle A and bottle B through the catheter by siphoning or other methods. The flow rate shall be about 700mL/min. Let the water naturally overflow from bottle A and B into the barrel until the level in the sampling barrel is more than 15cm above bottle A and B mouths.

5.4.2 Pretreatment of water sample If there are suspended substances in the water sample that can fix or consume oxygen, potassium aluminum sulfate solution can be used to flocculate. Fill a 1000mL stoppered bottle with the water sample to be tested and let the water overflow (according to the sampling procedure in 5.4.1). Pipette 20mL of potassium aluminum sulfate solution and 4mL of ammonia water into the water sample to be tested. Stopper, mix, and allow to settle. Aspirate the supernatant into a narrow-mouthed bottle. Analyze according to the determination procedure.

5.4.3 Oxygen fixation and acidification Use a slender glass tube to pipette about 1mL of the manganese sulfate solution. Insert the glass tube into the middle of bottle A. Put in the manganese sulfate solution. Then add 5mL of alkaline potassium iodide mixed solution and 2.00mL of potassium permanganate standard titration solution in the same way. Place bottle A under the water layer of the sampling barrel. After precipitation in bottle A, open the cork under water. Add 5mL of sulfuric acid solution to bottle A. Close the bottle tightly. Take out to shake well. First add 5mL of sulfuric acid solution to bottle B. Then add about 1mL of manganese sulfate solution, 5mL of alkaline potassium iodide mixed solution, and 2.00mL of potassium permanganate standard titration solution at the same position where sulfuric acid is added. There shall be no precipitation. Otherwise, retest. Close the bottle tightly. Take out. Shake well. Place bottle B under the water layer of the sampling barrel.

5.4.4 Determination Pour the solution in bottle A and bottle B into two 600mL or 1000mL beakers, respectively. Use sodium thiosulfate standard titration solution to titrate until light yellow. Add 1mL of starch indicator solution to continue titration. The solution changes from blue to colorless. Use the titrated solution to rinse the original bottle A and B. Continue to titrate until colorless as the end point.

5.5 Result calculation

5.5.1 The mass concentration of dissolved oxygen in the water sample (rho1) (as O2), expressed in milligrams per liter (mg/L), is calculated according to formula (1): Where, c - The value of the concentration of the standard titration solution of sodium thiosulfate, in moles per liter (mol/L); M - The value of the molar mass of oxygen, in grams per mole (g/mol) (M = 16.0); V1 - The value of the volume of sodium thiosulfate standard titration solution consumed by titrating bottle A water sample, in milliliters (mL); VA - The value of the volume of bottle A, in milliliters (mL); VA' - The value of the sum of the volumes of manganese sulfate solution, alkaline potassium iodide mixed solution, sulfuric acid solution and potassium permanganate standard titration solution added to bottle A, in milliliters (mL); V2 - The value of the volume of the standard titration solution of sodium thiosulfate consumed by titrating the water sample of bottle B, in milliliters (mL); VB - The value of the volume of bottle B, in milliliters (mL); VB' - The value of the sum of the volumes of manganese sulfate solution, alkaline potassium iodide mixed solution, sulfuric acid solution and potassium permanganate standard solution added in bottle B, in milliliters (mL). Calculation results are expressed to two decimal places.

5.5.2 If the water sample has been pretreated, the mass concentration of dissolved c - The value of the concentration of potassium permanganate standard titration solution, in moles per liter (mol/L); M - The value of the molar mass of oxygen, in grams per mole (g/mol) (M = 16.00); V - The value of the volume of the acid sodium indigo disulfonate solution pipetted, in milliliters (mL); 1/2 - The reaction coefficient of sodium indigo disulfonate and potassium permanganate converted to the reaction with dissolved oxygen.

6.2.8 Ammonia indigo sodium disulfonate solution: According to the required amount, mix equal volumes of ammonia-ammonium sulfate buffer solution and acid sodium indigo disulfonate stock solution. Due to the instability of the ammonia-based sodium indigo disulfonate solution, the solution shall be prepared when used.

6.2.9 Reduced sodium indigo disulfonate solution: Drain the water from the upper part of the silver-zinc reduction burette. Inject a small amount of ammoniacal sodium indigo disulfonate solution to wash the silver-zinc burette. Drain the washing liquid. Fill the burette with ammoniacal sodium indigo disulfonate solution. When the solution changes from blue to bright yellow, drain the blue solution at the tip of the burette. Then use. If the user is in a hurry, he can clamp the silver-zinc burette between the palms. Rub gently. Alternatively, hold the silver-zinc burette in hand and shake it up and down. It can also speed up the reduction of sodium indigo disulfonate. This solution shall be prepared at the time of use. After the solution originally stored in the burette is discarded, add a new solution to prepare. The use period is 4h.

6.2.10 Acid sodium indigo disulfonate standard solution: Pipette 50mL of acid sodium indigo disulfonate stock solution. Place in a 100mL volumetric flask. Use water to dilute to the scale. Each milliliter of this solution is equivalent to 20µg of O2.

6.2.11 Potassium permanganate standard titration solution: . After preparation according to GB/T 601, dilute 10 times.

6.3.1 Silver-zinc primary cells (electrolytic cells)

6.3.1.1 Preparation of sintered silver particles (porous silver particles) Weigh 50g of silver powder or precipitated silver. Spread in a 100mL porcelain evaporating dish. Flatten the surface. The thickness of silver powder is about 5mm. Put the porcelain evaporating dish into a 620°C high temperature furnace. Sinter for about 30min. Take out the silver block with a tool after cooling. Cut into silver bars with a width of 10mm and a length of 20mm. Put the silver bars back into the original evaporating dish. Then burn in an 800°C high temperature furnace for 30min. After taking out to cool, cut into porous silver particles with a particle size of 3mm~5mm.

6.3.1.2 Preparation of silver-zinc reduction burette (silver-zinc reduction battery) Take a 50mL acid burette. Pad about 10mm thick fiberglass on the bottom. Fill the burette with water. Remove air bubbles from the tip of the burette. Take the number of zinc particles with a particle size of 5mm~10mm (usually 7 particles are required). According to the ratio of one grain of zinc per 4mL of porous silver grains, fill the porous silver and zinc particles until the volume of the silver-zinc reducing agent is about 30mL. The top is covered with 4mL of porous silver particles. Vibrate from time to time during filling. Make the silver grains and the zinc grains fully contact without leaving air bubbles. The service life of silver-zinc reducing agent is generally not more than three months. After prolonged use, the silver particles become darker in color. It may pour the silver zinc mixture. Remove zinc particles. Heat the hydrochloric acid solution to dissolve the impurities. Then wash off the hydrochloric acid. Place porous silver particles in a porcelain evaporating dish. Dry on the stove first. Then put it in an 800°C high temperature furnace for 30min. Then it can restore the silvery white metallic luster.

6.3.2 Dedicated dissolved oxygen measuring bottle (dissolved oxygen bottle) The actual volume is about 300mL. It shall be colorless and transparent. Each bottle has the same volume. The stopper is a universal ground stopper.

6.3.3 Water sealing barrel The volume is 15L~25L. The barrel shall be at least 150mm higher than the dissolved oxygen bottle.

6.4 Test steps

6.4.1 Preparation of standard colors Because the standard dissolved oxygen is not easy to obtain, the dissolved oxygen standard color prepared by this method is prepared according to the "false color principle". That is, acidic sodium indigo disulfonate is added according to the amount assumed to be completely reacted with dissolved oxygen to form oxidized sodium indigo disulfonate (blue). The unreacted reduced sodium indigo disulfonate (yellow) is replaced with the corresponding picric acid to formulate the dissolved oxygen standard color. The volume (VD) of the acid sodium indigo disulfonate standard solution required for each standard color and the volume (VK) of the picric acid solution are calculated according to formula (4) and formula (5) respectively:

7.2 Reagents or materials

7.2.1 Water: GB/T 6682-2008, Grade two.

7.2.2 Sodium sulfite (Na2SO3).

7.2.3 Divalent cobalt salt: cobalt (II) chloride hexahydrate (CoCl2·6H2O) or other divalent cobalt salt.

7.2.4 Oxygen free water: Weigh about 25g of sodium sulfite and dissolve it in 500mL of water at room temperature. An appropriate amount of divalent cobalt salt can be added. Prepare it when required.

7.2.5 Nitrogen: The purity is not less than 99.999%.

7.3 Instruments and equipment Dissolved oxygen analyzer: It consists of measuring probe, host instrument and flow cell. The measuring probe anode is usually a silver electrode. The cathode is usually a platinum or gold electrode with a coating. The dissolved oxygen meter shall be equipped with temperature and atmospheric compensation devices. The operating temperature range of the instrument shall meet the requirements of the instrument manual.

7.4 Test steps

7.4.1 Calibration Calibrate according to the requirements of the instrument manual. Air calibration is usually available. The instrument shall be warmed up for at least 10min. Expose the sensor to the atmosphere. After confirming that there are no water droplets in the electrode film area, perform air calibration. Air calibration is best performed at saturated humidity. For example, place the electrodes at the junction of the water-gas- liquid surface or on a wet towel. When the response of the instrument is slow or the data is abnormal or unstable during the calibration process, it is necessary to check the electrode and the host of the dissolved oxygen analyzer. If necessary, replace the electrolyte and electrode membrane according to the instrument manual. Carry out zero check and calibration according to 7.4.2. Under laboratory conditions, other calibration methods can also be selected, such as saturated dissolved oxygen water calibration, water saturated air calibration, water standard oxygen calibration, standard gas calibration. The specific steps are carried out according to the requirements of the manual.

7.4.2 Zero check and calibration Perform the zero check of the instrument as required. When the reading is not zero or when the dissolved oxygen electrode membrane is replaced or the electrode is filled with electrolyte, zero calibration shall be performed. The following oxygen-free water method or high-purity nitrogen method can be selected. Perform zero calibration according to the instrument manual. - Oxygen-free water method: Immerse the oxygen electrode in deoxygenated water. Observe electrode response speed and test results. After the reading is stable, adjust the instrument reading to zero. - High-purity nitrogen method: Place the oxygen electrode in the flow cell. Tighten or squeeze. Guaranteed airtight and leak-free. Connect high-purity nitrogen through a hose to the flow cell inlet. Slowly open the outlet valve of the high-purity nitrogen cylinder. Adjust the intake air flow to meet the requirements of the instrument when measuring samples. Observe the electrode response speed and test results. After the reading is stable, adjust the instrument reading to zero.