Precise Determination of Traces of Pyrophosphate in Orthophosphates R. H. KOLLOFF Monsanto Chemical Co., St. Louis,
Mo.
H. K. WARD and V. F. ZIEMBA Parker Rust Proof Co., Detroit, Mich.
b A rapid, accurate, and exceptionally sensitive (0.002% pyrophosphate per 0.001 absorbance unit) colorimetric procedure for determining trace amounts of pyrophosphate in orthophosphates is described. The complexing effect of pyrophosphate on ferric iron is measured at 435 mp in the presence of orthophosphate by a modified thiocyanate colorimetric method. Interferences and the necessary corrections are given, including a heretofore undisclosed wave length shift of the ferric thiocyanate absorption spectrum in the presence of phosphate.
T
for a uniformly low pyrophosphate content in sodium orthophosphates for certain industrial applications has stimulated the development of better methods for the determination of trace amounts of pyrophosphate in the presence of orthophosphates. In a recent paper (1) Chess and Bernhart summarized the objectionable features of prior procedures and presented a method based on the 1, 10phensnthroline colorimetric test for iron. Unfortunately, traces of iron and aluminum, which are often present in commercial sodium phosphates, interfered with their method, causing serious rrrors which could not be eliminated by the recommended corrective measures. The precision and sensitivity of the method given in Table I proved to be serious limitations when working a t levels of 0.10% pyrophosphate or less. The Fe+3thiocyanate system has been applied to the determination of traces of pyro- and metaphosphates in dilute solutions ( 7 ) ) and to mixtures containing relatively equal amounts of pyroand tripolyphosphates (4). Until now, sppliclttions to binary phosphate mixtures with ratios of ort,ho- to pyrophosphate of more than 100 to 1 have not been believed possible ( 1 ) . The investigations described here, however, have shown that under t,he proper conditions this ratio of ortho t,o pyro can be c,ztc.ndcd t,o a t least 10,OOC to 1. 11) 'Table I, two of the bcst prior iiroccdures for trace3 of pyrop!iosphnte i:: art,hop!mph3te are mmpiired x i t h HE DEMAND
the new thiocyanate test. The thiocyanate system recommended here is from five to ten times w precise and at least ten times as sensitive as the best of the previous methods (1, 2). This is an empirical procedure particularly well suited for routine quality control work, where many samples of relatively constant composition must L. analyzed. When samples of widely varying composition are to be assayed, the method loses most of its speed because of the supporting analyses which must first be made (see Interferences). However, corrections may be made in the case of the most common interferences. The principle of the method is simple. Pyrophosphate reduces the intensity of color of the ferric thiocyanate complex- [F~(CNS),+~-"]-(where n equals 1 or 2), primarily by complexing some of the iron ordinarily available to the thiocyanate (3-7). This reduction of a known intensity is quantitatively measured with a spectrophotometer or colorimeter and related to per cent pyrophosphate by a standard curve. Orthophosphate also affects the color intensity but the absorbance is insensitive to minor changes in orthophosphate concentration, and small corrections may easily be made when required. The thiocyanate system
Table I.
reaches equilibrium almost instantaneously, even in the presence of orthoand pyrophosphate and, at equilibrium, the color is indicative of the pyrophosphate concentration. Conversely, this may be compared to the 1, 10-phenanthroline system which reaches equilibrium slowly and does not accurately indicate pyrophosphate concentration a t equilibrium. EQUIPMENT
Cary recording spectrophotometer, M14, 1-cm. fused silica cells (multiplier phototube high-voltage control, 3; slit control, 25; tungsten light source). A colorimeter, such as the Fisher Electrophotometer, equipped with a 425-mp filter and either 1.0- or 2.3-cm. cells may be used with excellent results in routine work. REAGENTS
All chemicals should be of ACS reagent grade. Standard Iron Solution, 100 p.p.m. of Fe+3, 0.6N HC104. Dissolve 0.1W gram of reagent grade iron wire in 50 ml. of 1 to 1 HCl. Add exactly 5.0 ml. of 72% HC104 and heat to dense white fumes, expelling all the HCl. To avoid any loss of HCIOn, cool the concentrated solution immediately upon the appearance of the perchloric acid
Summary of Precision and Sensitivity of Three Leading Analytical Procedures
Method Thiocyanate
Standard Deviation Repeatability (Table IV) sa = 0.0037% NaaPzO! Thiocyanate Repeatability (Labs. A B)s = 0.0051 % N ~ Z H ~ P ~ O ~ Thiocyanate Precision (Labs. A B ) s = o.0053y0 NaLU"07 1,lO-Phenanthroline ( 1 ) s = 0 . 0 2 to 0.03y0NadP207 Paper chromatography (2) s = 0.05 Na2HzP207b
+
+
hlethod
Sensitivity,
'70Na,P207,per 0.001 Absorbance Unit
Level (% Pyro) 0.02 to 0 . 2
0 . 1 to 0 . 4 0.1 to 0 . 4 0.1 to 0 . 2
0.1 to0.5 Level
(yoNarPZ07)
Thiocyanate 0.002 0 to 0 . 2 1,10-Phenaiit,hroline ( 1 ) 0.054 0 to 0 . 2 1 ,lo-Phenanthroline ( 1 ) 0.038 0 . 3t o 0 . 5 Paper chromatography ( 2 ) 0.02 0 . 1 to 0 . 5 a s is cstimated standard deviation. Staiidiird deviation obtained in analysis of commercial NaHJ'O, (Table 11); I iower s of C 1 . 0 2YsrPz07 ~~ has becri obtained in the analysis of commercial ?;a2€IP04.2H20.
VOL. 32, NO. 12, NOVEMBER 1960
1487
iI
2.400
2 200
I I I
2 000
CURVE
'
\
c
\
I I
\ \ \
\
\
\
I
I
I
1.800
1.600
1.400
1.200
z *
c II)
:: 1.000 ID
0.800
0.600
0.400
0 100 0.I
//
0.200
CURVE
I
1
I
300
350
400
I
450
W A V E LENGTH
Figure 1.
500
-7--550
. , 600
Imp1
EfFect of orthophosphate on ferric thiocyanate absorption spectrum
Cory recording spectrophotometer, M14, 1 -cm. silica cells Solution composition Peak NoH2- HClOb "4Fe+3, Curve Symbol mp p H Pod,% N CNS,M p.p.m. A 478 1.6 0 0.06 0.13 10 E 435 1.6 0.5 0.06 0.13 10 c 475 2.0 0 0.12 0.26 40 D 400 2.0 4 0.12 0.26 40
2 2 2 1
PROCEDURE
Sample Analysis. Dissolve exactly 0.5000 gram of NaH?PO4 (or 0.5917 gram of Na?HP04) in a 100-ml. volumetric flask (low actinic glass) containing approximately 25 ml. of H 2 0 . Adjust sample size of unknonns to give the equivalent (+0.5%) of the abovc orthophosphate content. I n the case of NalHP04, add a 7.0ml. aliquot of 0.631 HClO, to the flask to convert phosphate to dihydrogen form. If sample is IVaHJ'O,, ehminatr this step. The pH of the final solution in either case should be ea. 1.6. Add a 10-ml. aliquot of standard iron solution and a IO-ml. aliquot of 1.3231 amrnonium thiocyanate when using 1-rm. optical cella. For the 2.3-cm. cylin1688
e
ANALYTICAL CHEMISTRY
{.r
N.
4
0 5
P 0 1 2 I
Figure 2. Typical standard curves of ferric thiocyanate in presence of ortho- and pyrophosphates 1. 4% NoH2POd (also see curve D, Figure 1 ) X = 4 7 5 mp 2. 0 . 5 9 ~ N o ~ H P O ~ n o F e i n o r t h o p h o s p h a t e 3 0 59 0 Na2HP04, 100 p.p.m. of Fe in orthophosphate 4: 0 : 507NaHzPO,, no AI in orthophorphote 5. 0.5% NoH?PO&,0.170 A1203 in orthophosphote Curves 2-5, prepared as under procedure (Fisher Electraphotom. eter, 1-cm. cell, 425-mu filter)
__ ____ -_-
furncs. After cooling, add an additional 45.0 ml. of 72Yc HClO4 and dilute to 1 liter. Aluminum stock solution (for use in standard curve when analyzing samples Dissolve containing ca. 0.1% &&). 1.0576 grams of reagent grade A1 wire in ca. 75 ml. of 1 to 1 HCl, adding more acid only if nccded. Ji'hen dissolution is complete, add 10 ml. of 72% HC10, and heat to dense white fumes of HC104. Cool and dilute to volunic in a 1000-ml. volumetric flask. Dilute an aliquot to a convenient concentration (500 p.p.m. of A1203)bcforc using.
9.4
0.3
0.2 % Na H P 0
d\,
drical cuvettes (Fisher Elcctrophotometer) reduce the latter aliquot to 5.0 ml. of 1.32214 NH4CNS solution. Immediately after the addition of the KH4CNS, dilute to 100 ml. with HzO, mis well, and start the clock-timer. Read total absorbance a t ea. 435 mp with the spectrophotometer (Figure 1) or the 425-mp filter with the Electrophotometer between 10 and 30 minutes after starting timer. Compute the per cent of NazHZPsOi from a proper standard curve. The colorimetric analysis should be carried out a t a constant (*lo C.) and known temperature, and carried out a t the same temperature as was the standard curve (see Interferences). Standard Curves. Separate standard curves should be constructed for different types of phosphate samplesIVaH2POI, S a r H P 0 4 , or other watersoluble orthophosphates-using the proper orthophosphate salt. The standards should contain pyrophosphate-free orthophosphate cquivalent to 0.500 gram of KaH2PO( per 100 nil., as well as appropriate increments of reagent grade tctrasodirim pyrophosphate. 'iVhcn preparing the reagent blank, omit the pyrophosphate and substitute a 10-ml. aliquot of 0 . 6 S HClO, for the iron aliquot. A large numbrr of samplcs containing about the same ievcl of an interfering
impurity (Figure 2) should be analyzed with the aid of standard curye containing that impurity. Reagent grade orthophosphate may be checked for pyrophosphate by applying 150 pl. of a 2% (as NazHPOl. 2H20) solution to 30 spots in a line 1 inch from the bottom of a 9 X 5 inch sheet of S PE S 589 chromatographic paper (2). Carry eaniple and blank sheet through a 30-minute (3- to 3l/*inch) run in an acidic 2-propanol solvent. Develop the chromatogram, cut out the pyro band, and analyze for pyro P205 [(a),p. 1093, paragraphs 3 and 41. Calculate per cent pyro from a standard curve containing reagent grade hTarP20i. The standard curve need not be carried through the Chromatographic separation, but' should parallel the rest of the sample analysis, including the leaching and hydrolysis steps in the presence of equivalent amounts of chromatographic paper (blank sheet) (2). INTERFERENCES
The ions interfering with the colorimetric thiocyanate determination of iron [Cu, Bi, Ti, U, Mo> Co+', M n , Hg+3, Cd, Zn, Sb+3, CZO~-?,F- (5Jl generally are eithcr absent or are present a t noncritical levels in commcrcinl mono- or disodium phosphates. In applications to unknown orthophosphates, these interferences should be considered. Any anion that will complex iron will intcrfcrr-Le., long-chain phosnhatcs. Escept for orthophosphate, this . . y p of ~ interference has not been detritcd in
the commerc~ial mono- or disodiuni orthophosphates tested. Long-chain phosphates were not detected by paper chromatographic analysis ( 2 ) . Any cation which is complexed by pyrophosphate \vi11 interfere-Le., Ca +2, Mgf2, A1+3, Fet3. Aluminum alters the slope of the standard curve and changes the intensity of color. Small variations in aluminum level (0.01% &03) are insignificant; largcr variations (0.10% A1203) cause mcasurable errors. As long as the amount of aluminum is known and remains below ca. 0.4% A1203, adequate corrections may be made (Tables I1 and I11 and Figure 2). Ca+? and MgC2 were not investigated as they have not been found at int,erfering l ( w l s in mono- and disodium orthophosphate. Iron in the sample intc,rfercs by raising the absolute int e x i t y of color and changing the slope of the standard curve very slightly. Adequate corrections can bP made easily when the amount of iron is known and is btdo\v 500 p.p.m. Higher levels of iron were not investigated. but ran very probably be tolerated (Tables 11, 111, and IV, and Figure 2). Corrcctions for iron i n S H I C S S and HClO, reagents added to the sample solutions are grncrally very small (0,003y0 SaJ'J);) and often may be neglccted in routiiie work. Although t h r Fe+3 t1iiocyan:ite (:ompl(3 is c3omp:irativcly stable, the prepondtmnce of orthophosphate signifirantly lowers the absorbance of Fe(CKS),2+3-n. Compensation for orthopliosphate is made by construction of standard curves a t various phosphate levels (Figure 2). 'l'lie effect of variations in per cent orthophosphate. in the sample was minimized and srnsitivity to pcr rent pyrophosphatr was maximizcd by working a t a low p H (1.6), at a relatiwly high thiocyanate (0.13iM NH&?;Y) rind iron (10 p.p.m.) concentration, and a t a low sample concentration (0.5yo). Both ortho- and pyrophosphates have a pronounced effect on the wave length of the pe:A absorbancy of Fe (CNS),+3-", thc wave length decreasing with increasing pliosphate conccntration. This can be esplxinrd by the formation of complexes of Fe (CSS),+3-n with H2P207-2, H3P207-, and H2P04- (Figure 1). I n the absence of phosphate, the ferric thiocyanate systrm is relatively pH insensitive (4,5> 7') but p H becomes a criticma1 factor when phosphate is present (4.7 ) . The excess orthophosphate i n thcl tcast solutions acts as an excellent biiffer, ho\vtt\-er, and gives the rcquired pI1 control. 'lie pH rhosen is close to the optimum for ~rinxiniunisrnsitivity and accuracy. Lyltraviolet liglit r r d u c t , r the color intensity, presumably by catalyzing the rdiiction of Ft,+3t,o I++'. The color is st:il)lc fur at 1cn.d 30 minutcs xvhm
Table 11.
Typical Interferences Encountered with Thiocyanate Procedure When Analyzing Na2HP04 and NaH2POJ 70
Intcrfercnca +50 p.p.m. F e f 3
- 1"/c NaH2P04
0.1 0.4
0.09 0.35 0.09
0.1 0.4
+5" C. dev. from std. curve temp. (25' C.) +5" C. dev. from std. ciirvc temp. (2,s"C.)
0.087
-0.012
-0.05
0,349 0.09
-0.01 -0.01
0 39
0.1
...
0.164 ( NaH2P04)
0.1
...
0.149 (Na?HPOo)
+0.055
Accuracy and Precision of Thiocyanate Procedure in Analysis of Commerical NaHsPOd (MSP) and NaPHP04 (DSP)
c (LISP)
22
D (>LISP)
22
0 1
...
78A (DSP) 78B(I)SP)
-0.02 -0.024
0.088 0,382
.4ssay
(yoNa2H2P207)
Impiirity Lrvcl yo P , ~ , ~Thiocyanate, , Lab A Sample iZl?Oj Fe Run 1 Run 2 Av. A (RISP) 0 1 17 0 390 0 396 0 393 B (RISP) n 1 14 o 430 o 433
F (LISP)
(%
Na2H~P20i)
Lab B
0.39
____
E (AldP)
Av. Error
yo iia2H2P20i Found Lab A 0.07 0.37
0.4 0.1
+o. 10% A 1 2 0 2
Table 111.
h'a?
H2P2Oi Added
16 30
109 ...
95
0 140 0 039 0 425
0.183 0 .O G l b
Tlcocyanate, Lab B Run 1 Run 2 Av. 0.404 0 383
n.
0 059
0 429
0 41*3
0 13
0 OGI
0 061
0 424
0 42G
0.182
0 .051b
0.004* 0.064*
0.185
0 186
0 . 050*
...
0.064*
0 42
0 1:19 .__ O.Oi
0.420
0.180
0 420
0 138 0 110 1 4s .~
0 059
(2)' 0 40
0 394 0 424
0 434
0 14G
Referee Paper chrom.
0 30
0.15 0.07
...
0.08*
...
...
.4v. of 2 t o 3 determs. Yo I'JaoP20,. Repc,atability: samples 78A and B not included; av. range of duplicates ( 8 )= 0.00575; std. dev., s, = 0.0051. Precision: samples 78A and 13 not iircliided; av. range for sarnllk ( P ) = 0.011; std. dev., s = 0.0053. a
protertcd from light by low actinic glass. A 10-minutr exposure of the comp1r.s in a fused silica I-rm. optical cell to a fluorescent light (15-watt, 12 inchesfrom the cell) resulted in a 17% analytical error. Exposure to daylight, of course, would magnify this error. A 10-minute exposure in the same cell to a long-wave ultraviolet light resulted in approximately twice this error, or a 34% deviation from the true pyro value. The comples irradiatrd for 30 minutes in a 1-cm. silica cell in the Cary spcrtrophotometrr (W light, dit control, 25; A, 435 to 500 m ~ ) showed , no decompcsition. The tffcct a t shorter w v r lengths in the Cary KLS not inyestigated since absorbance ~waeurcmrntsarc not ordinarily uscd i n thiq rcgion. The f t w i i . thiocyanate phosphate system i, tcinpc'ratiire-sensitive, the xbsorbmc,y itwrcnsing by about 0.5% per O C. drop. Dropping tho temperature from 35" tn 0" C. t'auses about a 307, increasr i l l intcnsity of color and :thout n ;2cc incrcnse in srnsitivity.
Thus the optimum operating temperature would be ca. 0" C., but the difficulties involved in manipulations at this temperature were felt to outweigh the advantages over rooin temperature. DISCUSSION OF EXPERIMENTAL RESULTS
Table I11 sliows the exrcllt~ntagrecment obtainable between two laboratories when using independmtly prrpared standards and reagents-. The agreement with the referee method ( 2 ) is also very good, considering thc prccision of paper chromatography :it thrsc levels. Table IV gives t,he rrsults of one operator assaying nine commtwial XanHPO, samples in triplicatr. The population standard deviation estimatcd ( ~ ' = 0 . 0 0 4 % Sa,P,Oii is a good approximation of the optimum repeatability obtainablc with this procedure by a single operator in a given laboratory. Sincc the laboratory (.4) in qucstion wts air conditionrd and hacl VOL. 32, NO. 12, NOVEMBER 1960
1689
Table IV. Repeatability of Thiocyanate Procedure on Commercial NazHPOl
P.P.M. Fe 9 9 6 6 36 20 27 33 15
Assay ( 76 Na4P207) R u n 2 R u n 3 Av.
Run 1 0.010 0.229 0.031 0.021 0.050 0.046 0.018 0.036 0.081
0.019 0.017 0.230 0.230 0.031 0.035 0.021 0.024 0.057 0.064 0.056 0.046 0.018 0.018 0.043 0.046 0.088 0.084 Av. range ( R ) = 0.0062. Std. drv., s = 0.0037
0.015 0.230 0.032 0.022 0.057 0.049 0.018 0.041 0.084
little temperature fluctuation ( 3 ~ 0 . 5 C. ” per 24 hours), no constant temperature bath was used. Since there is a nearly linear relationship between temperature and absorbance, more precise temperature control ( j ~ O . 1O C.) should improve the repeatability somewhat (Table 11). All pyro values in Tables I11 and IV are corrected for iron and aluminum content and were spot-checked by paper
chromatography to verify the absence of other interfering species. Figure 1 shows the effect of orthophosphate on the thiocyanate-iron complex. Curve C (peak A = 475 mp) contains no phosphate and represents conditions normally recommended for the routine determination of iron. (For clarity, curve C is reduced by a factor of four.) Curve D (peak X=400 mp) represents the same conditions except that the solution contains 4% NaH2P04. Curve 1 of Figure 2 is a plot of absorbance us. per cent Na2HzPz07 a t 475 mp and is at the same conditions as curve D, Figure 1. These curves show the effect of applying the thiocyanate procedure directly without modification to the determination of pyrophosphate. The original absorbance peak is shifted from 475 (no phosphate) to 400 mp (with phosphate) and is reduced in intensity by about twentyfold. The sensitivity at 475 mp would be ca. 0.02% NadP207 per 0.001 absorbance unit. By properly adjusting the pH, the reagent and sample concentrations, and working at the correct wave length, the effects of orthophosphate can be
minimized (curves A , B, Figure 1) and the sensitivity increased to 0.002% Na4P207 per 0.001 absorbance unit (curves 2 to 5, Figure 2). Curves 2-5, Figure 2, also show the effects of two impurities, A1203 and Fe. These impurities react independently in the presence of one another, greatly facilitating the necessary corrections. LITERATURE CITED
(1) Chess, W. B., Bernhart, D. N., ANAL.CHEM.30,111 (1958). (2) Karl-Kroupa, E., Ibid., 28, 1091 (1956). (Also modifications thereof for
specific applications to trace analysis, private communications.) (3) Kobayashi, M., Tada, S., Shinagawa, M., J. Sei. Hiroshima Univ., Ser. A
21,27 (1957). ( 4 ) Maurice, J., BulE. SOC. chim. France 6,819 (1959). ( 5 ) Sandell, E. B., “Colorimetric Determination of Trace Metals,” p. 369, Interscience, New York, 1950. ( 6 ) Van Wazer. J. R.. Callis.’ C. F.. Chem. ‘ Revs. 58, 1011 (1958). (7) Wirth, H. E., IND. EKG. CHEM., ANAL.ED. 14,722 (1942).
RECEIVEDfor review April 6, 1960. Accepted July 15, 1960.
Determination of Bismuth as Bismuth Phosphate by Precipitation from Homogeneous Solution HARLEY H. ROSS and RICHARD B. HAHN Department o f Chemisfry, Wayne Sfafe Universify, Detroit, Mich. ,Metaphosphoric acid is used to precipitate bismuth as bismuth phosphate from homogeneous solution. The method is more rapid than standard methods, requires fewer manipulations, and has no critical separation steps. The metals usually alloyed with bismuth do not interfere, or they are easily separated. The range of bismuth which can be determined i s 1 to 250 mg., olthough larger amounts could probably be determined without changing the general method. The precipitaie obtained is dense, crystalline, and easy to collect and wash. The volume of the precipitate is estimated to be about ‘/a to ‘/w the size of the same amount of precipitate obtained in the conventional phosphate precipitation procedure. The method was compared with the standard method for the determination of bismuth in bismuth alloys. The results are in very close agreement.
oxybromide in acid solution (2). Phosphate methods are used frequently for the determination of bismuth (1, 8-6). The resulting bismuth phosphate precipitate is bulky and difficult to filter and wash. Many impurities are carried down in this precipitate. This work is concerned with the separation and precipitation of bismuth phosphate from homogeneous solution using metaphosphoric acid. This reagent hydrolyzes in acid solution forming orthophosphoric acid which precipitates bismuth phosphate. A similar procedure was developed for the precipitation of zirconium by Willard and Hahn (6). This paper discusses the general method, its useful range, interferences encountered, and a procedure for the determination of bismuth in bismuth alloys.
M
A standard bismuth nitra?e so!ution was prepared by dissolvkg 57.3 g r a m of reagent grade bismuth n,lr:itt: pentahydrate in 200 ml. of concentru ted nitric acid and diluting to 2 !iter> with distilled water. The solution \vas ,stTnd-
have been proposed for the determination of bismuth and for its separation from lead. Bismuth i s best separated from lead by the precipitation of bismutb oxychloride or 16%
ANY METHODS
ANALYTICAL CHEMISTRY
REAGENTS
ardized by precipitation and weighing as bismuth phosphate and by precipitation as the basic carbonate, weighed as BizOs, using the procedures given by Hillebrand and Lundell ( 2 ) . Solutions used for the studies of interferences were made by direct weight without standardization. A 10% metaphosphoric acid solution was freshly prepared each day by dissolving 20 grams of c . ~metaphosphoric . acid in 200 ml. of water which was acidified with 2 ml. of concentrated nitric acid. The solution was filtered before using. EXPERIMENTAL
A general procedure for the determination of bismuth was used throughout the course of the work except in the cases where only a single precipitation was required. Procedure. Weigh out a sample of the alloy which contains 100 to 200 ing. of bismiith. Add 15 to 20 mi. of concentrated nitric acid and, aftcr the reaction has ceased, warm and dLg& for about 15 minutes. Dilute with a n equal volume of distilied .vater and filter off a n y metastannic a:id through a Gooch crucible with ai? :isbestos miit. Wash the residue
