IKDIRECT X ~ U , Y S I S OF A FERRIC OXIDE
wr,
BY HARRY B. WEISER
In a paper by Professor Bancroft' a method n-as given for the indirect analysis ofla solid phase containing any number of components, without the removal of this solid phase from the mother liquor. The method consists in determining the composition of the original solution in terms of all the various components and then determining the composition of the mother liquor in terms of the same components after the separation of the solid. From the values so obtained it is possible to calculate the composition of the solid phase, providing one, a t least, of the components does not form a part of it. To calculate the composition of the phase from the data, the amounts of the various components before and after precipitation are expressed in terms of unit amount of the substance that does not enter into the solid phase. The difference between the values before and after precipitation gives the relative amounts of each of the components present in the separated phase. In order for this method to be generally applicable, it is evident that certain conditions must obtain : I . There must be an appreciable difference between the initial and final concentrations. 2 . It must be possible to analyze accurately for all componen ts. 3. The solid phase must separate and settle so as to leave a comparatively clear supernatant liquor. 4. One, at least, of the components must not form a part of the solid phase. Part I.-Method of indirect analysis of a gel, in n-hich one component of the system froin which it separates does ??of enter into the solid phase. It may be desired to find out the exact composition of a Joiir.
Phys. Chem., 9, 5.58
(IgOj).
Indirect A?inlysi.y of n Fesric Oridc Gel
537
gelatinous precipitate that cannot be removed from the mother liquor and determined accurately in the ordinary way. *Is an example of hon this may be done in a specific case by the indirect method above given, an analysis was made of the gelatinous ferric hydroxide obtained by adding caustic soda to a ferric chloride solution. 1,achs and Xichaelis' found that chlorine ion is not adsorbed a t all by blood charcoal in the presence of an excess of caustic soda. From the result of their investigation it was thought that perhaps chlorine would not be adsorbed by gelatinous ferric hydroxide n-hen the latter was precipitated from ferric chloride solutim iiilh An excess of sodium hydroxide. If we assume, then, that no chlorine is adsorbed in the presence of an excess of caustic soda, we have a t hand a11 the above conditions for ready analysis of a ferric hydroxide gel.
Experimental I X solution of ferric chloride was made up and the amount of iron and chlorine in unit weight of this solution determined. 2 . h solution of caustic soda was prepared and the strength per unit weight determined by titration n ith standard acid.' 3. To a weighed amount of the ferric chloride solution a considerable excess of caustic soda solution was added and the precipitated phase allowed to settle. 4. Portions of the clear supernatant liquid were pipetted off and analyzed for chlorine and alkali per unit weight. 5 The composition of the gelatinous ferric hydroxide was calculated from the date so obtained, assuming that no chlorine is adsorbed.
Lachs and Michaelis Zcit Elcktrochcmic, 17,2 , 91; (1911) It nas foilnd in conncction \ i i t h this \\ark that thc s ~ ~ d i uhydroxide m used contained about z j pcrcciit o f wdium chloridc, $0 that thc cauqtic soda s111ufionhad to be analyzcd f o r this irnpuritj pcr unit \\right and thc corrcction madc \chcn the composition of thc original solution hcforc mixing, cxprcsqcd in term9 nf thc bnrioiis constif iirnts iron, qodirini, Iiydrcluyl, c t c I
Harry B. IVPiscr
‘2 c 0 2
m N 3
Indiyect Aualysis
01
n
Fewic Oxidc Gcl
539
X further correction must be made in the hydroxyl content of the solution before the separation of the solid phase. Since analysis shows that the ferric chloride solution employed is not pure ferric chloride but contains slightly more iron than would correspond to the formula:FeCI,, a calculated increment of hydroxyl is added to the value obtained by analysis, in order to correct for the slight excess of iron in the ferric chloride solution. This -correctiongis a small one, in Determination KO. I , ' it is approximately 0.000002 of an equivalent of hydroxyl in a gram of solution. In the above table as well as the similar ones following, certain explanations are necessary. ( I ) In order to do away with so many decimal places and so facilitate tabulation, unit amount is taken as I O O O instead of I . Thus the composition of 1000 grams of the solution before and after mixing is given, instead of the composition of I gram. ( 2 ) '' Calculated " \,slues, express the composition of the solution in terms of unit amount (1000 grams) of chlorine. ( 3 ) '' Difference " gives the relative amounts of the various constituents present in the gel on the assumption that no chlorine is carried down. The formula is then calculated from these relative values. (4) The '' percent error " was calculated from the )act that the anion and cation concentration differed slightly. Thus in Iktermination No. I "
Fe
=
Sa =
3
x
9.301
Cations = Anions (OH)
Error
=
=
=
903 170 30 073
=
29.5So
= 27
-
=
2
o 493
=
I
6 percent
( 5 ) The sodium hydroxide solution used contained o 04660 gram of NaOH per gram of solution.
5 40
Harry B . Weiser
Indirect A ~iulyysis0 )
(2
Ferric Oxic?e Gel
'w
m
ll
u
I
u v ~-
5 42
-, 3 N N
-ccx
0
Iudirect Analysis
of u
Ferric Oxide Gel
543
t 1
1he above four determinations, made uiider similar coiiditions show the method that may be employed in determining and calculating the composition of a gel providing one of the components of the system from which it separates does not form a part of it. An examination of the composition of the gelatinous ferric hydroxide, as determined under the assumption that no chlorine is adsorbed shows that little or no vater is carried down by the gel. This is contrary to what we should expect and indeed contrary t o observed facts since a ferric hydroxide gel centrifuged until it is firmly matted together, shrinks and loses appreciable amounts of water on standing. Indeed in ‘‘ Determinations 3 and 4,” the amount of water expressed in terms equivalent to I gram of chlorine is greater after precipitation than it was before. The only logical explanation of this anomaly is that, even in the presence of an excess of caustic soda, chlorine is adsorbed by gelatinous ferric hydroxide. If the amount of chlorine as found by analysis of the mother liquor is less than it should be, i. e., if some chlorine is adsorbed, it is evident from the tables above that this n-ould cause a considerable error in the determination of the water as \Tell as of the sodium present in the gel. To show this effect even more strikingly an analysis was made in which just a trifle less sodium hydroxide was added than the amount necessary. I n such a case there is certain to be considerably more chlorine adsorbed than in the above cases where a considerable excess of sodium hydroxide is present. As can be seen from Table Tr,in which the result of this analysis is given, the xater content of the gel is quhe appreciably negative. Such an erroneous result is obtained because the calculations have been made under an erroneous assumption, for chlorine is unquestionably adsorbed in every case. The values obtained for the composition of the ferric hydroxide gel in each specific case are of course entirely wrong, since they are calculated on the basis of an assumption that the experiment at hand shows to be erroneous. However, the method is a general one and may be used with quantitative exactness
the aiialysis of a gel, providiiig (ill of the coiitlitioiis statctl a t the beginning of this paper are met. 111 the case untlcr coiisitleration all of the conipoiients of tlie system e11ter it1 to the solid phase so that the above iiiethod is inapplicable. Part 11. - Xethod o f indirect : t i i d y b i b of ;t gel iri \zliic.li all the components of the system from uliich it separates, enter into the solid plzace The next qiiestion that iiaturally presents itself is wliethei it would be possible by indirect analysis to determine the coniposition of a gel, \\-hen all of the components of the systeni from Jrhich it separates enter in -1s will be sho\\-n, this may be done accurately uiider certain conditions. Let us assume that we are dealing with the components 9, B , C, and D. In the solution before precipitation theii, there will be ,Y grams of -4, Y grams of B, 2 grams of C aiitl I T * grams of I>. If now the gel will settle firmly to the bottom, or if it can be made to settle to a firm homogeneous mass by contrifuging in a cylindrical graduate so that the volume of the supernatant liquid can be read, it is possible to determine the exact amount of the various constituents in all the mother liquor, thus : I . After the gel has been consistently matted together, read the volume of the supernatant liquid. 2 . Determine the density of the mother licluor \\it11 ;L pycnometer. 3. Pipette off portions and determine the amounts of the various constituents per unit weight of the solution. 4. From the volume, density and composition per unit weight, calculate the atnouiit of each constituent iii all o f the inother licluor. The reiult \rill be S’grains of ?7’ graiiis of 13, %’ grains of c‘ aiid 11.’ grains o f I ) . The c1ifiercnr.c 1jeLnceii the origiiial ;tml final ainounts o f the various coiistituents gives the amount of each present in tlie solid, thus: iii
Composition of original solution, S gram A - Y gram B gram C TI’ gram D Composition of liquor after prec., S’ gram X -C Y’gram B gram C IV’ gram D
+ +
+Z +2
+
Solid phasc. Y -- Y’g ~ a mX 1’ - 1’’ grain t Z -. 2’ gram 11- - 11” gram D From the above’discussion it is evident that nheii all the cornponents enter%to the solid phase, the same conditions before given as regards initial and final concentrations, possibility of accurate analysis, etc., must obtain. In addition the solid must be of such a nature that i t may be made t o mat together in a compact homogeneous mass, so that the volume of the mother liquor may be accurately read. Gelatinous ferric hydroxide prepared by precipitation of a ferric chloride solution with caustic soda, was found to fit all of the necessary conditions admirably, so that the above method could be employed readily for accurately determining its composition in any given case. It was even found by vigorous centrifuging for some hours that the gel was matted down sufficiently firmly to allow the mother liquor all to be poured off without stirring up a particle of the solid. In the interest of greater accuracy this latter procedure was followed, since, if correction is made for incomplete drainage, the weight of the mother liquor could be more accurately determined with the balance, than by volume-density determina tions. C
-
Experimental Solutions of ferric chloride and caustic soda were made up as before given and the strength of each per unit n-eight in terms of the various constituents was found. To a known weight of the ferric chloride solution in a cylindrical graduate, a known amount of caustic soda was added and then put into a centrifuge and centrifuged a t the rate of 1000 r . p. m. until the gel was pressed as a solid mass into the bottom of the cylinder It was assumed that the process was complete, when continuous centrifuging for z hours caused no further diminution in the mass of the gel. The mother liquor was then poured off and weighed (correction being made for incomplete drainage) and the exact total composition determined in terms of the five constituents. These values subtracted from the values representing the initial composition give the composition of the gel. The analysis follows:
3
0
z
l0
i
n
3
0
h
548
Harry B. IVeiser
In this determination the amount of sodium hydroxide was slightly in excess of the amount necessary. Under such conditions we would expect the adsorption of chlorine to be quite appreciable. The above analyses shows this to be true. Other analyses were made in which the excess of sodium hydroxide used \vas considerably more. The results are given on pp. 549-550. These determinations show the composition of the gelatinous ferric hydroxide under the conditions which prevail in each individual case. Chloride, alkali and water are all three found to be present, but the composition is not constant -the relative amounts of each constituent varying among themselves. This is exactly as we should expect, since the composition of a gel of this kind is certain to be iiifluenced to a greater or lesser degree by the conditions obtaining in each specific case. The chief factors influencing the composition of the gel under consideration are: I. The concentration of chlorine ion in the final solution. 11. The amount of caustic soda added in excess of the theoretical amount necessary. 111. The length of time of standing after precipitation. Generally speaking, the greater the concentration of chlorine ion in the final solution, the more would be carried down by the gel. If this were the only factor that influenced the amount of chlorine adsorption, me should expect it to be constant in all cases in which the final concentration of the solution with reference to chlorine ion, was constant. But this is not the only factor that influences the chlorine aclsorption. The caustic soda concentration also has an effect. The following table will shoxv this. In this table are brought together for comparison the several formulae of the gelatinous ferric hydroxide obtained above, under different conditions as regards chloride and alkali concentrations. Under the heading, “Excess of NaOH used,” is placed the number of grams of sodium hydroxide solution (0.0466
l
m h
h
c
I
Harry B . Weiser
550
z
0
5
E
Indirect Analysis of a Ferric Oxide Gel
55 I
gram NaOH per gram of solution) added over and above the amount necessary for complete precipitation of the iron. The figure in parenthesis represents the number of times more solution added than theoretically necessary. NaCl ” and ‘ I NaOH express the adsorbed constituents in terms of unit amount of Fe,O,. ‘ I
”
._ ~
No.
i,1
~
_
~
_
~
~
-.
Excess of NaOH -___Grams Times
Fe,O,
I
3 4 j
I
N aC1
(0
14)
(2
17)
026
(2
23.340 40 902
(2
22
52) j?) (6 3 )
-
~~
~
~
H*O
NaOH
~--__-1 joj 19 9 j 0
-
I
-~
2
~ - _ _
~
’
1 0
1
I o I (1 I 0
I o
1 ~
-
432 o 273 0 283
1
0 260
1
0
o 248
247 o 986
0
I I I
163
0j3 9S3
132 136 140 139 178
8 o S j
3
The above table shows the effect of the concentration o f sodium hydroxide on the amount of chlorine adsorbed. In No. I , in which there is only a very slight excess of sodium hydroxide, the amount of chlorine adsorbed is greater than in any succeeding case. In S o . j , in which the excess of alkali added is quite large, there is only about one-half as much chlorine carried down as in KO. I . Sos. 2 , 3 and 4 are intermediate betweeit these two extremes. It is interesting to note that the values for adsorbed chloride in Kos. 2 , 3, and 4 are but slightly greater than in No. 5, even though three times a s great an excess of alkali was used in the latter case as in the former ones. This shows that above a certain concentration, the excess of sodium hydroxide present has but slight effect in cutting down the ex tent of adsorption. ;Is regards the carrying down of alkali by gelatinous ferric hydroxide, we should expect that the amount likely to be carried down’ would be greater, the greater the excess present. l The following is interesting as a n experiment illustrating qualitativcly t h a t sodium hydroxide is adsorbcd b y gelatinous ferric hydroxide: 68 grams of a ferric chloride solution (0.00317 gram Fc pcr gram of solution) rcquircd for coniplcte precipitation of the iron 9.8 grams of a sodium hydroxide solution (0.0466 gram NaOH pcr gram of solution). When 0.5 gram more of this solution than necessary for complctc precipitation \vas addcd and thc supcrnatant liquid testcd for alkali with ~~henolphthalein, thc solution remained colorlcss.
This was found to be the case as shown in the above table. Since a gel which contains a large amount of water tends to shrink and lose some of this water on standing, the greater the length o i time of standing the less will be the rvater content. The chloride and alkali contents will likewise diminish on standing by reason of their diffusion into the Supernatant liquid. Since all of the above conditions influence the composition of gelatinous ferric hydroxide prepared in the manner described, i t is evident that the same values will not be obtained twice in succession, unless precautions are taken to have the concentrations of the reacting substances exactly the same and to allow the same time to elapse between precipitation and removal of the supernatant liquid for analysis. The question may naturally have arisen that if it is possible to pour off practically all of the supernatant liquor, why not analyze for the constituents of the gel directly without resorting to an indirect method a t all? The answer to this question is a t hand when we consider the difficulties involved in removing entirely 0.01gram of chlorine from a solution containing twenty times as much iron, or how impossible i t would be to analyze with anything like quantitative accuracy for from 0 . 0 2 to 0.06 of a gram of sodium in the presence of such a large excess of iron, unless a great amount of time were spent in working up a method that would be applicable quantitatively in the particular case a t hand. Then, too, this is a specific case in which the supernatant liquor could be poured off without stirring up the solid. The method is just as applicable by the volume-density determinations in cases in which the mother liquor can not be poured off'. Furthermore, cases may arise in which the difficulties of direct quantitative analysis are even greater than in the case under consideration. The two methods of indirect analysis giveii, describe the mode of attack in determining the composition of a gel. 'I'he methods are perfectly general in their nature and may be employed in any
case in which the conditions are such that an indirect method is applicable. I t is interestitig to note the magnitude of the error introtiticed in the gel analyzed, if the composition is calculated on the asstimption that no chlorine is adsorbed and the constituents expressed in terms of unit amount of chlorine, as ixfoi-e described in Part I . The follon.ing table gives the determination, So. j above. c:ilculated on this basis. IVliereas the correct composition is I ~ e , 3 0o .~2 i@~ SaC1. I ,583 NaOH, 178.3 H,O, if rve calciilate o n the assumption that 110 chlorine is adsorbed, a result is obtained shon-ing the amount of alkali carried doivn to be IC than one-third Tr-hat it should be, and the water content to he actudly negative. This is exactly Irhat \vas found and described in the first part of this paper.
Summary and Conclusions i . 1he compositim of a gel may he determined by 1'11,. ciii.c'ci ( ~ 7 ? ( 7 l j , ~ i ~The s , niethod employed is a general mie but is siihject to certain Ivell-defined limitations. lhese limitations are as follon-s: Case I. In Ivhich a t least one component of the system from which the gel separates does not enter into it. I . I t must be possible to analyze accurately for all conipoiielits. 3 . There must be an appreciable difference in the concentration of the soli!tiori before anti after the separation of the gel. ,;. The gel must settle sufficiently to allon. the niotlier liquor to be remos-ed aiid analyzed. Since by definition, a t least one component of the system from which the gel separates does not enter into it, the relati\-c amounts of the s-arious cmstitiients before and after precipitation are expressed in terms of unit amount of this tinadsorheti constituent. The difference hetn-een these s-dues gil-es the r ( ~ I t r t ! ' ' i amount ~ of each present in the gel. Gelatinous ferric. liydroxidc prepared hy precipitation from ferric chloritlc . 9
solution by caustic soda cannot be analyzed under this niethod. since all of the constituents enter into the gel. I t can he rletcrniined, lion-ever. tinder Case I1 iii which all the components of the system from \vhich the gel separate? enter into it. In this case conditions one and two are the same, and in addition the separated gel must be of such a natiire that it ivill mat together in a homogeneous inass, so that the voliinic of all the mother liquor can be read. 113th these conditions obtaining the exact total composition of the solution in ternis o f all the \-arious constittients before and after precipitation may he determined. The difference between these valtie:;. gives the (zbsoIutt~amount of each present in the gel. 2 . The composition of the ferric hydroxide gel, determined by the method given in Case I1 above, varies ividely under different conditions, chlorine, alkali and ivater beiiig carried down in varying amounts. Generally speaking, the amount of material adsorbed is proportional to the amount present. The adsorption of chloride is hon-e\-er appreciably lessened hy the presence of a11 excess of caustic soda, although it is not entirely prel-ented. I n conclusion, I express my sincerest thanks to Professor Ihncroft, at whose suggestion and under ivhose kindly supervision this ivork was done.
