1、BRITISH STANDARD BS 2690-104: 1983 Methods of testing Water used in industry Part 104: Silica: reactive, total and suspended IMPORTANT NOTE. It is essential that this Part be read in conjunction with the information in Part100 of this standard, “Foreword, scope and general requirements”, which is pu
2、blished separately. UDC 628.1:663.63.01:543.3:546.284-31BS2690-104:1983 This British Standard, having been prepared under the directionof the Environment andPollution Standards Committee, was published underthe authority of the Executive Board and comes intoeffect on 30November1983 BSI 09-1999 The C
3、ommittees responsible for this British Standard are shown in Part100 The following BSI references relate to the work on this standard: Committee reference EPC/37 Draft for comment 77/50234 DC ISBN 0 580 11741 3 A British Standard does not purport to include all the necessary provisions of a contract
4、. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 6 and a back cover. This
5、 standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Amendments issued since publication Amd. No. Date of issue CommentsBS2690-104:1983 BSI 09-1999 i Contents Page 0 Introduction 1 1 Scope 1
6、 Section 104.1 Reactive silica: spectrophotometric method (reduced molybdosilicic acid 2 Range 1 3 Maximum size of test portion 1 4 Principle 1 5 Interferences 1 6 Reagents 2 7 Apparatus 2 8 Sampling 2 9 Calibration 2 10 Procedure 3 11 Calculation 3 Section 104.2 Total silica: fusion and spectrophot
7、ometry 12 Range 3 13 Maximum size of test portion 3 14 Principle 3 15 Interferences 3 16 Reagents 3 17 Apparatus 4 18 Sampling 4 19 Calibration 4 20 Procedure 4 21 Calculation 5 Section 104.3 Suspended silica: filtration, fusion and spectrophotometry 22 Range 5 23 Maximum size of test portion 5 24 P
8、rinciple 5 25 Interferences 5 26 Reagents 5 27 Apparatus 5 28 Sampling 5 29 Calibration 5 30 Procedure 5 31 Calculation 6ii blankBS2690-104:1983 BSI 09-1999 1 0 Introduction Silica may be present in solution or in suspension. The majority of silica in solution is detected by the procedure described
9、in section104.1 “Reactive silica”. However, in some circumstances, silica may be present in solution in a non-reactive polymeric form. Suspended silica is commonly present in natural waters as finely divided clay, the smaller particles of which may pass through conventional water treatment plant. Th
10、us high quality boiler feed waters, with a low reactive silica content, can contain significant quantities of suspended silica which may subsequently pass into solution under the conditions existing within a boiler. Total silica may be determined by the procedure described in section104.2. Non-react
11、ive silica (soluble and suspended) is represented by the difference between reactive and total silica contents. Section104.3 describes the direct determination of suspended silica. It permits the determination of suspended silica at lower concentrations than is possible with the procedure for total
12、silica and is useful when the presence of a high concentration of soluble silica (reactive or non-reactive) limits the accuracy with which suspended silica can be determined, by difference between total and reactive silica. Only silica present in suspended particles greater than 0.1 4m in diameter i
13、s determined. This has been found to represent 95 % of the non-reactive silica present in some natural waters, but this value may vary, even for samples from the same source. The procedures described are for the analysis of industrial and natural waters subject to the limitations outlined in each se
14、ction. 1 Scope The methods described are for the determination, in industrial waters, of reactive silica (spectrophotometry reduced molybdosilicic acid), total silica (fusion and spectrophotometry) and suspended silica (filtration, fusion and spectrophotometry). Section 104.1 Reactive silica: spectr
15、ophotometric method (reduced molybdosilicic acid) 2 Range Up to50 4g SiO 2 , i.e.0.5 mg/L using a100 mL test portion. 3 Maximum size of test portion The maximum size of test portion is100 mL. 4 Principle Ammonium molybdate reacts with silica under acidic conditions to form molybdosilicic acid which
16、is reduced to molybdenum blue by 4-amino-3-hydroxynaphthalene-1-sulphonic acid. Tartaric acid is added after10 min to decompose molybdophosphoric acid that would otherwise interfere. The absorbance of the blue colour is measured at a wavelength of810 nm. Molybdosilicic acid occurs in two forms, and
17、, in a ratio dependent upon reaction conditions. The reduced forms have significantly different absorption spectra. Under the conditions specified, -molybdosilicic acid is the principal product but it slowly changes to the more stable -form. In the reduced state the -form is the more stable. 5 Inter
18、ferences Sulphate and nitrate above70 mg cause a slight decrease of the absorbance because of the formation of an appreciable quantity of the -molybdosilicate which occurs in the presence of high concentration of these anions. Molybdophosphoric acid is destroyed by the addition of tartaric acid befo
19、re reduction and the interference of2.5 mg of orthophosphate is less than0.5 4g of SiO 2 . Ferrous iron, sulphite and hydrazine all interfere in the presence of phosphate probably by reducing some molybdophosphoric acid before it is destroyed by tartaric acid. The presence of0.5 mg of phosphate toge
20、ther with25 4g of ferrous iron, or0.2 mg of sulphite, will give an absorbance equivalent to1 4g of SiO 2 . With0.1 mg of hydrazine there is a similar effect. In the absence of phosphate there is no interference with up to0.25 mg of ferrous iron, or2mg of sulphite or1 mg of hydrazine. Cupric copper c
21、auses increased absorbance because of its blue colour equivalent to0.07 4g of SiO 2per100 4g of Cu 2+present. Truly dissolved ferric iron causes increased absorbance equivalent to0.07 4g of SiO 2per200 4g of Fe 3+ . Coagulated ferric hydroxide causes no interference up to2 mg. Colloidal iron suffici
22、ent to give a visible yellow colour to the sample should be coagulated, e.g. by boiling in a silica-free vessel. Cyclohexylamine and morpholine do not interfere up to1 mg. Octadecylamine produces turbidity and increased absorbance equivalent to0.6 4g of SiO 2for0.1 mg of octadecylamine.BS2690-104:19
23、83 2 BSI 09-1999 6 Reagents 6.1 Low-silica water. Press a pad of cotton wool into the bottom of a polyethylene tube (see 7.1) and pour in a thick slurry containing approximately100 mL of mixed bed ion exchange resins with strong functional groups. Pass water downward through the column at a maximum
24、rate of 4L/h. Do not allow the water level to fall below the top of the resin bed. Run off about5 L to waste before collecting the water in a polyethylene container with precautions to exclude dust. Use this water throughout this method. The silica content of this water is determined by comparing th
25、e results obtained with the solution containing no added silica as described in clause 9 and the reagent blank described in clause 10. The difference between these results will be the reactive silica content of95 mL of the water. 6.2 Ammonium molybdate/sulphuric acid mixed reagent. Add62 mL of conce
26、ntrated sulphuric acid,98 % (m/m) (36n), slowly, with stirring, to about100 mL of water in a beaker (caution) and cool (solution A). Dissolve89 g of ground ammonium molybdate tetrahydrate, (NH 4 ) 6Mo 7 O 24 4H 2 O, in about800mL of water in a polyethylene beaker without heating (solution B). Add so
27、lution A to solution B with stirring, cool and make up to1 L. Store in a polyethylene bottle in the dark. If a blue colour appears in this solution, it need not be discarded unless the reagent blank has an absorbance of over0.02. 6.3 Sulphuric acid, 2.2n. Add62 mL of concentrated sulphuric acid,98 %
28、 (m/m) (36N), slowly, with stirring, to about900 mL of water in a beaker (caution), cool and make up to1 L. Store in a polyethylene bottle. 6.4 Tartaric acid.Dissolve280 g of tartaric acid in about800 mL of water and make up to1 L. Store in a polyethylene bottle. 6.5 Aminohydroxynaphthalene sulphoni
29、c acid, 2 g/L. Dissolve2.4 g of sodium sulphite heptahydrate, Na 2 SO 3 7H 2 O, and0.2 g of 4-amino-3-hydroxynaphthalene-1-sulphonic acid in about70 mL of water. Add14 g of potassium metabisulphite. When dissolved, dilute to100 mL and filter. Store in a polyethylene bottle. This solution is stable f
30、or about 1 week. 6.6 Standard silica solutions 6.6.1 Stock solution. Dissolve1.566 g of sodium fluorosilicate in400 mL of water in a polyethylene beaker, heating by suspending in a water bath until solution is complete. Cool and dilute with water to500 mL in a one-mark volumetric flask. Store in a p
31、olyethylene bottle. Alternatively, fuse0.500 g of powdered silica, previously ignited to1100 C for1 h, with5 g of sodium carbonate in a platinum crucible until all the silica is dissolved in the molten sodium carbonate. Cool and extract the melt with hot water. When all is dissolved, add2 g to3 g of
32、 sodium hydroxide, make up to500 mL, and store in a polyethylene bottle. 1mL = 1mg of SiO 2 6.6.2 Working solution. Dilute10.0 mL of the silica stock solution to1 L in a one-mark volumetric flask. Transfer to a polyethylene bottle. Prepare this solution on the day of use. 1mL = 10 4g SiO 2 7 Apparat
33、us 7.1 Ordinary laboratory apparatus, together with the apparatus specified in 7.2 to 7.8, fabricated from polyethylene or other silica-free plastics. 7.2 Ion exchange column; a tube about450mm long and25mm in diameter reducing to a6.5mm diameter at the base and fitted with a control valve. 7.3 Meas
34、uring cylinders, capacity25 mL, 50mL and100mL. 7.4 Wash bottles 7.5 Bottles, capacity500mL and125mL. 7.6 Beakers, capacity1000mL and150mL. 7.7 Burette 7.8 Pipettes 8 Sampling Store samples in silica-free polyethylene bottles. 9 Calibration Into a series of125mL polyethylene bottles add, from a polye
35、thylene burette, volumes of the silica working solution containing0, 10, 20, 30, 40 and504g of SiO 2 . Dilute each to100mL with water. Add to each bottle2.5mL of the ammonium molybdate reagent, mix and note the time. After10 min add2.5 mL of the tartaric acid reagent, mix and note the time. After a
36、further5 min add2.0 mL of the 4-amino-3-hydroxynaphthalene-1-sulphonic acid reagent, mix and leave for at least5 min.BS2690-104:1983 BSI 09-1999 3 Within4 h measure the absorbance of each solution using40mm cells at a known temperature between20 C and25 C in a spectrophotometer at the wavelength cor
37、responding to maximum absorption (approximately810nm, but the exact wavelength shall be checked for each spectrophotometer). Use water in the compensating cell. Deduct the reading for the blank solution containing no added silica from those obtained for the standard solutions. Plot a graph of the ne
38、t absorbances against the corresponding micrograms of silica. NOTEThe absorbance given by50 4g of silica in100 mL is approximately0.69 at20 C to25 C. (This decreases by approximately0.2 % per1 C temperature rise.) 10 Procedure Measure a suitable volume (V 1 ) of the filtered sample containing not mo
39、re than504g of silica into a125mL polyethylene bottle and dilute to100 mL with water. Place5mL of water into a second125mL polyethylene bottle to serve as a blank. Add to each bottle2.5mL of the ammonium molybdate reagent, mix and note the time. After10min add2.5mL of the tartaric acid reagent, mix
40、and note the time. At this point add95mL water to the blank. After a further5 min, add2.0mL of the4-amino-3-hydroxynaphthalene-1-sulphonic acid reagent, mix and leave for at least5 min. If it is necessary to correct for colour in the sample, carry a second portion of sample (V 1 ) through the proced
41、ure, adding2.5mL of sulphuric acid instead of ammonium molybdate reagent. Within4 h, measure the absorbances of the sample solution and blank solution using40mm cells in the spectrophotometer at a temperature within1 C of that at which the calibration graph was prepared and at the same wavelength. U
42、se water in the compensating cell. 11 Calculation Deduct the reading for the blank solution and, if used, the colour compensating solution from that obtained for the sample solution and read from the calibration graph the silica content of the test portion. If the silica content of the water used fo
43、r dilution of the sample is significant, correct the silica content of the test portion accordingly (see 6.1). Reactive silica (as SiO 2 ), mg/L of test sample = where m 1is the mass of silica in the test portion (in 4g) V 1is the volume of the test portion (in mL) Section104.2 Total silica: fusion
44、and spectrophotometry 12 Range Up to504g SiO 2 , i.e.1.0mg/L using a50mL test portion. NOTEContamination, particularly during the fusion stage, restricts the lower limit to about54g SiO 2under typical circumstances. 13 Maximum size of test portion The maximum size of test portion is50mL. 14 Principl
45、e The sample is evaporated to dryness in the presence of sodium carbonate, the residue fused and the fusion product dissolved in water. All silica present is then determined by the method described for reactive silica (see section 104.1). The slight depression of absorbance values by reagents introd
46、uced in the fusion step necessitates their inclusion in the calibration solutions. 15 Interferences This method is subject to the same interferences as the method for reactive silica described in section104.1 except that any volatile and readily oxidized substances are eliminated during the fusion s
47、tep. 16 Reagents 16.1 The reagents specified in 6.1 to 6.6, together with those in 16.2 to 16.6 16.2 Methanol 16.3 Hydrochloric acid , concentrated 36 %(m/m) (11n). 16.4 Hydrofluoric acid, 40 %(m/m) (20n). Caution. Hydrofluoric acid, even in dilute solution, should be handled with extreme care, part
48、icularly as the effects are often felt only several hours after contact. If splashes on the skin occur, the affected area should be flooded with copious amounts of cold water and medical advice sought immediately. 16.5 Sodium carbonate, 20g/L. Dissolve2 g of sodium carbonate containing not more than
49、104g of SiO 2per gram in100mL of water in a polyethylene beaker. Filter and store in a polyethylene bottle. 16.6 Sulphuric acid, 2n. Add56mL of concentrated sulphuric acid,98 %(m/m) (36n), slowly, with stirring, to about900mL of water in a beaker (caution), cool and make up to1L. Store in a polyethylene bottle. m 1 V 1 -BS2690-104:1983 4 BSI 09-1999 17 Apparatus 17.1 The apparatus specified in 7.1 to 7.8 together with the following (17.2). 17.2 Platinum crucibles,capacity approximately25mL and thoroughly cleaned as follows. Into each crucible place
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