Determination of free chlorine and total chlorine

2022-08-17
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Determination of free chlorine and total chlorine in water n, n

Abstract: the determination range of this standard method is 0 Mg/l free chlorine or total chlorine (calculated by Cl2) (3) the method of winding the sample directly on the reel for tensile test is applicable to steel wire ropes with rope diameter less than 30mm. For steel wire ropes with diameter greater than 30mm, it is recommended to adopt casting method. Free chlorine refers to chlorine in the form of hypochlorite, hypochlorite ions and elemental chlorine; Combined chlorine refers to a part of the total chlorine formed and existing by chloramine and organic chloramine; Total chlorine refers to chlorine in the form of free chlorine, combined chlorine or both. At ph6 Under 5 conditions, free chlorine reacts directly with N, n-diethyl-1,4-phenylenediamine to form a red compound. Titrate with ammonium ferrous sulfate standard solution until the red disappears, and calculate the content of free chlorine. React in the presence of excess potassium iodide, and then proceed according to the above method to calculate the total chlorine content

determination of chloride in water quality silver nitrate titration GB 11896 89

water quality determination of chloride

silver nitrate titration method

1 subject content and scope of application

this standard specifies the silver nitrate titration method for chloride concentration in water

this standard is applicable to the determination of chloride in natural water, as well as high salinity water with appropriate dilution, such as salt water, sea water, etc., as well as domestic sewage or industrial wastewater that has been pretreated to remove interferences

the applicable concentration range of this standard is 10 500mg/L chloride. Water samples above this range can be diluted to expand their determination range

bromide, iodide and cyanide can be titrated with chloride. Orthophosphate and polyphosphate have interference when they exceed 250mg/L and 25mg/L respectively. When the iron content exceeds 10mg/L, the end point is not obvious

2 principle

in the neutral to weak alkaline range (pH6.5 10.5), when titrating chloride with silver nitrate with potassium chromate as indicator, because the solubility of silver chloride is less than that of silver chromate, chloride ions are completely precipitated first, and then chromate is precipitated in the form of silver chromate, producing brick red, indicating the end of titration. The reaction of the precipitation titration is as follows:

Ag + Cl AgCl

2ag + CrO4 ag2cro4 (brick red)

3 reagent

only analytical pure trial production and distilled or deionized water are used in the analysis

3.1 potassium permanganate, C (1/5kmno4) = 0.01mol/L

3.2 hydrogen peroxide (H2O2), 30%

3.3 ethanol (C6H5OH), 95%

3.4 sulfuric acid solution, C (1/2h2so4) = 0.05mol/L

3.5 sodium hydroxide solution, C (NaOH) = 0.05mol/L

3.6 aluminum hydroxide suspension: dissolve 125g of potassium aluminum sulfate [kal (SO4) 2 12h2o] in 1L distilled water, heat it to 60 ℃, then slowly add 55ml of concentrated ammonia while stirring, place it for about 1H, move it to a large bottle, and wash the sediment repeatedly with the dumping method until the washing solution does not contain chloride ions. Dilute it with water to about 300ml

3.7 sodium chloride standard solution, C (NaCl) = 0.0141mol/L, equivalent to 500mg/L chloride content. It can also be designed according to the length of the sample: place sodium chloride (NaCl) in a porcelain crucible and burn it at 500 600 ℃ for 40 50min. After cooling in the dryer, weigh 8.2400g, dissolve it in distilled water, and dilute it to 1000ml in a volumetric flask. Suck 10.0ml with a straw and dilute it accurately to 100ml in a volumetric flask

1.00ml this standard solution contains 0.50mg chloride (c1-)

3.8 silver nitrate standard solution, C (AgNO3) = 0.0141mol/L: weigh 2.3950g of silver nitrate (AgNO3) baked at 105 ℃ for half an hour, dissolve it in distilled water, dilute it to 1000ml in a volumetric flask, and store it in a brown bottle

calibrate its concentration with sodium chloride standard solution (3.7):

accurately suck 25.00ml of sodium chloride standard solution (3.7) into a 250ml conical flask with a straw, and add 25ml of distilled water. Take another conical flask and measure 50ml distilled water as blank. Add 1ml potassium chromate solution (3.9) respectively, and titrate with silver nitrate standard solution under constant shaking until brick red precipitation just appears as the end point. Calculate the amount of chloride equivalent to each ml of silver nitrate solution, then correct its concentration, and then make the final calibration

1.00ml this standard solution is equivalent to 0.50mg chloride (C1)

3.9 potassium chromate solution, 50g/L: weigh 58 potassium chromate (K2CrO4) and dissolve it in a small amount of distilled water, and drop silver nitrate solution (3.8) until there is red precipitation. Shake well, let it stand for 12h, then filter and dilute the filtrate to 100ml with distilled water

3.10 take phthalein indicator solution: weigh 0.58 phenolphthalein and dissolve it in 50ml of 95% ethanol (3.3). Add 50ml distilled water, and then add 0.05mol/L sodium hydroxide solution (3.5) to make it reddish

4 instrument

4.1 conical flask, 250ml

4.2 burette, 25ml, brown

4.3 pipette, 50ml, 25ml

5 samples

collect representative water samples and place them in clean and chemically stable glass bottles or polyethylene bottles. There is no need to add special preservatives during storage

6 analysis steps

6.1 elimination of interference

if there are no following interferences, this section can be omitted

6.1.1 if the water sample is turbid and colored, take 150ml or an appropriate amount of water sample to dilute to 150ml, place it in a 250ml conical flask, add 2ml of aluminum hydroxide suspension (3.6), shake and filter, discard 20ml of the initially filtered water, and use a dry clean conical flask to take the filtrate for standby

6.1.2 if the organic content is high or the chromaticity is high, the water sample can be pretreated by Maofu furnace ashing method. Take an appropriate amount of waste water sample into a porcelain evaporating dish, adjust the pH value to 89, put it in a water bath and evaporate it dry, then put it into a Maofu furnace and burn it at 600 ℃ for 1h, take it out and cool it, add 10ml distilled water, transfer it into a 250ml conical flask, wash it with distilled water for three times, transfer it into a conical flask together, adjust the pH value to about 7, and dilute it to 50ml

6.1.3 for the lighter chromaticity produced by organic matter, 2ml of 0.01mol/l potassium permanganate (3.1) can be added and boiled. Then add ethanol (3.3) to remove excess potassium permanganate until the water sample fades, filter, and store the filtrate in a conical flask for standby

6.1.4 if the water sample contains sulfide, sulfite or thiosulfate, add sodium hydroxide solution (3.5) to adjust the water sample to neutral or weak alkaline, add 1ml of 30% hydrogen peroxide (3.2) to stop the test data, record, print and shake up. One minute, the experimental speed meets the requirements of all conventional metal and non-metallic material experimental speeds in China at present, and it will be heated to 7080 ℃ in an hour to remove excess hydrogen peroxide

6.2 determination

6.2.1 suck 50ml water sample or pretreated water sample with a straw (if the chloride content is high, an appropriate amount of water sample can be diluted to 50ml with distilled water), and place it in a conical flask. Take another conical flask and add 50ml distilled water for blank test

6.2.2 if the pH value of the water sample is in the range of 6.5 to 10.5, it can be titrated directly. The water sample beyond this range should use phenolphthalein as an indicator, and use dilute sulfuric acid (3.4) or sodium hydroxide solution (3.5) to adjust it to red and just fade

6.2.3 add 1ml potassium chromate (3.9) solution 1) and titrate with silver nitrate standard solution (3.8) until the brick red precipitation just appears, which is the titration end point

perform blank titration with the same method

note: 1) the concentration of potassium chromate in water samples affects the arrival of the end point sooner or later. Add 1ml of 5% potassium chromate solution to 50 100ml titrant to make the concentration of CrO4 2.6103 ~ 5.210-3mol/L. At the end point of titration, the amount of silver nitrate added bypasses the end point and can be eliminated by the blank measured value

7 expression of results

chloride content C (mg/L) is calculated according to the following formula:

where: V1 distilled water consumes silver nitrate standard solution, ml

v2 sample consumption of silver nitrate standard solution, ml

m silver nitrate standard solution concentration, mol/L

v sample volume, ml

8 precision and accuracy

the results of standard mixed samples containing 88, 29mg/L chloride determined by 6 laboratories (January 1987) are as follows:

8.1 repeatability

relative standard deviation in the laboratory 0.27%

8.2 reproducibility

the relative standard deviation between laboratories is 1.2%

8.3 accuracy

relative error 0.57%

the recovery rate of standard addition was 100.21 0.32%

appendix a determination of salt water and sea water with high salinity

(Supplement)

a1 for the determination of salt water or sea water with high salinity, the following methods can be adopted to expand its determination range 1):

a. increase the concentration of silver nitrate standard solution to 1ml standard solution, which is equivalent to 25mg chloride

b. dilute the sample, and the dilution can refer to table 1:

Table 1 dilution of high salinity sample

specific gravity

dilution

equivalent sampling amount, ml

1.000 ~ 1.010

take 50ml titration without dilution

50

1.010 ~ 1.025

take 25ml titration without dilution

25

1.025 to 1.050

25ml dilute to 100ml, take 50ml

12.5

1.050 to 1.090

25ml dilute to 100ml, Take 25ml

6.25

1.090 ~ 1.120

25ml to dilute to 500ml, take 25ml

1.25

1.120 ~ 1.150

25ml to dilute to 1000ml, take 25ml

0.625

note: 1) this range value is extracted from the annual book of ASTM standards, design D

additional notes:

this standard is proposed by the Standards Department of the State Environmental Protection Administration

this standard is drafted by the water quality test and research center of the Ministry of hydropower

Lu Guangsi, the main drafter of this standard

China national environmental monitoring station is entrusted with the interpretation of this standard

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