V O L U M E 2 7 , N O . 3, M A R C H 1 9 5 5 C0NCLI;SIOY
401
s
It will lie seen from Table I that the new method al1on.s the conil)iiied acetic acid cont,ent of secondary acetone-soluble wllulore acetate to be determined with great accuracy. Small tlifterences in the results ohtaineti by the new method and by the volumetric method can he justified by experiment,al errors and by the fact that salts and ot,her n-ater-soluble impurities, a1n:iys present in the commercial products, give iiworrect results in a tli:miet,rically oppoFite \my. The gravimetric. method can be used to help establish the :icc.ur:icy of other methods including those using saponification, : i n t i to analyze mixed esters :ind other esters.
LITERATURE CITED
Division of Cellulose Chemistry, Committee on Standards and Methods of Testing, A r a ~CHEM., . 24, 400-3 (1952). Dorbe, C., “Methods of Cellulose Chemistry,” pp, 285-9, Chapman & Hall, London, 1947. Eberstadt, O., dissertation, Heidelberg, 1909. Heuser. E., “Chemistry of Cellulose,” pp. 277 -9, Wiley, Sew York, 1947. Kruger, D., ”Zelluloseaaetate,” pp. 218-28, T. Steinkopf, Dresden, 1933. Ost, E., and Katayama, T., 2. a v g r w . rophosphate but not triphosphate at pH 6.5. The of the hexammine- and tris(ethy1enediamine)cobalt phosphates, precipitates dried at 110’ C. are C O ( ~ ~ ) ~ H Z P @ ~ O . ~silicates, € I Z O and su1f:iter is shown in Table I. Sulfate and silicaate and Co(en)s€IPzO~.Orthophosphate, trimetaphosphate, were included hecause thrir separation from t,he phosphates is and tetrametaphosphate are not precipitated. Trioften a problem. Tris( propylencdismine)cohalt( I11) chloride phosphate can be precipitated from a mixture which was also tested, but it did not preripitate any of the phosphatrls contains pjrophosphate, but some of the latter is coexcept the polyphosp1i:rte n-ith the longest chnin length. Trisprecipitated and some of the triphosphate is left in (ethylenediamirie)cobdt(III) chloride showed promise as a spc~solution. The distribution of plrophosphate and trirific precipitant. for of the phosphates and other anions testetl phosphate between precipitate and solution was deteronly pyrophosphte :inti triphosphate precipitated anti the.sc mined by phosphorus-32-tagged phosphates. Co(en)iprecipitates n-ere otitained at different p H values. With hcJxCIS may prove a valuable reagent for triphosphate, amniinecol~alt(II1)chloride, liyrophosphate and triphosp1i:itt’ notwithstanding the influence of plrophosphate on the both precipitated at d l pI-1 values, indicating that i~ niethotl precipitation of triphosphate. In contrast to Co(en)ifor one in t,he prcsence of the other was unlikely. VI?, hexamminecohalt(II1) chloride [Co(”&CIj] prechloride ( I ) iCo( SHa)6C‘l:i cipitates PjOlo----- and l’*O;----, instead of IbP3Materials. Hr?;snimir~ec~~balt(III) and tris(ethylenediamine)cohalt(III) chloride ( 1 4 ) [Co(en)3CI3] and HPzOi---, and the yield of both is increased were prepared according to puhlished procedures. Tris(prop.1h! increasing plI. Orthophosphate is also precipitated, enediamine)cot)alt(III) chloride [ C ~ ( p n ) ~ C l was a ] prepared i i ? . so Co(”&C13 is not a potentially valuable reagent. t>hesame method as Co(en)aCI~,making allowance for the different molecular weight of propylenediamine. -411 were recrystallized The triphosphate precipitate dried at 110’ C. is Na[Coand dried a t 110” C. hefore use. Their identities 1T-ei-e verified hy (Y C11)613(P3010)2. analysis. Sodium triphosphate v a s prepared from a commercial product liy salting it out of aqueous solution n-ith alcohol four times and, E S ~ l I M I S C C O B A L T ( I I I chloride ) has been suggehted as a reagent in qualitative microchemical tests for p~ rophosphate (3) and triphosphate ( I O ) . Recently the amperoTable I. Precipitations with Excess Reagent“ metric titration of pyrophosphate m ith this reagent has been CO(SH~)OC b II Co(en)~CIs b tiescrihed (6). The possibility of a stepwise variation in com~pH PH position, size, and charge made Kerner cations an attractive Sodium Pnlt in Solutiirn 12.3 7.5 L.5 12.5 7 . 5 4.5 class to investigate as specific precipitants for the various phosPolyphosplintrs phate anions. 1 I> .., = -1 1 5 = 31 I, This background led to a study of heuamminecobalt(II1) Tr.iphosphate X Pyrophosphate 0 and the related tris(ethy1enediamine) and (propylenediamine)0 Orthophosphate cohnlt(II1) ions as precipitants for use in the determination of 0 Tetrametaphosphate 0 Trimetaphosphate pyrophosphate and triphosphate in the presence of each other 0 Sulfate 0 Silicate, SiOt/KaiO = 2 . 0 and in the presence of orthophosphate, trimetaphosphate, and a I n 50 ml. of nierlianicnlly aeitated solution a t room temperature there other phosphates. l I o s t of this paper deals with the precipitawere 3 00 millinioles of cobalt re&ent and 0.250 gram of sodium salt except tion of phosphates, especially pyrophosphate and triphosphate, for triphosphate (1.65 niillirnoles) and sulfate, orthophosphate, pyrophosphate (2.50 millimolr-). T h e pH wns maintained constant with S a O H -,1 hexammine- and tris(ethylenediamine)cobalt(III) chlorides, or HCI. b X = precipitate; 0 = no precipitate; L = an oily liquid separated. the description of resulting precipitates, and the coprecipitation of pyrophosphate with the tris(ethylenediamine)cohalt( 111)
H
~
ANALYTICAL CHEMISTRY
402 -~
about pH 10, although the hexamminecobalt(II1) to triphosphate ion ratio was maintained, the gravimetric yield was 1 [ 4 0 M I . of solution, titrated with 0.0549M Co(NHajeClr1 to 2 % high and analysis showed a slight Taken, Millimoles (a) (bj (C) Results, M1. Rel. decrease in per cent nitrogen, cobalt, and KO. NarPzO; NasP30lo NaZHPOr Found Theor. Error. % p H a phosphorus, possibly due to the inclusion 1 ... 0.150 ... 4.09 4.10b ... 0 . 2 12 of some alkali. 2 ... 0.150 ... 4.13 4.10b ... 0.7 12 0.150 ... 4.13 4.10b , . . 0.7 11 Tris(ethylenediamine)cobalt(III) 3 4 0:209 0.150 ... 7.87 7.91 a t b ... -0.5 8.8 8.8 Phosphates. ilmperometric titrations 5 0.209 0.150 ... 7.93 7.91 a t b ... 0.2 6 0.209 0.300 12.01 12.01 a + b 0.0 8.5 (Table 111) indicated that for precipi7 ... ... O.'ljbb 2.69 0.OOa 2.73'0' ' ... 12 0.150b 8 ... 2.58 0.OOa 2 73 c ... IO tates of both pyrophosphate and tri0.150b 3.8i 3 81 a 9 0:ZOQ ... 6,54 a + c ... .. 10 0.209 ... 0.15Ob 6 37 3.81 a 6.54 a + o ... 12 p h o s p h a t e the tris(ethy1enediamine)11 0 , 2 1 8 ,,, 0.15Ob 3.55 3.97 a 6.70 a + c ... 8.7 cobalt(II1) phosphate ion ratio was . . to -~ p H was measured after titration. Before titration S a O H or NHs was added except t o sample 1 to 1. Analysis of precipitates dried numbers 4, 5 , 6, and 8,which were not adjusted, and t o 8, t o which was added +",-a buffer. a t 110" C. showed their compositions to b Only 10% alcohol was present. be C 0 ( e n ) ~ H P ~ 0and 1 C0(en)~H~P~0~0.2H2O. Analysis, calculated for Co(en)8HPZO;: X, 2 0 6 ; P, 14.9; titratable after air drying, was weighed as NajP3010.6HnO. The water loss H, 0.243; found: N, 20 3; P, 15.1; titratable H, 0.253. on heating was 22.8% (theory 22.7%) and the per cent phosphorus Analysis, calculated for C0(en)~HZP30~0 2H20: N, 15.8; P, 17.5; pentoxide as determined by titration after hydrolysis was 44.5% titratable H, 0.380; HzO, 6.80; found: S, 16.0; P, 17.5: titra(theory, 44.7%). Reagent grade sodium pyrophosphate, Naatable H, 0.377; HzO, distillation followed by Karl Fischer 6.64. Pz07, was recrystallized and dried. Preparation of radioactive pyrophosphate and triphosphate for tracer experiments is deThe same x-ray powder diffraction pattern was obtained for scribed elsewhere (11). Sodium trimetaphosphate, Na!P30s.Hz0, the air-dried triphosphate precipitate as was obtained after drying was recrystallized a t 50" C. from an aqueous solution of the a t 110" C. Crystals sucked dry on the sintered-glass filter concommercial material (Monsanto Chemical Co., St. Louis, Mo.). tained 7.9 to 8.4% water. Apparently the precipitate came Sodium tetrametaphosphate was salted out of a water solution of Cyclophos (Victor Chemical Works, Chicago, Ill.) with ethyl down as the dihydrate and was not dehydrated by oven drying. alcohol a t 35' C. Usually ill-formed tabular crystals showing parallel extinction The sodium silicate was weighed from an analyzed clear stock were observed but sometimes bluntly terminated needles pith solution made by diluting and filtering a commercial product. oblique extinction were also present. On the universal stage The silicon dioxide-sodium oxide weight ratio was 2.60. The polyphosphates were commercial samples characterized by the the needlelike crystals proved to be identical with the tabular average number of phosphorus atoms per ion, 5,assuming only crystals. Optical properties and x-ray powder diffraction data linear ions present. The chain lengths were calculated from the useful for identifying these and the pyrophosphate crystals titration between end points before hydrolysis, W , the titration have been published ( 7 ) . between end points after hydrolysis, S , and the determined Table 11. Arnperometric Titrations with Hexamminecobalt(II1)Chloride
'
orthophosphate ( 8 ) , 0, from the relation, 5 =
w2s+ 0'
~
This is
similar to the chain length formula used by Van Wazer (12) but differs in that it gives the average number of phosphorus atoms per - ion including orthophosphate. The polyphosphate with n = 10 was Calgon (Calgon, Inc., Pittsburgh, Pa.) and with ? =i 5.1 was Quadrofos (Rumford Chemical Works, Rumford, R. I.). Hexamminecobalt(II1) Phosphates. In the presence of sodium ion Jorgensen ( 4 ) and later workers (6) have shown that the precipitate with pyrophosphate, neglecting water of crystallization, is NaCo(XH3)8P2O7. With trisodium orthophosphate the precipitate CO(NH:,)~POI has been reported (2, 9). PH
Table 111. Titration of Triphosphate with Tris(ethy1enediamine)cobalt(III) Chloride 140 M1. of solution titrated with 0.0487M Co(en)aCIa and buffered with 0.15.M NaAc-1.8M HAC, p H 3.7 t o 4 11
Sample, Millimoles Na4P207 NasPsOla 0.2000 0 2000 a
Results" Used, ml. Rel. error, % 4 18 4 20
2 2
Theory, 4.11 ml.
Amperometric titrations of triphosphate with the reagent (Table 11) showed that the precipitate contained three hexamminecobalt(II1) ions per two triphosphate ions. The precipitate contained sodium, so that its formula is presumably Na[CO(NH~)~]~(P~O Analysis ~ O ) Z . of the precipitate made a t pH 9.0 and dried a t 110" C. supported this formula, although close checks for the theoretical nitrogen content were not obtained. Analysis, calculated for N ~ [ C O ( N H ~ ) ~ ] ~ ( P ~N, O ~24.9; ~ ) Z : P, 18.4; Co, 17.5; found: N, 23.8; P, 18.4; Co, 17.8. Above
Figure 1. Effect of pH on precipitations with Co("3)&13 Co(NHs)aClt 0.060M A . NaaPaOio 0.025M B . N a P 2 0 ~0.050M C. NasHPOd 0.050M
The pyrophosphate precipitate taken from the filter contained 4.3 to 4.5% water and after drying a t room temperature and 47% humidity contained 4.02% water, indicating a monohydrate (theory, 4.17%). The x-ray patterns of the air-dried and anhydrous precipitates were different ( 7 ) . Effect of pH on Completeness of Precipitation. The curves of Figure 1 show the per cent of theoretical gravimetric yield obtained when orthophosphate, pyrophosphate, and triphosphate were separately precipitated with hexamminecobalt(II1) reagent a t various definite pH values. The pH as indicated by a glass electrode was maintained constant with sodium hydroxide or hydrochloric acid. Data for similar experiments with tris(ethylenediamine)cobalt(III) reagent are given in Figure 2. The maximum yields of Co(en)3HP~07 and Co(en)~HzP8010. 2H20 were obtained a t about pH 6.5 and 3.5, respectively, where HP107---and HzP3010--- should be present in nearly the maxi-
V O L U M E 27, NO. 3, M A R C H 1 9 5 5 mum concentrations. This type of curve, where the yields pass through maxima a t certain pH values, contrasts with those obtained with hexamminecobalt( 111) chloride where the salts were normal phosphates and the yield remained high above a certain pH. The curves of Figure 2 suggest that tris(ethy1enediamine)cobalt(III) ions may be useful in detecting and determining triphosphate in the presence of pyrophosphate. Precipitations with Tris(ethy1ene d i a m i n e ) c o b a l t (111) Chloride. In solutions of pure salts, triphosphate p r e c i p i t a t e d almost quantitatively (98 to 99.570) between pH 3.0 and 4.0 and pyrophos4 8 12 phate precipitated in yields of around 93% PH at p H 7.0 (Figure 2 ) . Figure 2. Effect of pH on preHowever, when both cipitations with Co(en)aCla phosphates were in NaaPaOu ( A ) and NadPZOr ( B ) were 0.020 to 0.050M and Co(en)aCla was in excess the same s o l u t i o n , 0.021 to 0.060 M precipitation of triphosphate from solutions containing from about 3 to 1 to 1 to 1 molar ratios of triphosphate to pyrophosphate (Table IV, 20 to 40% sodium pyrophosphate) gave yields in excess of 100% [the yield being based on the amount of triphosphate present and the composition Co(en)jHzPsOlo2H201. In solutions containing excess pyrophosphate precipitation iTas far from complete (Table IV, 50 to 70% sodium pyrophosphate). Radioactive pyrophosphate, Ka4P3i07, was used to check for coprecipitation of pyrophosphate with triphosphate and Xa5P3;Ol0 was used in other experiments to determine the concentration of triphosphate ions in the solution. Results obtained with tagged triphosphate and tagged pyrophosphate are given in Table IV The values in parentheses were calculated assuming the pyrophosphate to be present as Co(en)sHP20; (see Discussion). Those for triphosphate in solution were calculated from the triphosphate taken, the weight of the precipitate, and the weight of Co(en)aHPzO, as determined from tagged pyrophosphate; the values for pyrophosphate in the solid which are enclosed in parentheses were calculated from the weight of precipitate and the lveight of Co(en)3H~P3OI02H20 in the precipitate as determined by tagged triphosphate. The solubility of Co(en)aH2PaOlo2H20, determined by the use of tagged triphosphate, under conditions of precipitation (room temperature, p H 3.5, 0.0018M excess C ~ ( e n ) ~ C lwas ~ ) ,3.01 mg. per 100 ml. The contamination of the Co(en)3H~P30102H20 precipitate by pyrophosphate is proportional to the mole fraction of the
Taken, NarPzO; 0 000 0 000 0.138 0.267 0.267 0.415
0.554 0.692 0 092 0.692 0 969 0 969
403 dissolved phosphate, before precipitation, which is pyrophopphate (Figure 3). Powder x-ray diffraction data for the oven-dried samples of Table I V did not show a second phase even for the precipitate with the greatest contamination. The absence of lines in the back reflection region and the width of the lines macle it impossible to measure accurately the slight changes which appeared as the amount of pyrophosphate in the sample increased. Houever, for the spacings of 2.63 and 2.40 A. there seemed to be a regular increase in the diffraction angle, 20, which amounted to about 0.5" going from a sample with 0% to one with ll.E17~ pyrophosphate as Co(en)aHP207. The pattern for a sample with 15.5% contamination was similar but showed some discontinuities such as the spacing formerly of 2.63 A. merging with one formerly of 2.57 A.
PYROPHOSPHATE, MOLE FRACTION OF DISSOLVED PHOSPHATE
Figure 3.
Pyrophosphate contamination of Co(en)lH~P3010.2HzO precipitate
Attempts to precipitate pyrophosphate in the presence of triphosphate a t a pH of 6.5 failed in a solution containing a mole ratio of one pyrophosphate to four triphosphate [O.O2001Cf Na*P&&, 0.0800X Ka5P3010,0.1200M Co(en)~Cla]and the yield was less than for pure pyrophosphate at a mole ratio of two pyrophosphate to three triphosphate (total phosphate concentration O.lOOOM, reagent 0.1200M). When more than half of the phosphate was pyrophosphate about the same yield as for pure pyrophosphate was obtained (93% for 0.20034 NaaP207 solution with reagent 0.0240M). The apparent solubility of c 0 ( e n ) ~ H P ~ ; 0 in , a solution 0.01OOM in P3OI0----and 0.0230M in cycess Co(en)3+++a t pH 7 was determined by the Pa2 tag to be 0.98 mg. of C O ( ~ ~ ) ~ H perP ml. ~ O ~In addition the precipitate must have contained triphosphate, for the precipitate contained only 94yo of the P207----required by its weight and formula. The purification by repeated precipitation of a radioactive pyrophosphatecontaminated precipitate, prepared by Table IV. Precipitation of Triphosphate in Presence of adding Co(en),Cls to a solution of pyroPyrophosphate with Co(en)aCls phosphate and triphosphate (315 ml., [Total volume 110 nil : pH 3 5 ; Co(en)zCls 1.20 millimoles] 16.0 millimoles of C0(en)~C13,14 0 milliFound, by Radioactive Count, Millinloles moles of NaOP3010,and 3.6 millimoles Millimoles ~ ~ ~ ~ \-iPld, . i ~ , , ~Triphosphate_ ~ ~ i ~ Pyrophosphate KarP3i0,), a t pH 2.5 is shown in FigNaaPaOlo % of Thcor Solid Solution Solid S o h tion ure 4. The precipitate was filtered off 1.000 99 1 1.01 0 00ci2 .... 1.000 98.7 1.01 0.00R2 .... a t each step and a sample was taken 0.900 100 ... (0.024) 0.0318 o'io7 for counting, the remainder being dis0.800 101 ... (0.02R) 0.0603 0.221 0.800 102 ... i o ,023) 0.0523 0.225 solved in dilute sodium hydroxide solu0.700 101 ... (0.044) 0.0622 0.360 0.600 101 i o 058) 0,0827 0.479 tion and reprecipitated by adjusting the 0.500 99.3 0:ii4 0 0512 (0,068) pH to 2.5. Q.3 5 0.500 ... 10.077) 0.0703 0 : iil 0,500 102 (0.054) 0.0812 0.601 Discussion. The purification of the 0 300 01: 5 o.'im 0.131 (0.040) 0.300 72.7 ... (0.116) 0.0432 o.iib pyrophosphate-contaminated t r i p h o s ___-___ phate precipitate by repeated precipita-
ANALYTICAL CHEMISTRY
404 tions (Figure 4) is much slomer than would be experted for a simple mixture of components, one of n hich (pyrophosphate) is soluble in the absence of the other under the conditions of the precipitation. A straight line would represent a distribution of pyrophosphate between precipitate and filtrate which did not change with composition of the previpitate, or, in other words, a rate of purification proportional to the amount of impurity present. -4lthough the reprecipitations can be made rapidly, the number of reprecipitations necessary t o get a relatively pure product is too great for this to be a convenient method for carrying out an isotope dilutioii dctermination of triphosphate
30 \
\ \
\ \
c
IO:
\
5.0-
r0
c r m
1.0 :
u C
Y
0
0
0.52
4
6
8
1
0
NUMBER OF REPRECIPITATIONS
Figure 4. Purification of Co(en)3f€d'&~2HzO
The reasons for use of C O ( ~ ~ ) ~ H as Pthe ~ formula O~ for pyrophosphate in the solid should be explained. It is the formula of the solid precipitating from pyrophosphate solutions between p H 6 and 8 although at p H 3.5 no precipitate is formed. Norc acid was liberated in precipitating a pyrophosphate-triphosphate mixture than was liberated in precipitating pure triphosphate] as would be expected if the precipitated ion was HP207---. It therefore appears reasonable t o assume Co(en)3HPz07 as the formula for pyrophosphate in the solid, although direct proof for its existence a t low p H values was not obtained. I n precipitations from solutions containing up to 0.692 millimole of pyrophosphates per 0.500 millimole of triphosphate (Table IV), the ratio of triphosphate in the solution to pyrophosphate in the solid ranged from 0.5 to 1 and averaged 0.7. Difference figures were used in calculating the values of the ratio so that they are not very accurate and involve the foregoing assumption of the formula of the pyrophosphate in the precipitate. However, for the composition containing 0.500 millimole of triphosphate, the ratio can be calculated directly from experimental data and is 0.68. If it can be assumed from this evidence that 1.5 Co(en)3HP207precipitates for each Co( en)3H2P301a that does not, the moles of phosphorus in the solid remain constant, the slight observed increase in yield with increased contamination of the precipitate x i t h pyrophosphate is explained, and a little extra C ~ ( e n ) ~ is C lused ~ in agreement with amperometric titration results for solutions containing pyrophosphate and triphosphate (Table 111). When pyrophosphate was in greatest excess, about three triphosphate ions were in solution for every pyrophosphate ion that mas in the precipitate. Most of this change was caused by the low yield (high solubility) of the precipitate-that is, it con-
tained 4 moles of triphosphate per mole of pyrophosphate and dissolved to such an extent that the ratio of triphosphate in solution to pyrophosphate in the solid 11as raised from 0.5 t o l to 3. ANALYTIC4L APPLICATIOh-S
Amperometric Titration Method. Solutions of the phospliates were titrated with Bolutions of Co(en)sCls or Co(NHB)&ls as a convenient means of studying t,he stoichionietry of precipitation and of evaluating their use as analytical reagents. The titration cell designed by Laitinen and Burdett ( 5 ) which provides for continuous stirring by a gas stream n-as used. The required voltage was supplied and current was measured by a Leeds & Sorthrup Electrochemograph, Type E. The recommended ( 6 ) applied potential of -0.65 volt 2's. standard calorncl electrode Ti-as used for titrations with Co(?;Ha)&ls; an applied potential of -0.80 volt 1's. standard calomel electrode as chosen for Co(en)3Cls titrations after inspection of appropriate polarogranis. Gelatin (0.01%), supporting electrolyte (O.lO.V sodium nitrate), and alcohol (usually 20%) were added t o the phosphate solutions. The supporting electrolyte was omitted from certain experiments in d.hich an acetic acid-acetate buffer \vas used. Volume corrections were made but were never large. Titrations with Hexamminecobalt(II1) Chloride. The aniprrometric titrations of pyrophosphate with hexamminecohalt(II1) chloride solution as described hy Laitinen and Burdett (6) were found to be satisfactory, except that other phosphates interfere ' more than indicated. They state that sodium triphosphate does not precipitate in 0.1.V concentration and 20% alcohol. The data of Figure 1 indicate triphosphate is likely to int,erfcre hecause in their recommended p H range of 9 to 12 it is obtained in as good yields as pyrophosphate. This ip confirmed by the data of Tahle I1 which show that t,riphosphate titrates quantitatively in the prrsence or absence of pyrophosphate. Orthophosphate is also more likely to interfere than indicated. Laitinen and Burdett (6) have indicated that 4 X 10-3.11 pyrophosphate can be determined in the presence of 4 X 10-*51 orthophosphate if only 10% alcohol is present. However, great difficulty was experienced in titrating solutions which contained orthophosphate because it precipitated slonly and in amounts depending upon the pH. From pH 10 t,o 12 most of the orthophosphate, alone or with pyrophosphate, precipitated (Tahle 11). \Then sodium monohydrogen phosphate-pgroptloel,hate mixturcs n-ere titrated with no p H adjustment or in a solution buffered at p H 8.7, little orthophosphate precipitated (Talde 11). If alooliol does not change the shape of the curve of yield 2's. pH appreciably, Figure 1 indicates that the best p H for precipitating pyrophosphate and triphosphate but not orthophosphate lies bet,ween about 8 and 10. I t is certain that abovc ],IT 10 orthophosphate interferes. .it, lower pH values its presence const,itutes a potential source of interferencc. Titration of Triphosphate with Tris(ethy1enediamine)cobalt(111) Chloride. When triphosphate is titrated with t'his reagent, orthophosphate and other possible interfering substances remain soluhle excbept for pyrophosphate. I t coprecipitates, as desrrihed prtviously, in amperonietric titrations. As can be x e n from Table I11 the titration is satisfactory for triphosphate alone but is spoiled when pyrophosphate is present in about a tenth of the inolar conrentration of triphosphate. However, the reniarlialde freedom from interference by othcr phosphates, sulfate, and silicate suggests that a study of ways of minimizing or correcting for the coprecipit,ation would he north while. A colorimetric method for triphosphate has been developed along thepe lines (13). ACKNOWLEDGRlENT
The authors wish to thank 0. T. Quimby of this coinpnny, ~ h supplied o the pure trimetaphosphate and tetranietaplios-
V O L U M E 2 7 , NO. 3, M A R C H 1 9 5 5
405
pliate and who has made many helpful suggestions. Also they arc indebted t o 11. W.Lampe of the radiochemical laboratory. LITERATURE CITED (1’ DJerrum, J., and McReynolds, J. P., “Inorganic Syntheqes.” 1-01.11,p. 216, JIcGraw-HI11 Book Co., iYew York 1946. (2) Braun, C. D., ”Cntersuchungen Uber ammoniakalische Kohaltverbindungen.” thesis, Gottingen, 1862. (3) IIynes, TV. A . , and Yanowski, L. K., Mikrochemie, 23, 1 (1937). (4) Jorgensen, S. 11..J . prakt. Chern. (2) 35, 440 (1887). ( 5 ) Lsitinen, H. A , , and Rurdett, L. W.,ANAL.C H m r . . 2 2 , 833 (1950). ( 6 ) Ibid., 23,1265 (1951). ( 7 ) JIcCune, H. W., and \Tilkins. K.,I b t d . , 26, 1524 (1954). (SI JIartin, J. B., and Doty, D. JI., I h i d . , 21, 965 (1949).
W‘., “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. XIV, p. 856, Longmans, Green, London, 1935. (10) Keuberg, C., and Fischer, H. A , , Enzymologia, 2 , 241 (1938). AN.\I,. CHEM., (11) Quimby, 0. T., Alabis, A. J., and Lampe, H. W., (9) JIellor, J.
26, 661 (1954). (12) Van Wazer, J. R., J . Am. Chem. SOC.,72, 647 (1950). (13) Weiser, H. J . , paper presented before the Division of Analytical Chemistry, a t the 126th Meeting of the AMERICAN CHnr1c.a SOCIETY, New York, N. Y., September 1954. (14) Work, J. B., ”Inorganic Syntheses,” Vol. 11, p. 221, McGrawHill Book Co., iYew York, 1946. R E C E W E Dfor review hfarcli 13, 1954. Accepted S o v r m b e r 12, 19.54. Presrnted before the Division of Physical and Inorganic Chemistry at the 121tli I I r r t i n g of tlir A\II:RICAS Cm,vrr.tr, R o r r E T r . Chicago. III., Srpteniber 1953.
Statistical Comparison of Three Methods for Determining Organic Peroxides C O N S T A N T I N E RICCIUTI, J. E. C O L E M A N , and C. 0. WILLITS Eastern Utilization Research Branch,
U. S. Department of Agriculture, Philadelphia 78, Pa.
The polarographic method for determining h?droperoxides was compared with the more commonlj used Wheeler iodide and stannous chloride chemical methods. The Latin square experimental design and statistical analjses were used to determine the relative accuracy and precision of the results obtained. The three methods gave results which were not significant]! different for high purity Tetralin hydroperoxide. For two hydroperoxide samples of lower purity and for three samples of autoxidized methyl oleate, the chemical iiiethods gave values which were significantlj higher than those bj the polarographic method. With pure 11jdroperoxides the three methods apparently > ield identical results, but with impure products the polarographic method may give more reliable values because i 1 is more specific than the chemical procedures.
given to the stability of the peroxide samples with respect to time and other conditions, such as exposure to air and room temperature during sampling. T o make the statistical comparison of the three methods and to include the factors of stability, a series of statistically designed experiments based on a 3 X 3 Latin square arrangement ( 2 , 4 , 10) was conducted. STATISTICAL DESIGN O F EXPERIMENT
The 3 X 3 Latin square was designed to include the three methods, three aliquots of each peroxidic material being analyzed, and the three different times a t which the analyses were made. This randomized block arrangement lyas repeated for each of the six perovidic materials included in the stud!.. The Latin square arrangement is as follow: RIethods
B
.lRS.4RD and Hnrgrave (1) recently presented a critical
review of chemical methods used for determining organic perosides and reported that these methods have many sources of error. For example) the methods for ferrous ion oxidation are unrc.lial)le unless carried out carefully under controlled conditioil3 : the commonly used iodide oxidation methods are subject to error because of the addition of iodine to olefinic double bonds and the effect of sample size. Barnard and Hargrave developed a modified stannous chloride procedure, which when tested on perosides and hydroperoxides of high purity (99 t o 100%) gave theoretical values with an nverage standard deviation of only 0.32%. The present investigators have used extensively a modification of the Wheeler (8)iodide methods for determining organic perosides. Recently they developed a polarographic method, which, i n contrast with chemical methods, distinguishes between pei,ositles and hgdroperosides and is specific for determining Imtli. It was hoped that a statistiral comparison bet,n-een the st:innous chloride, Wherler iodide, and polarographic methods might explain some of the anomalous results iThic’h the present investigators had observed between the last two methods. They believed that a statistical appraisal might also show the relative precision and accuracy of the three methods. T o make a proper statistical evaluation of the three methods, not only should a varietj. of peroxidic samples and a sufficient nunilier of replicates he included but ronsideration should he
I I1 I11
Aliquot8
1 RI W F
2 W F >f
3
F M W
v here I, 11, and I11 are the polarographic, iodide, and stannous chloride methods, respectively; 1, 2, and 3 are the three undiluted aliquots of a perolidic sample; and 11, W,and F are the days (Monday. \Tedriesda)-, and Friday) on which the analyses were made. An analvsis of variance as described by Snedecor ( 7 ) was then applied to the data obtained by the Latin square arrangement so that the effect of methods (polarographic us. chemical and Wheeler iodidr 2s. stannous chloride), aliquots, times, interaction, and interaction within the three individual methods could be evaluated. I n this experiment the three undiluted aliquots of each original material were transferred to separate containers and each of these aliquots were analyzed by the three methods on the three different days. The Snedecor F ratio, obtained ~ I J -dividing in turn the mean s q u a r e for the methods, aliquot., times, etc., for each sample t)> the mean square for interaction, n a s compared to critical P values a t the 5% level to determine whether the mean squares 4 ere statistically qignificant or not. T o determine if a diff rrenre exists between the values obtained hy the three methods, the least significant difference was calculated. This consisted in comparing the mean values of the three sets of duplicates of the three undiluted aliquots for one mt=thod and one sample with the corresponding mean values 01)-
