Determination of Tin in Hydrogen Peroxide Solutions by Atomic Absorption Spectrometry E. J.
AGAZZI
Shell Development Co., Emeryville, Calif.
b Atomic absorption spectrophotometry has been successfully applied to the determination of tin in hydrogen peroxide solutions. A sample diluted to contain 45% or less hydrogen peroxide i s injected directly into an oxyhydrogen flame, and absorption of the tin 2863-A. line by the tin atoms in the flame is measured. Tin concentration i s obtained by reference to a calibration curve prepared by injecting known concentrations of sodium stannate in hydrogen peroxide into the flame. The concentration of hydrogen peroxide i s not critical. As little as 0.05 mg. per liter of tin in 90% hydrogen peroxide can be determined. The effects of other materials added to hydrogen peroxide solutions (phosphate, pyrophosphate, sodium nitrate) were established.
H
peroxide solutions containing 70 to 90% hydrogen peroxide undergo slow decomposition; therefore, for long-term storage, it is necessary to add stabilizers such as sodium stannate, sodium pyrophosphate, phosphoric acid, and sodium nitrate. The amounts added depend upon the intended use of the peroxide solution and range from less than 2.0 to 50 mg. per liter of tin, less than 0.1 to 40 mg. per liter of phosphorus, and about 100 to 400 mg. per liter of sodium nitrate. For proper performance and storage of the hydrogen peroxide, the concentration of stabilizers must be maintained over a narrow range. This report is concerned with the determination of tin. Solutions of sodium stannate in hydrogen peroxide are unstable and frequent testing is required. The tin content of hydrogen peroxide is normally measured by decomposing the peroxide and measuring the tin content of the resulting aqueous solution by spectrophotometric or polarographic methods. The chief difficulty encountered in these methods is that stannic ions in hydrogen peroxide tend to form polymers, which are unreactive. I t is necessary to treat the solution to decompose the polymers. One of the most effective treatments is to heat the solution to fumes of sulfuric acid. However, one is never sure that all of the tin polymers have been decomposed YDROGEX
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ANALYTICAL CHEMISTRY
and the extra manipulations required plus the need for decomposing peroxide are time-consuming. Thus, use of atomic absorption spectrophotometry for the determination of tin in hydrogen peroxide has been investigated. The author is unaware of an)- specific analytical application of atomic absorption to the determination of tin. Gatehouse and Rillis ( 3 ) measured the limits of detection of many metal ions and report a limit of detection (amount of metal which gives an absorption of 1%) of 5 p.p.m. for tin. This limit of detection is too high for many formulation>. The sensitivity of atomic absorption measurements can be increased by increasing the absorption pathlength. A long path can be obtained by directing the atomizer-burner into a long quartz tube and passing light to be absorbed axially through the tube ( 2 ) . I31 use of such a device, the limit of detection (concentration which gives 1% absorption) of tin was lowered to 0.025 p.p.m. Conditions have been established for the determination of tin in hydrogen peroxide by diluting the peroxide] injecting it into a hydrogen-oxygen flame] and measuring the absorption of the tin 2863-A. line. Other materials added to hydrogen peroxide as stabilizers gave no interference. EXPERIMENTAL
Apparatus. The apparatus is shown schematically in Figure 1. Modulated (60 cycle) light from a hollow cathode tube emitting the tin
2863-A. line is focused on the entrance of the absorption tube. T h e light goes through the tube and enters the monochromator (Jarrell-Ash, Ebert Model 8200 equipped with 100-micron entrance and evit slits), and 2863-A. tin line is detected by the photomultiplier (1P28). Output of the photomultiplier is amplified by an a x . amplifier and displayed on a recorder. Normally, sample is drawn into the flame by the Beckman atomizer-burner. With 45% hydrogen peroxide] the peroxide decomposed in the stainless steel capillary of the burner through which the peroxide flowed, and oxygen gad formed in decomposition caused an uneven aspiration. The sporadic operation led to a variation in the absorbance. The effect was greatly decreased, but not eliminated, by forcing the sample to the burner with a motor-driven syringe. Absorbance, which is dependent upon the sample flow rate, is taken as the average of the recorded trace, and can be established with good accuracy. Maximum absorbance is observed between 2.5 and 5 ml. per minute. A sample flow of 2.5 ml. per minute was used. To minimize decomposition of hydrogen peroxide] the syringe was all glass and connection to the burner was a Teflon needle (Hamilton Co., Inc., Whittier, Calif.). The burner was a Beckman large bore atomiaerburner (Beckman Instrument, Inc., Fullerton, Calif.). Hydrogen and oxygen flow rates are controlled by the regulator unit used with Beckman flame photometers. The burner is positioned so the flame enters the quartz absorption tube (40 cm. long
C o o l i n g A i r Blast
I
Hollow C a t h o d e L a m p (Sn)
R
I
AbsorDtion Tube
Photomu1 t i p l i e r
Recorder Motor Driven Syringe
Figure 1.
Atomic absorption apparatus with absorption tube
and 9 mm. in diameter). A vigorous air blast is directed against' the tube as shown t,o prevent the flame from melting a hole in the tube. X second vigorous air blast, directed across the tube exit, blows the hot' exit gases away from the monochromator and terminates the absorption path. Air outlets are 1I4-inch copper tubing, and the outlet a t the tube exit has its end flattened to spread out the air blast. Air at 30 p s i . is supplied to these tubes. Procedure. APPARATUSPREPARATION. T u r n on the hollow cathode power supply (60 c.p.s.) a n d adjust output to 5.0 ma. T u r n on the hollow cathode lamp, a.c. amplifier, and recorder, a n d allow them to warm u p for hour before use. Apply 750 volts to the photomultiplier tube and align the apparatus so light from the hollow cathode lamp focuses on t h e quartz tube entrance a n d enters the monochromator. Set the wavelength dial on the monochromator to 2863 A. and adjust until the recorder shows maximum transmittance. Adjust the amplifier gain to give a reading of 100 on the recorder. CALIBRATION.Prepare a st'andard tin solution containing 10.0 mg. per liter of tin by dissolving 22.5 mg. of sodium stannate (r\'a2Sn03.3HzO) in 1 liter of approximately 45y0 hydrogen peroxide solution. Use hydrogen peroxide free from tin. Tin solutions are unstable and should be discarded after 2 days. Dilute the 10.0 mg. per liter tin standard with amroximatelv 45% hvdrogen peroxide to prepare CO-ml. portions of solutions containing 1.0, 0.5, and 0.2 mg. per liter of tin. Fill 10-ml. syringes with these solutions and fill one syringe with 45% tin-free hydrogen peroxide. Remove the burner from the tube and turn on both the cooling air blast and the air blast a t the tube exit. Turn on oxygen to the burner, then turn on the hydrogen and immediately light the burner. Place a syringe in the syringe drive and attach it to the burner. Insert the burner into the absorption tube and adjust the amplifier gain to give a reading of 100 on the recorder. Turn on the syringe drive and record the absorption for about 10 seconds. T u r n off the drive and, if necessary, readjust to 100. Again, turn on the drive for about 10 seconds. Repeat until consecutive readings agree to about 1% transmittance. Remove the syringe, replace it with another, and determine the per cent transmittance. When all of the standards have been measured, convert transmittances to absorbances. Subtract the absorbance for the tin-free peroxide from the others and make a plot of absorbance us. concentration of tin in milligrams per liter. If desired, the calibration curve can be extended. The author observed linearity up to 2 mg. per liter of tin. Calibration curves should be prepared with each batch of samples. I
0.30
! m
absorbing atoms and, hence, the higher the absorbance. However, Fuwa and Vallee ( 2 ) have shown that little is gained 0.25 by decreasing the diameter below 1 cm. The optimum length of tube depends 0.20 upon how far from the burner flame tin atoms can evist as free atoms. Distribution of tin atoms within the A 0.15 absorption tube was examined by mounting a tube vertically and directing 0.10 light from the hollow cathode source along the diameter of the tube. With 0.05 the tube shifted vertically, absorbance a t different distances from the burner I I could be measured. Because the absorption path was only 9 mm., it was 0 IO 20 30 40 50 D i s t a n c e from B u r n e r E n d o f Tube necessary to u5e a concentrated (100 mg. per liter) tin solution to achieve Figure 2. Distribution of tin atoms significant absorbance. The distribudong absorption tube tion of tin atoms is shown in Figure 2. Concentration of tin atoms is highest about 15 cm. from the burner end of the ANALYSISOF SAMPLE. By suitable tube and then drops off. The curve dilution with water or hydrogen perindicates there is little gain in absorboxide solution, adjust the sample to ance to be realized by extending the contain 2 mg. per liter or less of tin and about 45% hydrogen peroxide. tube length beyond 35 to 40 cm. Fill a syringe with the sample, inject Light from the hollow cathode lamp it into the flame, measure the transundergoes multiple reflections from the mittance as described above, and deinner surface of the tube, so the effective termine the tin concentration in the pathlength is longer than the tube sample solution from the calibration length. A highly reflective inner surcurve. face such as clear fused quartz is therefore desirable. With use, the inner surRESULTS A N D DISCUSSION face of the tube will gradually be Study of Variables. OPTIMIZATION coated. The tube is still usable in this OF GAS FLOWRATES. Both acetcondition, but sensitivity will be deylene-oxygen and hydrogen-oxygen creased. Coating of the walls is one mixtures were tried for production of reason for the need of frequent calibratin atoms. T h e acetylene-oxygen tion. combination was not satisfactory. BURNERPOSITION.The burner is Acetylene flames rich in oxygen were placed so the flame can be directed into too hot and melted holes in the abthe tube and the burner top does not sorption tube. Flames rich in acetsignificantly obstruct the beam from the ylene deposited soot in t h e tube, hollow cathode lamp. The inclination changing the effective pathlength (see of the burner to the horizontal is section on Absorption Tube). important; for example, with the burner Optimum flow rates of hydrogen and inclined at about 15O, 1 mg. per liter of oxygen were determined by measuring tin gave an absorbance of 0.1. At about the absorbance of an aqueous tin soluan inclination of 30°, the absorbance tion at various combinations of hydroincreased to 0.25. Highest absorbance gen and oxygen flow rates. These tests was found with the burner tilted so a hot gave the optimum flows for this apspot formed about 1 cm. from the tube paratus, but a t these flows the flame was end. hot enough to melt holes in the absorpCONCENTRATION OF HYDROGEN tion tube despite the cooling air blast. PEROXIDE. The highest concentration The oxygen flow rate was, therefore, of hydrogen peroxide in the samples was decreased, and the hydrogen flow rate 90%. Because the handling of such conmaintained at optimum until the flame centrated peroxide is hazardous, samples could be operated without melting the are diluted to an arbitrary 4501, hydrotube. Absorbance observed a t these gen peroxide. The concentration of gas flows was 9570 of optimum. An peroxide is not critical. Peroxide reoxygen flow of 3 liters per minute and a sults in an absorbance about 10% higher hydrogen flow of 7.1 liters per minute than that observed with aqueous soluwere used. The best gas flows may be tions, and the effect is essentially conslightly different for other apparatus stant from 10 to 45% peroxide. and should be established experiInterferences. Sodium pyrophosmentally. phate, phosphoric acid, and sodium ABSORPTIONTUBE. The diameter, nitrate are also added to hydrogen length, and reflection from the inner peroxide as stabilizers. T h e effects walls of the absorption tube all affect the of these materials on the determinaabsorbance. The smaller the diameter, tion of tin are shown in Table I. the higher will be the concentration of Sodium phosphate was used instead of VOL. 37, NO. 3, MARCH 1965
365
Table 1. Effect of Stabilizers on Determination of Tin by Atomic Absorption Spectrometry
Stabilizer added NaNOa Na3P04 Na4P207
Amount added to 0.6 mg. per liter of tin, Recovery in mg. per liter of tin, yo 500 123 250 98 100 as P 50 203.a P 91 100 as P 75 20 as P 91
phosphoric acid because phosphate ion was expected to be the interferent. Two concentration levels for each compound were tested. At the higher concentration interference was obtained. Sodium nitrate enhanced production of tin atoms. Both phosphate and pyrophosphate decreased the concentration of tin atoms, presumably by the formation of thermally stable tin phosphate and tin pyrophosphate. At lower concentration, sodium nitrate showed no interference and the phosphorus com-
pounds gave results low by 9%. I n most peroxide samples, the concentration of phosphorus compound is even lower than the lowest concentration given in Table I, so no significant interference is to be expected from phosphorus compounds. CONCLUSIONS
The method was applied to a number of samples of hydrogen peroxide and comparison made with a spectrophotometric method ( I ) (Table 11). Agreement was good except for sample E47 where atomic absorption gave a higher value. Because of the difficulties encountered with polymer formation, it is believed the atomic absorption result is the more correct. Data indicate a standard deviation for this method of 0.03 mg. per liter tin or 10% of the amount present, whichever is greater. The method described is rapid and requires only that the hydrogen peroxide be diluted, a syringe be filled, the sample be injected in the flame, and the absorbance be measured. The spectrophotometric method, on the
Table II. Comparison of Atomic Absorption Method with Spectrophotometric Method
Tin. me. - Der . liter Atomic Spectroabsorption photometric 1 i n -.6 0.08 0.13 0.44 0.43 I
Sample No., 90% H202 E47 A146 EA64 A63 A96 A147
0.00 0.08
1.4
0.05 0.08 1.3
other hand, requires from 3 to 5 hours per determination depending upon how rapidly the hydrogen peroxide can be ss,fely decomposed. LITERATURE CITED
(1) Agazzi, E. J., Boone, D. J., Shell
Development Co., Emeryville, Calif., unpublished data, 1959. (2) Fuwa, K., Vallee, B. L., ANAL. CHEW 35, 942 (1963). (3) Gatehouse, B. M., Willis, J. B., Speclrochim. Acta 17, 710 (1961). RECEIVEDfor review October 7, 1964. Accepted December 15, 1964.
Determination of Azide ion by Hydrogen Ion Titration after Oxidation with Nitrite RAY G. CLEM and E. H. HUFFMAN Lawrence Radiation Laboratory, University of California, Berkeley, Calif.
b Measurement of the hydrogen ion consumed in the nitrite oxidation of azide is the basis for a simple and accurate method for determining azide ion in the range from 0.05 to 1.5 mmoles. The method is applicable to the direct determination of soluble azides, such as sodium azide, to the determination of silver azide after metathesis with sodium thiocyanate, and to the determination of lead azide after distillation of hydrazoic acid. Palladium azide requires chloride as a complexing agent in order to recover all of the hydrazoic acid by distillation. The precision is comparable to that of other weak acidstrong base titrations. Chloride, thiocyanate, nitrate, and perchlorate do not interfere.
A
has been analyzed by a variety of reactions. Macro methods, both gravimetric (9) and volumetric ( 7 , I f ) are based upon the low solubility of silver azide, and upon the quantitative oxidation of azide to ZIDE ION
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ANALYTICAL CHEMISTRY
nitrogen with cerate ( I ) permanganate (16), iodine (6) and nitrite (IS, 15) as titrants. Semimicro methods make use of the red complex FeNsi2 in a spectrophotometric determination (14) and of the Kjeldahl determination of nitrogen after thiosulfate reduction (22). A gas volumetric method involves the measurement of nitrogen liberated by cerate oxidation (3). These methods were developed for application to sodium azide and to primer components. Silver precipitation methods usually give low results because of the appreciable solubility of silver azide. The various redox titrimetric methods use titrants which must be standardized periodically. The color of FeNa+2 is unstable, varies with the pH, and is of no use in the presence of thiocyanate, a constituent of some primers. The Kjeldahl method is subject to positive errors; any ammonium ion or organic nitrogen present is counted as azide. The nitrogen volumetric method requires equipment and skills which may not be available for a n occasional analysis of azide. A recent paper re-
ported the analysis of palladium azide by a thermal decomposition technique in which the nitrogen evolved was measured (2). This method was not very satisfactory, as some samples were lost by explosion during analysis. The method presented here was developed to overcome most of these difficulties and is based upoii the stoichiometric consumption of two hydrogen ions for each azide ion oxidized by nitrite in a reaction which has been used in a different way ( 5 ) . This simple approach does not appear to have been used before, though it offers some advantages. Large amounts of chloride, thiocyanate, nitrate, ,and perchlorate do not interfere under the conditions specified and the titrants, perchloric acid and sodium hydroxide, are stable. I t is not necessary to standardize the unstable nitrite solution, as it is used in excess. As in some other methods, the azide must be in solution, and metal ions which hydrolyze at p H 8-9 must be absent; methods for meeting these conditions in the analyses of three insoluble azides are used a n d
