Benzenephosphinic Acid as Analytical Reagent Gravimetric Determination of Ferric Iron JAMES E. BANKS' and
D.
A.
SKOOG
Deparfment of Chemisfry and Chemical Engineering, Stanford University, Stanford, Calif.
b In 0.1 - to 1 .ON solutions of mineral acids, iron is quantitatively precipitated b y benzenephosphinic acid. The precipitate is readily dried at low temperatures to constant weight and has a composition of three phosphinate ions per ferric ion. In several respects it is more satisfactory than the basic oxide precipitate for the analysis of iron. However, the precipitate is more soluble than the basic oxide precipitate and losses of iron amounting to 0.1 to 0.5 mg. are encountered. These losses result in an analytical error of 0.1 to O.3y0 relative for quantities of iron greater than 60 mg.
T
PRRPARATIOK and properties of benzenephosphinic acid were first described in 1874 by Michaelis and Ananoff (4). I n a subsequent publication (3) Michaelis reported briefly on the reaction of certain cations with this compound; these include ferric, silver, barium, lead, mercurous, and mercuric ions. Of particular interest is t'he compound with ferric ion, m-hich IWIS reported to be insoluble even in fairly strong solutions of hydrochloric arid and was described as a white granular substance with a composition of three phosphinate ions for each ferric ion. Benzenephosphinic acid has recently become amilable commercially ( 5 ) $ anti it seemed north while to investigate the applicability of this compound as a precipitating agent for various metallic ions. As a result of this work, it has hecome apparent t'hat the substance has some intcrcsting and useful properties as a reagent for the separation and determination of ferric iron in the presence of c e i h i n other heavy metal ions. I n many respects ferric benzenephosphinate is a more satisfactory precipitation and n-eighing form for iron than the classical basic oxide precipitate. The physical properties of the compound are markedly superior; it can be readily freed from moisture by drying a t relatively low temperatures t80give a nonhygroscopic residue which has the theoretical composition of three phosphinate ions per ferric ion. Furtherniore, in mineral acid solutions t,he rcagent is selective in it's behavior and allo~rs exrellent separations of iron HE
Present address, OMS Box Girtland .?ir Force Base. N.M.
173,
from certain other metals from nhich i t is not readily separated by procedures involving hydrolytic precipitation of the iron However, the reagent is by no means specific, and precipitation and coprecipitation phenomena prevent its application t o some separations which can be accomplished by hydrolysis procedures. The precipitate is also more soluble than the basic oxide precipitate and precautions must be taken t o aroid errors arising from solubility losses. The physical and chemical properties of ferric benzenephosphinate and the variables which affect these properties have been investigated, and a procedure n hich has been developed for the separation and gravimetric determination of iron is described here. The application of this procedure to the separation of iron from a number of cations as well as anions is given by Banks, Pennell, and Skoog (1). REAGENTS AND SOLUTIONS
0
t
Benzenephosphinic acid. CsH5P--OH, I
H
is a white crystalline compound soluble in water to the extent of 7.7 grams per 100 grams at 25.5' C. (6). Solutions of the reagent are stable in air, but i t is reported that oxidation of the compound to benzenephosphonic acid occurs easily with such reagents as hydrogen peroxide and nitric acid. The compound is a fairly strong monobasic acid. rln approximate value for its acid dissociation constant was obtained by potentiometric titrations of 0.1F solutions with standard solutions of sodium hydroxide using a glasscalomel electrode system. ilt 21' C. a value of 0.012 n-as obtained. The titration curve for the compound indicates the presence in the commercial henzenephosphinic acid of a second weaker acid, in a concentration estimated to be about 1%. Rased on the assumption that this foreign acid was benzenephosphonic acid, purification was attempted by leaching with ethyl ether. Benzenephosphonic acid is reported to be considerably more soluble in this reagent than benzenephosphinic acid. The treatment resulted in a n improvement in the appearance of the titration curve, and samples of the benzenephosphinic acid leached twice with ether were found to have a purity
of 99.4 to 99.8% on the basis of titration data. Benzenephosphinic Acid Solution. T h e solid benzenephosphinic acid was obtained from Victor Chemical Works, Chicago, Ill. T o purify it, t h e solid was placed in a flask, covered with diethyl ether, and allowed t o stand for about 1 d a y with intermittent shaking. T h e ether was then decanted and the process repeated. After filtration the excess ether n as removed under vacuum. Standard solutions were prepared b y diluting weighed quantities of the purified reagent to measured volumes in volumetric glassware. The concentrations of such solutions were frequently checked by potentiometric titration with a standard sodium hydroxide solution, I n every case where this v a s done the concentrations were found to lie within 99.4 to 99.8y0 of the theory. Approximately 0.2F solutions of the reagent were prepared for use in the recommended procedure b y dissolving 28.4 grams of the purified compound in distilled water and diluting to approximately 1 liter. Ferric I r o n Solutions. Weighed quantities of reagent grade ferrous ammonium sulfate hexahydrate TT ere dissolved in 1N sulfuric acid and treated n i t h a few milliliters of 30% hydrogen peroxide t o oxidize t h e iron t o t h e ferric state. T h e solutions were boiled for several minutes t o remove the excess peroxide and then diluted to volume. I n some cases hydrochloric acid was used rather than sulfuric acid. The exact concentration of the solutions was determined by titration of the iron with a standard solution of potassium dichromate after reduction with stannous chloride. Barium diphenylaminesulfonate was used as an indicator. Barium Chloride, 1%. Approriniately 1.0 gram of barium chloride dihydrate n a s dissolved in 100 ml. of distilled water. RECOMMENDED PROCEDURE
Use a sample of such a size as t o contain 10 to 250 m g of iron (preferably greater than 50 mg.). After solution of the sample, the iron should be present in the ferric state and in about 50 ml. of a solution which contains either sulfuric or hydrochloric acid. The acidity should be such that upon addition of the precipitating agent, the solution will be 0.5- to 1.ON in the mineral acid. Any ferrous iron in the solution can be readily oxidized by
vot.
29, NO. I , JANUARY 1957
109
addition of several drops of 30% hydrogen peroxide, followed by boiling for several mjnutes to destroy the excess peroxide. Heat the solution to boiling and add dropwise a 0.2P solution of benzenephosphinic acid. A sufficient volume of this solution should be added to precipitate the iron completely and then a 50 to 200% excess of the reagent added. The follon-ing table will serve as a guide to the volume of reagent to be used. If the approximate weight of iron is not known, the disappearance of the yellow color from the solution can be used as a measure of the stoichiometric amount of the reagent, and half again as much reagent should then be added. Benzenephosphinic
Iron, Mg.
Acid ( 0 . 2 F ) , 111.
10 to 20 20 to 40 40 to 80 80 to 160 160 to 250
13 30 6.3
€4
100
Digebt the precipitate for to ll/z hours and cool the solution to room temperature. Filter through a niediumporosity glass filtering crucible which has b e m previously dried and weighed. K a s h the precipitate once or twice in the beaker with cool distilled water before quantitative transfer of the precipitate. Finally, wash the precipitate with 5-ml. portions of distilled water until one of these washings gives a negative test for sulfate with a 1% barium chloride solution. If hydrochloric acid was used as the mineral acid, wash until free from chloride as indicated b y a dilute silver nitrate solution. Ordinarily three to four washes are sufficient; excess washing is to be avoided. D r y the precipitate for 2 hours a t 110" C., cool in a desiccator, and weigh. Usually this is sufficiently long to give a constant weight. The neight of iron is calculated from the following equation: Weight of iron
=
weight of precipitate
x
0.1166
is slow enough so that it causes no difficulty in determining the weight of the residue after drying. Kumerous experiments showed that under a fairly wide range of conditions, the precipitate had a composition of exactly three phosphinate ions per ferric ion. These experiments involved tnking known quantities of iron, precipitating. and determining the neight of dried precipitate as well as the weight of iron remaining in the filtrate and washings, Solubility. Ferric benzenephosphinate is not appreciably soluble in water. 1N solutions of mineral acids, or organic solvents such a s benzene, petroleum ether, chloroform, carbon tetrachloride, acetone, and diethyl ether. T h e compound dissolves readily in concentrated hydrochloric or sulfuric acids. Quantitative solubility measurements in 0.1N sulfuric acid vere made by equilibrating the solid n ith the acid solutions and then determining the iron concentration colorimetrica!ly using o-phenanthroline as a reagent. The procedure was as follori s. Ferric benzenephosphinate n as prepared by addition of a dilute solution of benzenephosphinic acid to a hot. sulfuric acid solution of trivalent iron. The resulting precipitate was digested for several hours, filtered, and washed thoroughly with hot water until the washings were free of sulfate ion. I t was then dried for 2 hours a t 110' C. Approximately 100-mg. samples of the dried compound were suspended in 500-ml. portions of 0.100-V sulfuric acid. Some of the samples were refluxed for 1 t o 2 hours; others were shaken mechanically for 3 hours a t room temperature. Finally, all of the samples were placed in a water bath maintained a t a temperature of 25.1' =k 0.1' C. After several days 50-ml. aliquots of the supernatant liquids were withdrawn, filtered, and analyzed for iron by the o-phenanthroline method. The procedure of Fortune and Mellon
The crucible can be cleaned for reuse b y scrubbing with water, immersing for a few minutes in hot, concentrated nitric acid. and finally rinsing n i t h distilled water. PROPERTIES
OF
FERRIC BENZENEPHOSPHINATE
Fcrrir benzenephosphinate. n hen formed by s l o ~addition of a dilute solution of benzenephosphinic acid to a mineial acid solution of ferric ion, is a hite, easily filteiable piecipitate. Rapid precipitation yields a product of small enough particle size to render filtration difficult. The compound can be dried to constant weight b y heating at 110" C. for 1, 01 a t the most 2 hours The dried compound slonly absorbs moisture a t loom temperature, however, the rate a t nhich this occurs 110
Table 1.
ANALYTICAL CHEMISTRY
Sample -4
(8) was followed with a Bausch & Lomb spectrophotometer used for the photometric measurements. I n preparing the standard curve for the analysis, benzenephosphinic acid was introduced into the standard iron solutions in a ratio of 3 parts to 1 part of iron in order to correct for any effect this compound might have on the color of the solutions.
The results of the solubility determination are given in Table I. The solubility equilibrium n as approached from both the undersaturated and the supersaturated states and the tn o approaches check rather well. The data in Table I indicate that the solubility of the precipitate is lon enough so that. by taking advantage of the common ion effect, it should lie possible to make satisfactory recoveries of iron from 0.liV solutions of sulfuric acid. I n fact, solubility losses in 1N sulfuric acid can be kept low enough for quantitatiITe work. EFFECT O F VARIABLES
A number of experiments were carried out in which knoll-n quantities of iron n ere precipitated with benzenephosphinic acid under a variety of conditions in order to establish the variables which affect iron recovery, particle size of the precipitate, and composition of the precipitate I n all cases, the precipitates n ere isolated by filtration m ith medium-porosity glass filtering crucibles and dried to constant weight at 110" C. I n most cases. the iron concentrations of the filtrates and washings \yere determined spectrophotometrically in order to dptermine whether departures from theoretical results arose from variations in the composition of the precipitate or from solubility effects The following general procedure was used in these studies. I n moit cases, 25-ml. aliquots of a standard iron solution, containing about 2.5 mg. of iron per ml., were transferred to 100- or 250-ml. beakers and enourh
Solubility of Ferric Benzenephosphinate in 0.1OON Sulfuric Acid
Days of Standing a t Treatment before Standing 25.1" C. Refluxed 60 minutes S
Solubility 11g./100 G.F.W. X ml. 10'/literb 1.12 2.01 1.13 2.02 1,lO 1.9; 1.13 2.02 0 95 1 70
10
-
B
Refluxed 60 minute3
6
Cn
Shaken 180 minutes, room temp. Shaken 180 minutes, room temp.
15
D
13
15 Av.
*
Disregarded in computation of averages. G.F.W. = gram formula neight.
1 11 1 09 1 11
1 99 1 95
1.99
Table 11.
carried out froni 0.SN hydrochloric acid, and tjhe rerults from these experiments are compared with results from 0.5N sulfuric acid in Table 111. The apparent recoveries of iron from the hydrochloric acid medium are slightly lower than from similar precipitations from sulfuric acid. The filtrates from these experiments were not analyzed for iron. but it seems probable that the lower results from the hydrochloric acid solutions are a result of complex formation betneen the ferric ions and chloride, leading t o an increase in solubility of the precipitate. I n most of the work reported herein, precipitations were carried out from sulfuric acid solutions; honerer. the use of hydrochloric acid solutions would probably not lead to serious errors.
Effect of Sulfuric Acid Concentrations on Recoveries of Iron (63.6 mg. of iron taken for each test)
H80,
Fe, Calcd. from Precipitate,
Diff. between Fe Taken, and Calcd.,
Fe Found in Filtrate and Washings,
Sormalit y
XIg.
Mg.
Mg.
0.10
63 2 63 3 63.2 63.1 63.1
-0 -0 -0 -0 -0
4 3 4 5 5
0 0 0 0 0
1 1 1 1 1
0.50
G3.5 63.4 63 4 63.4
-0 -0 +0 1 0
1 1 2 2
0 0 0 0
3 2 2 2
1.00
63.i 63 3 63.4
70 1 -0 3 -0 2
0 3 0 2 0 3
Excess
6-L- sulfuric acid was added to give the desired acid concentration after addition of the precipitating agent. The solution was heated to about 80' C. and 0.1F benzenephosphinic acid was added dropwise and with stirring. Generally a 50% excess of the precipitating agent m s added. The resulting mixture ivas digested a t about 70" to 80' C. for 1 hour, allowed to cool, and filtered throuqh a medium-porosity glass filtering crucible. The precipitate was washed with 5-id. portions of water until one of these failed to cause turbidity in 10 ml. of 1% barium chloride. This usually required 20 to 30 ml. in addition to a like volume used to transfer the precipitate to the crucible. The combined washing. and filtrate a e r e reserved for colorimetric iron analysis. The crucible and precipitate were oven dried to conqtant w+$it a t 100' C. Usually 1 to 2 hotirs was required for this, The iron concentration of the combined filtrate and washings was determined after destruction of the excess benzcnephoq2hinie acid. This mas RCcomplished by addition of 20 nil. of coneentratrd nitric acid 10 nil. of concentratcd sulfuric acid, and 10 ml. of 607, perchloric acid. The solution n a s evaporated slonly to a rolume of about 2 nil., then diluted with water and analyzed for iron by the method of Fortune and llellon (6),u sing ' ophenanthroline as a reagent. For these measurements a Bausch & Lomb Spectronic 20 spectrophotometer was uscd. Acid Concentration. Precipitation of known quantities of iron from 0.1-, 0.5,a n d 1 . O N sulfuric was carried oiit. I n each case SOY, excess of t h e precipitating agent was used. T h e results of these experiments (Table 11) indicate a n increase i n t h e solubility losses of iron with increases in acidity. This, of course, is t o be expected inasmuch as benzenephosphinic acid is a weak acid. However, even in 1N sulfuric t h e losses are not very serious.
For 0.5- and l.OAl- solutions of acid the iron content of the filtrates and n ashings checks, within the experimental error, with the differences betneen the iron taken and the iron calculated from the weight of precipitate (column 4). This would indicate that the precipitate formed under these conditions approaches rather closely a theoretical composition. Hon ever, in the precipitation from 0.l.Y acid solution. there appears to be a small but significant difference betn een the iron found in the filtrate and the loss calculated from the precipitate n.eight. This difference could be accounted for by assuming some coprecipitation of ferric sulfate or basic ferric benzenephosphinate, either of vhich nould result in a precipitate n hich was lighter than theory. From the practical standpoint, one coiicludes from these data that the best analytical results are to be obtained by carrying out precipitations from solutions 0.5- to 1.ON in acid concentrations. Under these conditions solubility losses of 0.2 to 0.3 mg. of iron are to be expected. Some precipitations of iron TT ere
Table IV.
Precipitating
A
Agent.
number of experiments were undertaken t o determine t h e effect of ewess precipitating agent on t h e recoreries of iron. On t h e basis of solubility studies described earlier, i t appeared desirable t o reduce t h e solubility of ferric benzenephosphinate, and i t was of interest t o determine whether excess precipitating agent n ould accomplish this n-ithout having other undesirable effects on the composition of the precipitate. I n these experiments, precipitations were carried out from
Table 111. Comparison of Iron Recoveries from 0.94 Sulfuric Acid and 0.5N Hydrochloric Acid
Fe, Calcd.
from Precipi- Difference, tate, Mg. Ng.
Acid
Fe Taken, Mg.
HC1
63 8
63 63 63 63
4 5 4 5
-0 -0 -0 -0
4 3 4 3
H2S04
63 5
63 63 63 G3
3 3 3 3
-0 -0 -0 -0
2 2 2 2
Effect of Excess Precipitating Agent on Recoveries of Iron (63.6 nig. of iron taken for each test)
Mole Ratio, Benzenephosphinic .4cid to Iron 3.33
Excess
Benzenephosphii$c Icid,
(c
11
Fe, Calcd. from Precipitate,
Ditf. betmen Fe Taken and
Fe Found Filtrate, Mg.
JIg.
Calctl., 11g.
62.9 63.1
-0 5
0 4 0 8
-0 i
4.50
30
63 5 63.5
-0 -0
1 1
0 3 0.2
6.05
101
63.6 63.7
0.0 +O. 1
0.2 0.2
9 10
203
63 8 63 8
i o
0 2 0 2
+o
2 2
VOL. 29, NO. 1 , JANUARY 1957
11 1
TableV. Effect of Temperature during Precipitate Formation on Recoveries of Iron
Temperature, C.
Taken,
Fe
Precipitate,
hlg.
11g.
63 0 63 3 63 5* 63 Oh 63 Oh 63.0 63.0' 63.5' 63.5' 63.0' 63.0'
63 0
O
22
80
Fe, Calcd. from
6.5 63 65 66
2
2 8
2 62.8 62.8 63.4 63.3 62.9 62.9
Iliff.. 1Ig. +2 0 +1 7 +l 7 +2 8 $3 2 -0.2 -0.2 -0.1
-0.2 -0.1 -0.1
a Precipitations from hydrochloric rather than sulfuric acid. * Iron solutions added to benzenephosphinic acid.
Table VI.
a t about 0.2 nig. with reagent excesses of 50% or greater. I n all probability, the losses found under these conditions are due almost entirely to solubility in the wash liquid where no excess precipitating agent is present; solubility of the precipitate in the mother liquor itself is negligible. There is a continual increase in the weight of precipitate with increasing amount of reagent. despite the fact that the actual solubility losses become constant nith reagent excesses of 50% or greater Furthermore, with the 11 and 50% excess of reagent, the difference betn een the theoretical and actual weight of precipitate correlates fairly well with the measured solubility losses, whereas, with greater excesses of reagent, the weights of precipitate are larger than expected. I n all probability, this means that, with a large ewess of the berizenephosphinic acid. some of the
Effect of Rate and Order of Mixing on Recoveries of Iron
Rate of Addition Very rapid
Order of Mixing Reagent to Fe
Dropwise
Reagent to Fe
Dropwise
Fe to reagent
Fe Taken, Mg. 63.6 63.6 63.6 63.0 63.0 63.0
Fe, Calcd. from Precipitate, Mg. 63 5 63.6 63.5 62.8 62.9 62.9
Diff ., hIg. -0.1 0.0
-0.1 -0.2 -0 1 -0.1
tating agent exceeds that needed for complete precipitation of iron by n factor of 50 to 100%. Temperature during Precipitation. I n several eyperiments undertaken to determine 11hether precipitations should be carried out at room or elevated temperatures, t h e precipitates were formed b y slow addition of a 50% excess of t h e reagent t o a 0 . 5 s acid solution of t h e iron. T h e resulting precipitates were not digested before filtration. The data in Table 1,s110w that markedly high results were obtained in every case where precipitate formation occurred at room temperature; therefore, elevated temperatures should be used for quantitative m-ork. At room temperature considerable coprecipitation of the benzenephosphinic acid apparently occurs. Rate of Addition a n d Order of Mixing. In Table VI are found d a t a for precipitations of iron from hot 0 . 5 s sulfuric acid when the rate and t h e order of addition of the reagent mere varied. T h e results indicate t h a t neither of these variables affects t h e results significantly. Digestion. It n as found desirable to digest t h e ferric benzenephosphinate precipitate for at least 1 hour prior t o filtration; such treatment had a considerable effect on t h e ease with which the precipitate could be filtered and, furthermore, seemed t o give somewhat better recoveries of iron (Table VII).
RESULTS
0.5.V sulfuric acid and the filtrates and washings were analyzed for residual iron. From the data (Table IV) on the iron found in the filtrates, it is apparent that an excess of precipitating agent is effective in reducing solubility losses. However, these losses become constant
Table VIII. Table VII.
Effect of Digestion on Iron Recoveries
(63.6 mg. of iron taken for each test) Fe, Fe Calcd Found Digestion Precipiin Time, tate, Diff, Filtrate, Minutes Mg, Mg hlg. 0 632 -04 0 2 632 -04 0 3 20 63.5 -0.1 0.1 63.2 -0.4 0.2 60 63.5 -0.1 63.5 -0.1 0.1 120 63.4 -0.2 0.2 63.4 -0.2 0.2
112
ANALYTICAL CHEMISTRY
The recommended procedure was tested on several solutions containing known quantities of iron. The results given in Table VI11 show that there is a systematic negative error in the calculated results that correlates fairly well nith the iron found in the filtrnte
acid itself coprecipitates and is not completely removed during the washing operation. On the basis of these experiments, it appears that the best analytical results are obtained if the amount of precipi-
Fe Taken, Mg.
12 63 25 34 63 50
Analysis of Solutions Containing Known Concentrations of Iron
Fe, Calcd. from Precipitate, Mg. 12 51
254.2
Error, c 10
12 52
-0 11
1 0 0 9
25 23 25 15 63 34
-0
11
0 4
-0 IC) -0 16 -0 10 -0 21 -0 22 -0.1
0 8
63 31 63 29
127.1
Diff ., JIg. -0 12
63 27 127.0 126.9 127.0 126.8 126.7 253.8 263.7
-0.2
-0.1 -0.3 -0.4
-0.4 -0.5
Fe Found in Filtrate, hIg. 0.1 0.1 0.1 0.1
0 3 0 3 0 3 0 4 0.1
0.2
0.1
0.2 0.3 0.2 0.2
0.3 0.3 0.2
0.2 0.4 0.4
0.4
and washings. This loss amounts t o 0.1 to 0.2 mg. for quantities of iron less than 100 mg. and rises as high as 0.5 mg. n i t h larger amounts. However, in terms of percentage error resulting from these losses, more accurate results w e obtained when the quantity of iron is greater than 100 mg. Extensive studies have been made
on the use of benzenephosphinic acid for the separation of iron from other heavy cations and the effect Of common anions on recoveries of iron by the reagent (1). LITERATURE CITED
(1) Banks, J. E., Pennell, J. F., Skoog, D. .4..ASAL. CHEJI.29, 113 (1957).
( 2 ) Fortune, K.B., lIellon, hf. G., I?*.D. ENG. CHEM., ANAL. ED. 10, 60
(1938). (3) Michaelis, A., iinn. 181, 265 (1876). (4) Michaelis, A,, Ananoff, J., Ber. 7, 1688 (1874). ( 5 ) Victor Chemical Works, Chicago, Ill.. “New Victor Chemicals,” 1963. RECEIVED for review July 20, 1956. Accepted October 27, 1956.
Benzenephosphinic Acid as Analytical Reagent Separation of Ferric Iron from Certain Anions and Cations JAMES E. BANKS’, JOHN
F. PENNELL2, and
D. A. SKOOG
Department of Chemistry and Chemical Engineering, Stanford University, Sfanford, California
b Benzenephosphinic acid separates ferric iron satisfactorily from large excesses of manganese, nickel, cobalt, cadmium, magnesium, the alkaline earths, arsenic, phosphate, citrate, tartrate, oxalate, and chloride. The separations are comparable to or better than existing methods. The precipitate forms rapidly, is easily filtered and washed, and provides a satisfactory weighing form for the iron. Because of coprecipitation, iron cannot be separated satisfactorily from aluminum, chromium and vanadium. Other elements which form insoluble precipitates with benzenephosphinic acid include titanium, molybdenum, zirconium, cerium, uranium, tungsten, tin, bismuth, silver, and mercury.
F
BI~NZENEPHOSPHIKATE is a satisfactory weighing form for trivalent iron, and a procedure utilizing the formation of this compound can be carried out in acid solutions (1). Michaelis (Z)> in describing the properties of benzenepliosphinic acid, indicated tliat the heavy metal salts of blie acid irerc generally not soluble in water and that silver! mercurous, mercuric, and ferric ions formed insoluble precipitates from mineral acid solutions. KO other data are available in the literature regarding the solubility of the salts of benzenephosphinic acid, and it was not possible to judge how selective a reagent it would be for iron. Therefore, experiments were carried out to determine what other ions would affect the deterniination of iron, either by
ERRIC
Present address, O M S Box 173, Iiirtland Air Force Base, N. M. * Present address, Calavaras Cement Co., San Andreas, Calif.
precipitation with the reagent, by coprecipitation during formation of the precipitate, or by complex formation with the benzene phosphinate or ferric ions, REAGENTS AND SOLUTIONS
Benzenephosphinic Acid Solution. The method described previously (1) was used. Ferric Iron Solutions. These were prepared and standardized as described previously (1). Cation Solutions. Solutions of t h e various cations studied 11ere prepared by dissolving suitable quantities of t h e reagent glade salts in water. K h e r e necessary, sulfuric acid n as added t o preyent hydrolysis. I n most cases t h e chlorides, sulfates, or nitrates of t h e cations were used. Hon-ever. solutions of arsenic(II1) and arseniciV) were obtained from sodium metaarsenite and sodium metaarsenate, respectively; chroniium(J-I) from potassium chroniatr; vanadium
Table I.
(73 from aiiimonium metavanadate; molybdenum(V1) from ammonium molybdate; tungsten(V1) from sodium tungstate; and uranium(V1) from uranyl acetate dihydrate. The solution of vanadium(1V) was prepared b y reduction of a n acid solution of vanadium(\’) with sulfite, followed by boiling to remove the excess sulfur dioxide. The solutions of zirconium (117) and titanium(1V) mere prepared b y fusing the corresponding oxides with sodium pgrosulfate and dissolving t h e fluxes in 2N sulfuric acid. Anion Solutions. These were prepared b y dissolving suitable n eights of t h e reagent grade alkali metal or ammonium salts in water. Buffer Solution, pH 4. A solution approximately 0.75F in acetic acid and 0.12F in sodium acetate v a s prepared. T h e p H of this buffer v a s then adjusted b y additions of glacial acetic acid or sodium acetate such t h a t n hen diluted with 1 volume of water and 1 volume of 0.05F benzenephosphinic acid, a solution of pH 4.0 = 0.2 n a s obtained.
Qualitative Precipitation Tests with Benzenephosphinic Acid
Coinposition of Solutions 0 . 5 5 HzSOd
Ions Forming Precipitates Fe(III), Ce(IV), hIo(TI), U(VI), W(l-I), Ti(IV), %r(IS’)
O , l a l - HC10,
Fe(II), Fe(III), Bi(III), Sn(II), Sn(IV), Ag(I), Hg(II)
Acetate buffer, pH 4
Fe(II), Fe(III), Bi(III), Sn(II), Sn(IV), Hg(I1)
Ions XotlPrecipitating
Ca(II)a, Sr(II)Q, Ba(II)O, Mg(11),Cu(I1) Cd(II), Mn(II), Xi(II), Co(II), Cr(III), Cr(W), Al(III), V(IV), V(V), -4s(III), As(V). Ca(II), Sr(II), Ba(II), hTg(131, Cu(II), Cd(II), Nl(II), CO(11), Zn(II), Pb(II), Mn(II), Al(III), Cr(III), .4u(III). Ca(II), Sr(II), Ba(II), Mg(II), Cu(II), Cd(II), Ni(II), Co(11), Zn(II), Pb(II), Mn(II), ill(III),Cr(III), Au(II1).
Solutions 0.5-l’ in HCl.
VOL. 29, NO. 1 , JANUARY 1957
e
113
