UOP 946-1996 ARSENIC IN PETROLEUM NAPHTHAS BY HG-AAS.pdf

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1、ARSENIC IN PETROLEUM NAPHTHAS BY HG-AASUOP Method 946-96SCOPEThis method is for determining total arsenic in petroleum naphthas by Hydride Generation-AtomicAbsorption Spectroscopy (HG-AAS). The method is quantitative to one mass-ppb (ng/g) when a 100-gsample is used. This method yields results equiv

2、alent to, and can be used in place of, UOP Method 296. Themethod was validated by spiking with triphenylarsine, As(C6H5)3; for which 100% recovery was obtained atthe 80 mass-ppb level.OUTLINE OF METHODThe arsenic is extracted from the naphtha in two steps, using 5% sodium hypochlorite solution and t

3、hen70% sulfuric acid. The combined extracts are digested with nitric acid and hydrogen peroxide to destroyorganic matter. An aliquot of the resulting sulfuric acid solution containing the arsenic is diluted andreduced with potassium iodide. The diluted and reduced solution is analyzed by HG-AAS to d

4、etermine thearsenic content.APPARATUSReferences to catalog numbers and suppliers are included as a convenience to the method user. Othersuppliers may be used.Balance, readability 0.1-mgBalance, top-loading, readability 0.01-gBeaker, 250-mL, borosilicate glass, Fisher Scientific, Cat. No. 02-540KCyli

5、nders, graduated, 25-, 100-, 250- and 1000-mL, Fisher Scientific, Cat. Nos. 08-554C, E, F and H,respectivelyFlask, Kjeldahl, 300-mL, Pyrex brand, Fisher Scientific, Cat. No. 10-110DIT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TODETERMINE THE APPLICABILITY OF RE

6、GULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH ANDSAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THISPROCEDURE IN THE MANNER PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS(MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL O

7、F THE MATERIALS USED INTHIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTIONEQUIPMENT (PPE). COPYRIGHT 1996 UOP LLCALL RIGHTS RESERVEDUOP Methods are available through ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken PA 19428-2959,United Stat

8、es. The Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Service atserviceastm.org, 610.832.9555 FAX, or 610.832.9585 PHONE.2 of 9946-96Flasks, volumetric, Class A, 50-, 100- and 1000-mL, Fisher Scientific, Cat. Nos. 10-210-5B, 5C and 5G,respectivelyFunnel, s

9、eparatory, Squibb with Teflon stopcock, 500-mL, Kimax brand, Fisher Scientific, Cat. No. 10-437-10DHeater, transformer-controlled, wire coil, Fisher Scientific, Cat. No. 11-427Pipets, Mohr, 1- and 5-mL, Kimax brand, Fisher Scientific, Cat. Nos. 13-664D and F, respectivelyPipets, volumetric, Class A,

10、 5- and 10-mL, Fisher Scientific, Cat. Nos. 13-651-1E and 1K, respectivelySpectrophotometer, atomic absorption, having hydride generation capability. A Perkin-Elmer Model 4100ZL, equipped with a FIAS 400 flow injection analysis system and AS90 autosampler, was used indeveloping this procedure.Temper

11、ature indicator, noncontact, infrared measuring, Fisher Scientific, Cat. No. 13-904-60REAGENTS AND MATERIALSAll reagents shall conform to the specifications established by the Committee on Analytical Reagents ofthe American Chemical Society, when such specifications are available, unless otherwise s

12、pecified.References to water mean deionized or distilled water, except where noted.References to catalog numbers and suppliers are included as a convenience to the method user. Othersuppliers may be used.Ammonium oxalate, saturated solution, ACS, Alfa Aesar, Cat. No. 36228Arsenic, standard solution,

13、 0.5 g/mL (mass/vol-ppm). Pipet 5 ml of 10-g/mL arsenic standard solutioninto a 100-mL volumetric flask. Fill to the mark with water, cap and invert several times to thoroughlymix the contents. Discard after each use.Arsenic, standard solution, 10 g/mL (mass/vol-ppm). Pipet 10 mL of 1000-g/mL arseni

14、c stock solutioninto a one-liter volumetric flask. Dilute to the mark with water. Cap and invert several times to mixthoroughly. Discard after one week.Arsenic, stock solution, 1000 g/mL (mass/vol-ppm), SPEX CertiPrep, Cat. No. PLAS2-2XBoiling chips, carborundum, Fisher Scientific, Cat. No. 09-191-1

15、2Bottles, amber, 237-mL (8-oz), Fisher Scientific, Cat. No. 03-320-4CHydrochloric acid, 10% (by volume). Carefully mix one portion of concentrated hydrochloric acid withnine portions of water.Hydrochloric acid, concentrated, 37%, Certified ACS, Fisher Scientific, Cat. No. A144-212Hydrogen peroxide,

16、30%3 of 9946-96Lamp, EDL, arsenic, Perkin-Elmer, Cat. No. N305-0605. An arsenic HCL lamp may be substituted if aninstrument is used that cannot accommodate an EDL, but performance is somewhat diminished due tolosses in both intensity and stability (increased noise).Nitric acid, concentrated, 70%, Ba

17、ker Ultrex II, J.T. Baker, Cat. No. 6901-05Nitric acid, dilute (1:1). Add one part concentrated nitric acid, carefully, to one part water and mixthoroughly.Pipets, polyethylene transfer, standard bulb, disposable, for use with nitric acid and hydrogen peroxide,Fisher Scientific, Cat. No. 13-711-7Pot

18、assium iodide, ACS Grade, Alfa Aesar, Cat. No. 11601Potassium iodide, stock solution. Dissolve 10 0.1 g of potassium iodide in 100 mL of water. Store in anamber bottle. Discard any solution over two days old and make new.Sodium borohydride, Aldrich Chemical, Cat. No. 21,346.2Sodium borohydride reduc

19、ing solution, 0.4% in 0.05% NaOH. Using a top-loading balance, weigh 4 0.1 g of sodium borohydride and 1 0.05 g of 50% sodium hydroxide into a 250-mL beaker. Add 100mL of water and quantitatively transfer to a one-liter volumetric flask with water. Fill to the mark withwater, cap and invert several

20、times to thoroughly mix. Depending of specific instrument requirements,other concentrations may be preferable (see Determination of Arsenic by HG-AAS).Sodium hydroxide, 5% (by volume). Carefully mix 1 portion of 50% sodium hydroxide with 9 portions ofwater.Sodium hydroxide, 50%, Fisher Scientific, C

21、at. No. SS410-4Sodium hypochlorite (NaOCl), 5% neutral solution, arsenic-free, Alfa Aesar, Cat. No. 33369Sulfuric acid, concentrated, Baker Ultrex, J.T. Baker, Cat. No. 6902-05Sulfuric acid, 65%. Add cautiously, with stirring, three volumes of concentrated sulfuric acid to twovolumes of water.2,2,4-

22、Trimethylpentane (isooctane), Spectranalyzed, 4-L, Fisher Scientific, Cat. No. O300-4PROCEDURESamplingThe glassware used for this procedure is to be cleaned by filling it with warm (about 60C) 1:1 nitric acidcleaning solution for at least one hour after each use. Rinse with at least four portions of

23、 tap water and threeportions of deionized or distilled water. Reserve for this use only.Collect samples in amber glass bottles, as samples must not be stored exposed to light during shippingand storage. Light promotes the loss of arsenic to the container walls. If amber bottles are not available,wra

24、p clear bottles in dark paper or otherwise protect from exposure to light. Samples should be analyzed asquickly as possible to minimize possible analyte losses.4 of 9946-96When samples have been stored longer than two weeks, sample from the bottle and determine the arseniccontent as described under

25、Extraction. If the whole sample is taken, rinse the bottle with sodiumhypochlorite solution and sulfuric acid as described under Extraction. Use these rinses to perform thesample extraction and proceed with the decomposition and analysis.If only a fraction of the sample is taken, transfer the excess

26、 sample into another nitric acid cleaned amberbottle. Approximate the volume fraction of sample taken; for example, if a liter of sample was received andapproximately 130 mL was taken as the sample, approximately 13% of the original sample was used. Rinsethe original sample bottle with sodium hypoch

27、lorite and sulfuric acid as described under Extraction.Decompose and analyze these washings separate from the sample. Multiply the arsenic concentrationobtained for the bottle washes by the volume fraction of sample taken, and add this number to the samplearsenic value obtained from the instrument p

28、rintout. Use this combined value as M in CALCULATIONS todetermine the total arsenic in the original sample.ExtractionThe quantity of sample taken for analysis should contain less than 10 g of arsenic. Typically, a sampleweight of 100 g (about 130 mL) is analyzed. Smaller quantities may be analyzed i

29、f insufficient sample isavailable, but will yield a correspondingly higher limit of quantitation. Samples containing higher levels ofarsenic must be diluted with isooctane to reduce the arsenic content to this level. If this is required, aweighed amount of sample is diluted with isooctane to produce

30、 the final dilution mass of 100 g.1. Place the sample, weighed to the nearest 0.01 g, into a 500-mL separatory funnel.2. Add 15 mL of 5% sodium hypochlorite solution to the funnel, cap and shake vigorously for 5 minutes,periodically releasing pressure, carefully, away from the face.3. Allow the laye

31、rs to separate and transfer the sodium hypochlorite (lower) layer to a 300-mL Kjeldahlflask. Wash the stem of the funnel into the Kjeldahl flask with 2 mL of water added to the funnelwithout shaking, draining the water into the Kjeldahl flask.4. Add 15 mL of 70% sulfuric acid to the sample in the fu

32、nnel, shake for 5 minutes, periodicallyreleasing pressure carefully as before, and combine this extract with the sodium hypochlorite extractin the Kjeldahl flask. Again wash the stem of the funnel into the Kjeldahl flask with 2 mL of wateradded to the funnel without shaking, draining the water into

33、the Kjeldahl flask.5. Add 10 mL of water to the sample in the funnel, shake for 5 minutes, periodically releasing pressurecarefully, and combine this extract with the others in the Kjeldahl flask. Wash the stem of the funnelinto the Kjeldahl flask with 2 mL of water added to the funnel without shaki

34、ng, draining as before.DigestionTotal oxidation and complete removal of nitric acid and oxides of nitrogen are essential to the completerecovery of arsenic. It is also necessary that the solution be kept from more than slight darkening during thedigestion to avoid possible loss of arsenic, because c

35、arbonization-reduction will counteract the intendedoxidation.5 of 9946-961. Add, cautiously, 25 mL of concentrated nitric acid to the Kjeldahl flask and swirl the flask tothoroughly mix the contents.2. Add 2 or 3 carborundum boiling chips to the flask and allow the mixture to stand at room temperatu

36、refor at least 30 minutes, or until any initial reaction has subsided.3. Warm the contents on the heater set at approximately 250C, in a hood, to start the digestion.CAUTION: Wear safety goggles and use a safety shield during the oxidation process.4. Increase the heater temperature to approximately

37、500C, raising the temperature slowly to avoidexcessive foaming, after the initial evolution of nitric oxide slows down. If the oxidation becomes too rapid and foaming occurs through the neck of the Kjeldahl flask, the foam canusually be broken and losses prevented by inserting a cold glass rod into

38、the foam. Foaming generallymay be prevented by cautious addition of reagents. The temperature during this nitric acid digestion should be such that gentle reflux occurs but not so highthat the nitric acid boils out before most of the oxidation has occurred.5. Continue the digestion until practically

39、 all the nitric acid is gone, that is, when boiling has stopped andthe brown fumes characteristic of nitrogen oxides are no longer present.6. Add, cautiously, more nitric acid in 2- to 5-mL portions dropwise if there is any evidence ofdarkening due to the presence of unoxidized carbon. Repeat these

40、additions until there is no furtherevidence of brown fumes. Allow most of the remaining nitric acid present to distill out. It is veryimportant that nitric acid be present in excess during oxidation or loss of arsenic will occur.7. Add, cautiously, 5 mL of hydrogen peroxide, dropwise, allowing each

41、drop to run down the neck ofthe inclined flask. Continue adding hydrogen peroxide until fumes of nitrogen oxide are no longerevolved and the solution becomes colorless.8. Allow the sulfuric acid to concentrate to the release of fumes of sulfur trioxide, and reflux in theKjeldahl flask until beads of

42、 sulfuric acid and returning streams of condensing sulfuric acid begin toappear in the neck of the flask. Continue this refluxing for at least 5 minutes, longer periods will do noharm. Prior to this fuming, the heater temperature should be no higher than the 500C recommended above. If the sulfuric a

43、cid solution darkens upon concentration, repeat the addition of nitric acid followed byhydrogen peroxide, Steps 6 and 7. Continue this procedure until the concentrated sulfuric acid solutionremains clear and colorless (no further darkening occurs), Step 8.9. Add, cautiously, 5 mL of hydrogen peroxid

44、e, dropwise, to ensure that any residual nitric acid has beenremoved.10. Remove the flask from the heater and allow to cool for 5 minutes. Add 15 mL of saturated ammoniumoxalate, slowly, to remove the last traces of nitric acid and oxides of nitrogen. Place the flask on theheater, bring the sample t

45、o a boil and allow the vapors to boil out, then fume again for 5 minutes.11. Transfer the contents of the Kjeldahl flask to a 50-mL volumetric flask, quantitatively, with water.Allow the 50-mL flask to cool to room temperature. Bring to volume with water, cap and invertseveral times to thoroughly mi

46、x the contents.6 of 9946-9612. Determine the amount of arsenic present in the resulting sulfuric acid solution of the sample asdescribed under Sample and Standards Preparation for Hydride (Arsine) Generation, Determinationof Arsenic by HG-AAS and CALCULATIONS.Preparation of Blank SolutionThe blank s

47、olution should be prepared after the sample digestion, so that the reagent additions to thesample can be properly matched.1. Place 15 mL of 70% sulfuric acid in a 300-mL Kjeldahl flask, add 15 mL of 5% sodium hypochloritesolution, 25 mL of concentrated nitric acid, two to three boiling chips, and bo

48、il to the release of fumesof sulfur trioxide. If additional nitric acid is added to the sample during the sample digestion, then an equal amount ofconcentrated nitric acid must be added to the blank.2. Add 10 mL of hydrogen peroxide, dropwise, to the solution, evaporate to fumes of sulfur trioxide,

49、andreflux for at least 5 minutes.3. Remove the flask from the heater and allow to cool for 5 minutes. Add, slowly, 15 mL of saturatedammonium oxalate solution and place the flask on the heater. Bring the sample to a boil and allow thevapors to boil off. Fume for an additional 5 minutes.4. Allow the blank solution to cool to room temperature. Carefully add 5 to 10 mL of water to the flaskand then transfer quantitatively to a 50-mL volumetric flash. Bring to volume with water, cap andinvert several times to thoroughly mix the contents.5. Reserve the blank solution for use