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ELECTRON MICROGRAPHS AS AN AID IN TEACHING GRAVIMETRIC ANALYSIS ROBERT B. FISCHER University of Illinois, Urbana, Illinois
Tm
'ELECTRON microscope has revealed much new information along various lines of research, yet one of its most imvortant contributions has been the direct observation-of phenomena previously known only by indirect.means. In the study of gravimetric analysis, students are told that certain procedures must be followed and that snecific nrecautions mbst be taken. In most cases valid reasons may be given, yet direct visual indication of the specific requirement is often lacking. The reason is that various phenomena having to do with the formation of gravimetric precipitates take place upon a microscopic or submicroscopic scale. The electron microscope. now provides a means of direct observation of many things previously not resolvable because of their extremely small size. A series of electron-microscope studies of precipitates has been performed a t the University of Illinois, and the first result of these investigations is a series of micrographs which have proved of value as a teaching aid in quantitative analysis. The results reported in t.his article do not in themselves reveal any new information, but they do present direct visual observation of the effect of several factors on precipitate formation which are commonly discussed in first courses in quantitative analysis. For all electron micrographs presented herein, the specimen was purified of dissolved substances by ion diffusion through a thin membraqe to insure that the particles observed are truly those desired and not a residue formed by evaportion of the sdlution in which the desired precipitate was formed. The scale marker on each picture represents one micron. Classification of Precipi' tates. All precipitates may be classified into the groups gelatinous, curdy, and crystalline. The individual crystals of a crystalline precipitate are often large enough to be seen with an optical microscope. The ultimate particles of a curdy precipitate are generally crystalline in nature, but the actual particles are more or less porous aggregates of extremely minute crystals rather than larger individual rigu.. 1. H Y ~ ~ Orelo,. ~ . a crystals. A gelatinous pre~ . i ~..~i~it.t. ~ t ~ cipitate ~ ~ contains ~ ~ an indefi-
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484
nite amount of water as an 1' integral part of the solid particles. - In Figures 1,2, and 3 are shown electron micrographs of these three types of precivitate. Thev are hvdrous , , Fe,O,,~. A&l. - , "and B~SO,. respectively. (Since specil I.igYFe 2. AgC1, a Pre. mens must be placed incipitate side a vacuum chamber for electron microscope observation, the hydrous FezOs doubtless lost some of its water before being photographed. Various tests have indicated that it is not completely dehydrated, however, by the vacuum, and a "gelatinous" appearance is clearly evident.) Effect of Rate of Addition of Precipitating Agent. Laboratory instructions for a number of gravimetric analyses state that very slow mixing of reagents is required for proper crystal growth. For this reason MgNH4P04is precipitated by slow neutralization of an acid solution containing the necessary ions rather than by direct mixing of the reagents, while BaS04 is p r e cipitated by the dropwise addition of the precipitating agent. The electron micrographs of Figures 3 and 4 show BaSOl crystals as formed by very slow and by rapid mixing of the reactants. The slow process in this case consisted of mixing' t h e Ba++ and SO4-ions by ion diffusion through a thin membrane, whilz the rapid mixing was accomplished by quickly pouring one solution into the other. Much larger, betterformed crystals of BaSOl are formed by the slow mixing of the reactants. Thus, the importance of the rate of mixing the necessary solutions to form a gravimetricprecipitateisevident. Effect of Order of Addition of Reactants. The order of mixing reactants can be an important consideration, as exemplified by the common sulfate determination by precipitation as BaS04. Figures 4 and 5 show particles of this precipitate Fisure 3. Baso., a c v t d formed by direct addition ~ ~ t p ~ ; ; ~ ~ ~ ; ~ ; b y ~ o ; ~ a ~ of SO&-- to Ba++ and .ion
OCTOBER, 1947
v,;
Pigum 4.
,
,.-,
,
.
- .
-
.
BaSOa Formed by Rapid Addition of Room Temperature
. .i ' .
SO.--
a .
to Bai' et
vice versa, with a11 other conditions being held constant so far as conveniently possible. A larger crystal size is obtained from the latter process. Since large crystals are desirable to aid in easy filtering and to minimize surface adsorption by lessening the total surface, the latter process appears advisable. It should be mentioned t.hat some foreign ions coprecipitate more readily by one procedure than by the other. Cation coprecipitation is more pronounced when Ba++ is added to S04C-and anion roprecipitation when the reverse process is followed. Therefore, for an analysis in the presence of foreign ions which readily coprecipitate, other factors may become of greater significancethan the crystal size. Importance of Hydrogen Ion Concentration. Most gravimetric determinations present specific requirements for the acidity of the precipitating medium a t the time the precipitate is formed. The reasons vary from one determination to another, and the BaS04 determination will here be used as an example. Strong acidity is to be avoided in this analysis because of exc~ssivesolubility of the desired precipitate, yet a slight acidity, besides compensating for a coprecipitation through increased solubility, also yields larger,
Figure 7. Figure 8. B&Oc Formid in 0.06 M HCI by Slow Diffusion
formed crystals through slower crystal formation. This factor is illustrated by a comparison of Figures 3 and 6, in which BaSOa is formed in neutral and in slightly acid solutions, respectively. Again all other factors were held as constant as possible to insure a justifiable comparison. Although the magnitude of differcnce is not great, the crystals from the acid solution do appear larger on an average. Coprecipitation by Adsorption. Although coprecipitation is a factor of importance in nearly all gravimetric determinations, it is especially pronounced in the Bas04 precipitation. Surface adsorption is only one of several mechanisms of coprecipitation, and it may be in the form of mono- or diatomic layers or it may be on a larger scale. In Figures 3, 7, and 8 are shown Bas04 crystals precipitated in the absence of interfering ions, in the
Figure 5.
BaSO, Formed by Rapid Addition of Bai+ Tempant"?* ' .
toso,-- at Room
presence of a significant concentration of Fe+++ ions, and in the presence of N03-ions, respect,ively, all other conditions being constant for all three. When interfering ions are absent, the crystals are seen to be well formed with smooth edges and square corners. In the presence of the interfering cation or anion, the crystals,
BaSOi
o ~ r n e di n
Presence of F e + + + by Slow
. Diffusion
Figure 8. BaSOt Formed i n Prasence of NO.- by Slow Diffusien
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486
Figure 9.
and Ob.
Bas04 Formed by Rapid Addition of S O 4 to for a Faw ~ i n u t . ~
Ba++and Dipsted near IW'C.
although larger, are much more poorly formed and formed, but the larger ones are much in evidence in contain various protuberances from the crystals as this picture. Thirdly, the increase in particle size well as a general scattering of extremely minute, amor- mQ be caused by a coagulation, or coming together phous-appearing material. The preparation of. these into clusters of the smaller particles, as illustrated specimens clearly indicates that the extra material in by Figure 9b. It is obvious that any layers of adthe latter two is caused by the coprecipitating foreign sorbed foreign ions on the surface of the smaller parion. ticks will be trapped as these particles coagulate Coagulation by Heat-Digestion. Digestion of a since the clusters appear to be very d e n s e n o doubt precipitate is a common operation in many gravimetric too dense in many parts to permit penetration of a determinations. The process is illustrated by Figures subsequently used wash solution. This phenomenon is 4, 9a and 9b. Figure 4 is of Bas04 formed by direct known as occlusion and is a common mechanism of mixing of the reagents a t room temperature, while the coprecipitation in gravimetric analyses. other two are of the same suspension after heating Effect of Concentration of Precipitating Agent. Alnear the boiling point for a few minutes. These elec- though the concentration of a solution used to form a tron micrographs illustrate several factors. In the gravimetric precipitate is nota.very critical quantity, first place, much larger pieces of precipitate are formed an approximate specification is ' generally given in by the digestion process, rendering s more filterable laboratory instruction sheets, and the thoughtful stusubstance. Secondly, the increase in dent often wonders why such a value particle size may be caused by a growth was chosen. Figure 10 contains electron of larger crystals as seen in Figure 9a. micrographs of AgCNS precipitated with 0.1 N AgN08 and various concentrations, No doubt the crystal growth is accomplished by small particles dissolving of KCNS as indicated, all ot.her factors ~d reprecipitating on the larger ones. being kept constant. As the concentraThe crystals are not necessarily perfectly (Continued on page 516) , ,,
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.. ..,.. . .:-:., ...'.'.........1 '
Fig10.. AgCNS Psacipiteted born 0.1 M AsNOz end 0.1 N KCNS
F i m m lob. AgCNS Pz&pitatad from 0.1 M AgNO, and 0.01 N KCMS
Figuse loc. AgCNS Prrcipitated from 0 . 1 M AsNOa and 0.031 NKCNS
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ELECTRON MICROGRAPHS AS AN AID IN TEACHING GRAVIMETRIC ANALYPIS (Conlimed from page 4,Yh') q .
tion of the one reagent is decreased, startmg with the Although tlm electroil mirrographs as prtwntrd in most concentrated one, the crystal size is first increased t.his report do not in themmarkedly and then decreased as t,he one reactant's selves prove any new facts, ,concentration is further reduced. This indicates that the frnitf~dnessof further there \is an optimum concentration for the formation investigation along this line ofmlarge crystals. Therefore, reagent. concentration is is indicated. These picsignificance. .fures do present direct visi;Postprecipitation. Postprecipitation is the process of ble evidence of a number of formation of a second pwcipitate upon the crystal factorsencoui~teredingravisurfaces of one substance aftrr that material has prr- metric quantitative analysis cipitated. Fignre 11 illust,ratw the precipitation of that have hithert,~heen AgClon the surfacr of AgCNS crystals. To ohtain this ascertained primarily by specimen, .Ag+ was added (slowly by ion diffusion) to a indirect means. A picture solution containing both C X - and C1-. The thiocya- of a phenomenon along with nate precipitated first because it is less soluble and then a description is often of more Ithe AgCI formed on the surface of the AgCNS crystals, value to a student than a $8 Seen in the micrograph. Clg-stals of AgCNS simimere explanation in words, larly foimed except for the absence of C1- in the medium and t,hiv is t,he realm in have smooth surfaces, as in Figure lob, SO the surface ~ h i c hthe present article 3 1 A(yC1 Postprosipiparticles in Figure 11 mnst he of thr.chloride. should hr of value. t.t.d on surfac. of AVCNS
