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2 Modern Instrumental Methods for Analysis of Soluble Silicates JONATHAN L. BASS

Downloaded by CORNELL UNIV on September 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch002

The PQ Corporation, Research and Development Center, Lafayette Hill, PA 19444

Modern a n a l y t i c a l instrumentation has been used i n the l a s t 25 years f o r determining commercially important c h a r a c t e r i s t i c s o f soluble silicates, and the nature o f silicate species i n silicate glasses and s o l u t i o n s . The c l a s s i c a l wet methods for assay o f silicate s o l u t i o n s are alkali titration and g r a v i m e t r i c determination of silica, which can a l s o be determined, with l e s s e r p r e c i s i o n , by the alkali fluosilicate method. The a l t e r n a t i v e instrumental assay methods, X-ray f l u o r e s c e n c e , atomic spectroscopy and t h e r m o t i t r i m e t r y , will be compared with the classical methods f o r p r e c i s i o n and ease o f measurement. Instrumental methods have g r e a t l y extended the ability o f the analyst to detect trace c a t i o n s and anions i n s o l u b l e silicates. The scope and l i m i t a t i o n s , i l l u s t r a t e d by some a p p l i c a t i o n s , of atomic and X-ray fluorescence spectroscopy, i o n s e l e c t i v e e l e c t r o d e s , and other l e s s common methods f o r impurity a n a l y s i s will be d i s c u s s e d . The techniques o f i n f r a r e d , Raman, X-ray p h o t o e l e c t r o n , and sputter induced photon spectroscopy, used f o r identification of silicate species will be briefly reviewed.

Sodium s i l i c a t e was the 45th l a r g e s t volume chemical produced i n the United States i n 1980, according to the 1981 Chemical and Engineering News Survey (I). Obviously, the a n a l y s i s o f t h i s m a t e r i a l as w e l l as the other major s o l u b l e a l k a l i s i l i c a t e , potassium s i l i c a t e , i s v e r y important commercially. This paper w i l l b r i e f l y review the modern a n a l y t i c a l instrumental methods that are used to determine the q u a l i t y o f commercial s o l u b l e s i l i c a t e s and instrumental

0097-6156/82/0194-0017$06.00/0 © 1982 American Chemical Society

Falcone; Soluble Silicates ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

SOLUBLE

18

techniques that are used i n s t r u c t u r a l c h a r a c t e r i z a t i o n s i l i c a t e s as glasses and i n s o l u t i o n . Assay of Soluble

of

Silicates

C l a s s i c a l Wet


SILICATES

Chemical Assay Methods

Before d e s c r i b i n g the instrumental methods, i t i s important to discuss the c l a s s i c a l wet chemical techniques that form the foundation of a n a l y s i s of soluble s i l i c a t e s . The most e s s e n t i a l chemical property of s i l i c a t e i s the content of a l k a l i and s i l i c a i n e i t h e r the glass or s o l u t i o n . The standard method f o r determining the a l k a l i assay involves t i t r a t i n g a d i l u t e d s i l i c a t e s o l u t i o n with h y d r o c h l o r i c acid to e i t h e r a methyl orange or methyl orange-xylene cyanole end point ( 2 ) . The mixed i n d i c a t o r gives a more d i s t i n c t end p o i n t . S i l i c a content may be determined by e i t h e r the p r e c i s e , tedious gravimetric s i l i c a procedure (2) or the more r a p i d but l e s s p r e c i s e f l u o s i l i c a t e method ( 3 ) . The gravimetric method involves p r e c i p i t a t i o n of the s i l i c a with a c i d , c o l l e c t i n g the p r e c i p i t a t e , ashing, v o l a t i l i z i n g the s i l i c a with h y d r o f l u o r i c acid and determining the weight l o s s a f t e r v o l a t i l i z a t i o n . The f l u o s i l i c a t e method involves r e a c t i n g s i l i c a i n a p r e v i o u s l y n e u t r a l i z e d s o l u t i o n with sodium f l u o r i d e to form sodium f l u o s i l i c a t e and sodium hydroxide by the following reaction: Si(OH)

4

+ 6NaF =

Na SiF 2

6

+ 4NaOH

and t i t r a t i n g the hydroxide with h y d r o c h l o r i c a c i d to the methyl orange end p o i n t . S i l i c a t e to soda r a t i o s can a l s o be determined r a p i d l y for q u a l i t y c o n t r o l purposes by an a l k a l i t i t r a t i o n and a measurement of e i t h e r s p e c i f i c g r a v i t y or r e f r a c t i v e index and v i s c o s i t y which are c o r r e l a t e d to S i 0 / N a 0 r a t i o s using control charts. The c o n t r o l charts are based on samples p r e v i o u s l y analyzed by the p r e c i s e gravimetric method. The s p e c i f i c g r a v i t y method i s more commonly used i n commercial practice. 2

2

Assay by Instrumental Methods The character of chemical a n a l y s i s has changed d r a s t i c a l l y since World War II with the advent of s o p h i s t i c a t e d o p t i c a l systems and e l e c t r o n i c d e t e c t i o n devices, which have been combined into instrumentation now commonplace i n many i n d u s t r i a l l a b o r a t o r i e s . The major advantages of instrumental



2.

BASS

Instrumental

Methods

for

Analysis

a n a l y s i s are speed, s e n s i t i v i t y , v e r s a t i l i t y and r e l a t i v e ease of automation. There are two venerable methods o f instrumental s i l i c a t e a n a l y s i s that predate World War I I , flame photometry f o r the a l k a l i metals (4) and s p e c t r o s c o p i c d e t e c t i o n o f the s i l i c o m o l y b d a t e acid complex i n the v i s i b l e spectrum ( 5 ) . The spectroscopic method has been adapted f o r automated determination o f s i l i c a t e i n detergents ( 6 ) . The s i l i c o m o l y b d a t e method i s a l s o u t i l i z e d to monitor the l e v e l of monomeric s i l i c a i n s i l i c a t e s o l u t i o n s ( 7 ) . More r e c e n t l y developed instruments are capable of determining both a l k a l i and s i l i c a (8-11). Atomic spectrometric instruments determine the t o t a l amount o f an a l k a l i i o n , i n c l u d i n g that due to n e u t r a l s p e c i e s . Therefore the a l k a l i assay by these methods may be g r e a t e r than a t i t r a t i o n method. Atomic absorption (AA) and plasma emission spectroscopy (PES) i n v o l v e decomposition o f the ions i n s o l u t i o n to the atomic s t a t e . In the case o f AA, atoms and ions o f the element being analyzed are v o l a t i l i z e d i n t o the path o f a l i g h t beam emitted from a lamp, and absorb t h i s l i g h t , whose wavelengths are c h a r a c t e r i s t i c o f valence e l e c t r o n i c t r a n s i t i o n s i n the atomic s t a t e . PES i n v o l v e s the e x c i t a t i o n o f valence e l e c t r o n i c t r a n s i t i o n s o f atoms and ions v o l a t i l i z e d i n a plasma a r c . X-ray f l u o r e s c e n c e (XRF) involves e x c i t a t i o n o f core e l e c t r o n s by i n c i d e n t X-rays, followed by X-ray emission at wavelengths that are c h a r a c t e r i s t i c o f the elements present i n e i t h e r s o l u t i o n or s o l i d . F i n a l l y , the thermal t i t r a t o r i s capable of d e t e c t i n g both a l k a l i and s i l i c a by sensing a temperature increase i n an a d i a b a t i c system. In the case o f a l k a l i , the increase i s caused by the heat o f r e a c t i o n due to n e u t r a l i z a t i o n with a c i d , and f o r s i l i c a , by the heat produced during the f l u o s i l i c a t e r e a c t i o n . Comparison o f Wet

Chemical and Instrumental Methods

When choosing whether to use a wet chemical or instrumental methods f o r assay o f a l k a l i s i l i c a t e , the analyst must weigh the compromise between the u s u a l l y higher p r e c i s i o n of wet chemistry and the speed and v e r s a t i l i t y o f an instrument. In a d d i t i o n , the purchase o f an instrument i n v o l v e s a s u b s t a n t i a l c a p i t a l expense with higher operating annual expenses due to the requirements f o r p e r i o d i c maintenance and more expensive s u p p l i e s . However, i f a l a r g e volume o f analyses are run, the cost per sample may be lower using an instrument. Table I summarizes the r e l a t i v e p r e c i s i o n o f the v a r i o u s assay methods. These tabulated values are c o n s e r v a t i v e estimates; experienced a n a l y s t s may achieve b e t t e r p r e c i s i o n between d u p l i c a t e analyses. The t a b l e i n d i c a t e s a 3 to 5 f o l d advantage i n p r e c i s i o n f o r wet chemistry i n most cases.


S O L U B L E SILICATES

20

TABLE I RELATIVE PRECISION OF WET CHEMICAL INSTRUMENTAL ASSAY METHODS Wet

Chemical

Gravimetric s i l i c a +0.05% Fluosilicate

r e a c t i o n +0 .3%


A l k a l i T i t r a t i o n ^0.1%

AND

Instrumental Atomic absorption and emission S i l i c a +2% A l k a l i +1% X-ray Fluorescence S i l i c a +0.5% A l k a l i +1%

Thermometric t i t r a t i o n

Silica Alkali

+0.3% +1%

Silicomolybdate

Silica

+0.5%

However, i n the commercial world the u l t i m a t e i n p r e c i s i o n i s o f t e n not needed to s a t i s f y the s i t u a t i o n a l a n a l y t i c a l requirements. Because the g r a v i m e t r i c procedure involves many time consuming steps, the f l u o s i l i c a t e r e a c t i o n i s g e n e r a l l y p r e f e r a b l e as the usual s i l i c a wet chemical assay method. I t i s not as p r e c i s e as a normal a l k a l i t i t r a t i o n because of the d i f f i c u l t y of observing the end p o i n t . Instrumental methods p l a y an important r o l e i n s i l i c a t e assay when the content of a s p e c i f i c a l k a l i ion i s required or when a large volume of samples j u s t i f i e s the cost of labor saved by using an instrument. For example, the t i t r a t i o n method cannot d i s t i n g u i s h between sodium and potassium i n a mixed a l k a l i s i l i c a t e . A drawback o f atomic, molecular and emission spectroscopy as assay methods i s the extensive d i l u t i o n required to lower analyte concentrations to the l i n e a r o p e r a t i n g range of the instrument 9). This c o n t r i b u t e s a d i l u t i o n e r r o r which reduces the p r e c i s i o n of the a n a l y s i s . An advantage of X-ray fluorescence i s that samples can be analyzed without d i l u t i o n . It i s necessary to use a l k a l i r e s i s t a n t hardware. U n f o r t u n a t e l y , X-ray fluorescence i s the most expensive instrumental assay method. Several papers have appeared w i t h i n the l a s t s e v e r a l years d e s c r i b i n g the a p p l i c a t i o n of thermometric t i t r a t i o n s for s i l i c a t e a n a l y s i s (11). The instrumentation i s l e s s expensive than spectrometers but has not yet r e c e i v e d widespread use i n the U.S. s i l i c a t e i n d u s t r y . However, somewhat analogous procedures are commonplace f o r a n a l y s i s of c a u s t i c and alumina in the Bayer process streams of the aluminum i n d u s t r y (12). The method r e q u i r e s comparison against standards whose assay has been determined by other methods.


2.

BASS

Instrumental

Methods

for

Analysis

The s i l i c o m o l y b d a t e a c i d complex method i s used f o r in-process monitors f o r s i l i c a content up to 50 ppm i n process water. In combination with an automatic sampling and d i l u t i o n system, such a monitor could assay f o r s i l i c a i n a process stream with a p r e c i s i o n o f 0.5 to 1%, r e l a t i v e .


Instrumental

Techniques f o r S i l i c a t e Impurity

Analysis

The use o f modern a n a l y t i c a l instruments has g r e a t l y expanded the a n a l y s t ' s a b i l i t y to determine i m p u r i t i e s i n s i l i c a t e s . Wet chemical methods u s u a l l y are f a r too tedious, s u f f e r from s u b s t a n t i a l i n t e r f e r e n c e or are not s e n s i t i v e enough f o r impurity a n a l y s i s . Even some instrumental techniques are subject to i n t e r f e r e n c e s , r e q u i r i n g separation to be used i n the a n a l y t i c a l procedure. The analyst must a l s o decide on the s e n s i t i v i t y required since lowering d e t e c t i o n l i m i t s u s u a l l y increases the cost o f a n a l y s i s and the s o p h i s t i c a t i o n o f the a n a l y t i c a l procedure. Impurities o f major s i g n i f i c a n c e i n a l k a l i s i l i c a t e s are i r o n , alumina, calcium and magnesium, c h l o r i d e , s u l f a t e , carbonate and t i t a n i a . They may o r i g i n a t e as i m p u r i t i e s i n raw m a t e r i a l s , be added from the manufacturing equipment, or be absorbed from the atmosphere. The degradation o f product q u a l i t y may be manifested as undesirable c o l o r , t u r b i d i t y i n s o l u t i o n , c o r r o s i v e n e s s , l o s s o f a l k a l i n i t y or a l t e r e d r e a c t i v i t y o f products made from the s i l i c a t e (e.g., i r o n o r s u l f a t e may poison a s i l i c a - b a s e d c a t a l y s t manufactured from a silicate solution). Several instrumental techniques are a v a i l a b l e f o r d e t e c t i o n o f both c a t i o n i c and anionic i m p u r i t i e s i n a l k a l i s i l i c a t e s , with d e t e c t i o n l i m i t s ranging down to the parts per b i l l i o n or i n some cases, parts per t r i l l i o n l e v e l . The i n v e s t i g a t o r must be aware that these s e n s i t i v i t i e s are achieved using the analyzed sample. I f s u b s t a n t i a l d i l u t i o n i s required to b r i n g the o r i g i n a l m a t e r i a l i n t o the instrumental operating range, then the d e t e c t i o n l i m i t i n t h i s as-received sample i s f a r h i g h e r . For example, i f one can determine the presence o f element A at the 1 ppb l e v e l i n s o l u t i o n but a s i l i c a t e r e q u i r e s 1000-fold d i l u t i o n before i t can be analyzed, then the d e t e c t i o n l i m i t i n the o r i g i n a l s i l i c a t e i s 1 ppm. S i m i l a r l y , i f a separation procedure i s r e q u i r e d , the d e t e c t i o n l i m i t i n the o r i g i n a l m a t e r i a l i s higher than i n the a l i q u o t being analyzed. Table I I summarizes the c a p a b i l i t y o f s e v e r a l instrumental methods f o r d e t e c t i o n o f i m p u r i t i e s . This t a b l e provides broad guidance; i n the case o f a p a r t i c u l a r s p e c i e s , the analyst must consult the l i t e r a t u r e or perform experiments to f i n d the a c t u a l d e t e c t i o n l i m i t f o r that s p e c i e s .


Falcone; Soluble Silicates ACS Symposium Series; American Chemical Society: Washington, DC, 1982. ppb-ppm i n s o l u t i o n ppt-ppb i n s o l u t i o n ppb-ppm i n s o l u t i o n ppm as r e c e i v e d ppb-ppm i n s o l u t i o n ppb-ppm as r e c e i v e d ppb a f t e r s e p a r a t i o n

Separation, d i l u t i o n Separation, d i l u t i o n Dilution Direct Separation d i l u t i o n D i r e c t , separation

Flame AA Furnace AA Argon Plasma X-Ray Fluorescence Ion S e l e c t i v e Electrodes Neutron A c t i v a t i o n

Detection Limits ppm i n s o l u t i o n ppb to ppm i n s o l u t i o n Β, Ρ ppm i n s o l u t i o n ppm as r e c e i v e d 0.5% carbonate i n g l a s s ppb-ppm as r e c e i v e d ppb a f t e r s e p a r a t i o n

Sample P r e p a r a t i o n Separation, d i l u t i o n Separation, d i l u t i o n Dilution Direct Direct D i r e c t , separation

Technique

Ion Chromatography Ion S e l e c t i v e E l e c t r o d e s Argon Plasma X-Ray Fluorescence Raman Spectroscopy Neutron A c t i v a t i o n

D e t e c t i o n o f Anions

Detection L i m i t s

Sample P r e p a r a t i o n

Technique

D e t e c t i o n o f Cations

INSTRUMENTAL TECHNIQUES FOR SILICATE IMPURITY ANALYSIS

TABLE I I


2.

BASS

Instrumental

Methods

for

Analysis


D e t e c t i o n o f C a t i o n i c Impurities Probably the most commonly used instruments f o r c a t i o n impurity a n a l y s i s o f s i l i c a t e s are flame atomic a b s o r p t i o n spectrophotometers and i o n s e l e c t i v e e l e c t r o d e s . In most cases, s e p a r a t i o n o f s i l i c a i s r e q u i r e d to reduce i n t e r f e r e n c e s . The sample may a l s o have to be d i l u t e d t o b r i n g the analyte c o n c e n t r a t i o n w i t h i n the l i n e a r o p e r a t i n g range. For c a t i o n s , the atomic absorption spectrophotometer i s more v e r s a t i l e than i o n s p e c i f i c e l e c t r o d e s . I f the analyst i s concerned with the presence o f heavy metals, then a c c e s s o r i e s such as a hydride system f o r the elements that form high vapor pressure compounds, e.g., Sb, and a mercury vapor c o l d trap are u s e f u l . I f a l a r g e number o f elements are to be determined, a s u b s t a n t i a l investment i n hollow cathode and e l e c t r o d e discharge lamps must be made. Several gas mixtures w i l l also be r e q u i r e d . The flame atomic absorption spectrophotometer has d e t e c t i o n l i m i t s ranging from the ppb to ppm l e v e l , depending on the element analyzed. Improved s e n s i t i v i t y can be achieved with the use o f the g r a p h i t e furnace which has lower background and atomizes more e f f i c i e n t l y than the flame. In most cases a three order o f magnitude improvement i n s e n s i t i v i t y i s achieved. However, t h i s improvement i n s e n s i t i v i t y r e q u i r e s more c a r e f u l sampling, handling and u l t r a h i g h p u r i t y reagents to be used i n sample p r e p a r a t i o n . The c a l i b r a t i o n procedure i s a l s o more t e d i o u s . In the l a s t 6 to 7 y e a r s , argon plasma emission (PES) instrumentation has been commercialized with d e t e c t i o n l i m i t s u s u a l l y intermediate between flame and furnace AA. The two most common types o f plasma instruments are the i n d u c t i v e l y coupled plasma (ICP) and d i r e c t c u r r e n t plasma (DCP). Although the ICP i s somewhat more s e n s i t i v e i n terms o f reported d e t e c t i o n l i m i t s than DCP, the former cannot t o l e r a t e as high a d i s s o l v e d s o l i d s content as the l a t t e r . Therefore, on the o r i g i n a l s i l i c a t e m a t e r i a l s , the d e t e c t i o n l i m i t s are s i m i l a r . Another advantage o f PES compared to AA i s that commercial PES spectrometers can be configured f o r simultaneous m u l t i - e l e m e n t a l a n a l y s i s , while the c u r r e n t commercial m u l t i - e l e m e n t a l AAs are s e q u e n t i a l . The base p r i c e of PES equipment i s higher than AA but i f the sample load i s h i g h , the increased p r o d u c t i v i t y o f multi-elemental PES may r e s u l t i n a lower cost per a n a l y s i s . Table I I I l i s t s s p e c t r a l l i n e s that are u s e f u l f o r the spectroscopic a n a l y s i s o f major components and i m p u r i t i e s i n soluble s i l i c a t e s .


24

SOLUBLE

SILICATES

TABLE I I I TYPICAL SPECTRAL LINES FOR ATOMIC SPECTROSCOPIC ANALYSIS OF MAJOR AND TRACE ELEMENTS IN SILICATES


ELEMENT

PES

AA

Iron

248

Silicon

252

252

Magnesium

285

280

Titanium

365

335

Sodium

295

590

Calcium

211

397

Aluminum

309

396

Potassium

383

770

nm

238

nm

Two l e s s commonly used techniques, X-ray fluorescence (XRF) and neutron a c t i v a t i o n a n a l y s i s (NAA) have the advantage that as-received samples can be analyzed, glasses as w e l l as s o l u t i o n s . Both are more expensive than the p r e v i o u s l y mentioned techniques. The NAA technique that produces the greatest s e n s i t i v i t y r e q u i r e s i r r a d i a t i o n i n a research nuclear r e a c t o r and hence i s r e a l l y p r a c t i c a l only when d e t e c t i o n of low l e v e l s o f unusual c a t i o n s i s r e q u i r e d . Sodium s i l i c a t e i s somewhat more d i f f i c u l t to analyze than many other m a t e r i a l s because of the formation of the r e l a t i v e l y long l i v e d r a d i o n u c l i d e N a ^ whose emissions i n t e r f e r e with the d e t e c t i o n of other elements. Nevertheless we were able to determine, i n a sample of sodium s i l i c a t e , that many heavy elements of t o x i c o l o g i c a l concern were undetectable down to the ppm to ppb l e v e l i n the u n d i l u t e d s i l i c a t e (13). An XRF spectrometer can be configured to perform s e q u e n t i a l multi-elemental analyses. It i s less s e n s i t i v e to the elements of lower atomic number. A l s o , since the X-rays penetrate only to a depth of about 10 um, the sample must be homogeneous. S o l i d samples must be presented to the X-ray beam with a f l a t s u r f a c e . However, the r e l a t i v e ease of sample p r e p a r a t i o n and the a b i l i t y to run glasses and s o l u t i o n s with only minor d i l u t i o n make X-ray fluorescence a u s e f u l technique where a n a l y s i s f o r a wide range of i m p u r i t i e s is required.


2.

BASS

Instrumental

Methods

for

Analysis

D e t e c t i o n o f Anionic Impurities D e t e c t i o n o f anionic i m p u r i t i e s i n a l k a l i s i l i c a t e s has not been as f u l l y developed as f o r c a t i o n s . The anion o f greatest concern i s carbonate which i s absorbed from the atmosphere. P o t e n t i a l l y , carbonate could o r i g i n a t e from the soda ash or potash raw m a t e r i a l used i n s i l i c a t e manufacture but under normal furnace o p e r a t i o n the ash should be thoroughly decomposed. The standard c l a s s i c a l method f o r carbonate a n a l y s i s i n v o l v e s a c i d i f i c a t i o n and b o i l i n g o f the s o l u t i o n to r e l e a s e C 0 which i s adsorbed on A s c a r i t e * The procedure i s time consuming and subject to e r r o r s r e s u l t i n g from d i f f i c u l t i e s i n maintaining uniform flow. Two instrumental methods that show promise are i o n chromatography (14) and l a s e r Raman spectroscopy (15). Using a s i z e e x c l u s i o n column, carbonate has been determined down to the ppm l e v e l . This technique has not yet been a p p l i e d to s o l u b l e s i l i c a t e s , which may r e q u i r e s e p a r a t i o n o f the s i l i c a . Laser Raman spectroscopy has been a p p l i e d to carbonate determination down to the 0.5% l e v e l i n potassium s i l i c a t e g l a s s , using bands at 1770, 1428 and 575 cnf"l.


2

Using the proper choice o f separation column, i o n chromatography appears to be a p p l i c a b l e f o r the s e q u e n t i a l multicomponent a n a l y s i s o f other anions such as s u l f a t e , c h l o r i d e , f l u o r i d e and n i t r a t e . The d e t e c t i o n l i m i t s w i l l be s u b s t a n t i a l l y lower than the c l a s s i c a l g r a v i m e t r i c and potentiometric methods c u r r e n t l y used. Ion s e l e c t i v e e l e c t r o d e s are a v a i l a b l e f o r c h l o r i d e and f l u o r i d e . A s i l i c a t e sample r e q u i r e s s e p a r a t i o n i n order to remove interferences. As i n the case o f c a t i o n s , NAA and XRF permit d i r e c t a n a l y s i s f o r impurity elements that may be present i n an anionic form. XRF i s capable o f d e t e c t i n g P, S, CI, Br and I. NAA can determine CI, Br and I at the ppm l e v e l i n the as-received s t a t e , depending on the m a t e r i a l and at lower l e v e l s using radiochemical s e p a r a t i o n . F i n a l l y , argon plasma emission spectroscopy can determine the presence o f two other elements, which can be present as anions, Β and P. The technique i s f a r more s e n s i t i v e f o r the former element which can be detected at the ppb l e v e l i n s o l u t i o n , while Ρ can be detected at the ppm l e v e l . Both elements can also be analyzed by atomic absorption spectroscopy, but with l e s s s e n s i t i v i t y . A p p l i c a t i o n s o f Advanced Instrumentation to S i l i c a t e Structural Analysis The l a s t 25 years, and e s p e c i a l l y the l a s t 10, have seen the a p p l i c a t i o n o f advanced, expensive instrumental techniques


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to the s t r u c t u r a l c h a r a c t e r i z a t i o n of s i l i c a t e s both i n the glass and s o l u t i o n s t a t e s . Other c o n t r i b u t o r s to t h i s symposium have discussed e x t e n s i v e l y the use o f nuclear magnetic resonance (NMR) spectroscopy and t r i m e t h y l s i l y l a t i o n combined with g a s - l i q u i d chromatography (GLC), g e l permeation chromatography (GPC) and mass spectroscopy (MS) to analyze the nature of s i l o x y b r i d g i n g i n s i l i c a t e s o l u t i o n s . This paper w i l l b r i e f l y d e s c r i b e the r e s u l t s o f some other techniques that are l e s s f r e q u e n t l y used. V i b r a t i o n a l spectroscopy, both l a s e r Raman (16) and i n f r a r e d (16, 17), can be a p p l i e d as a u s e f u l supplement to the data developed by NMR and TMS f o r c h a r a c t e r i z i n g s i l i c a t e species i n s o l u t i o n . The number of bands observed i n v i b r a t i o n a l spectroscopy depends on the symmetry o f the s i l i c a t e species present. Protons attached to the Si-0 bonds lower the symmetry compared to the S i O ^ i o n . In t h i s way, M a r i n a n g e l i , et a l (15), assigned seven l a s e r Raman bands i n a sodium m e t a s i l i c a t e s o l u t i o n adjusted to pH 14 to the presence of S i 2 ( 0 H ) 2 . The s p e c t r a are shown as Figure 1. As the pH was lowered, s h i f t s of bands to higher frequencies (930-1000 cm"" ) were observed. In unadjusted sodium m e t a s i l i c a t e s o l u t i o n (pH 13.3), i n f r a r e d bands a t t r i b u t e d to the transformation of Si02(OH2)~ i n t o SiO(OH)" and the dimer Si2C>3(0H)4 appear. These bands were assigned by analogy to bands observed i n the i n f r a r e d s p e c t r a of c r y s t a l l i n e s i l i c a t e s . When the s o l u t i o n i s further a c i d i f i e d , bands at higher frequencies (1000-1120 cm~l) assigned to polymeric species were observed. These s h i f t s were also observed i n the i n f r a r e d by Beard (17) who studied s i l i c a t e s of d i f f e r e n t s i l i c a to a l k a l i r a t i o s . He also observed changes i n i n t e n s i t i e s , over a period of s e v e r a l days, f o r s i l i c a t e s o l u t i o n s produced by dissolving s i l i c a in a l k a l i . These changes were a t t r i b u t e d to depolymerization o f the high molecular weight s i l i c a t e species o r i g i n a l l y formed. The nature of the s i l i c o n - o x y g e n bond i n a l k a l i s i l i c a t e glasses as the sodium content increases has been i n v e s t i g a t e d by e l e c t r o n spectroscopy f o r chemical a n a l y s i s (ESCA) and h i g h r e s o l u t i o n X-ray fluorescence spectroscopy (18-21). ESCA has shown that the binding energy o f oxygen Is e l e c t r o n s of non-bridging oxygen i s about 2ev l e s s than that of b r i d g i n g oxygens. This r e s u l t i s i l l u s t r a t e d by the deconvoluted 0^ ESCA spectrum i n Figure 2 (18) . At low sodium c o n c e n t r a t i o n , sodium i s a s s o c i a t e d with non-bridging oxygens ( i . e . , the network terminating oxygens). However, at higher sodium c o n c e n t r a t i o n s , the number of oxygen atoms with t h i s lower binding energy as i n d i c a t e d by the peak i n t e n s i t y i s l e s s than the number of sodium ions i n d i c a t i n g that some of these ions are d i s p e r s e d i n the network (18). In a d d i t i o n , the chemical 2

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3

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Figure 14 (a); 2.5 M 2.5 M

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1. Raman spectra of aqueous solutions of 2.5 M Na SiO$, 4 M NaOH, pH 2.5 M Na SiO , 0.5 M NaOH, pH 13.4 (b); 2.5 M Na SiO , pH 13.3 (c); Na SiO , 1.25 M HCl, pH 13 (d); 2.5 M Na SiO , 2.5 M HCl, pH 12.5 (e); Na SiO , 3.75 M HCl, pH 11.5 (f). (Reproduced, with permission, from Ref. 16. Copyright 1978, Multiscience Publications Ltd.) 2

2

2

s

2

s

2

2

s

s

s

525

530

535

540

545

Binding Energy (E.V.) Figure 2. 30% Na 0 2

Binding energy (EV). ESCA O(ls) spectrum of a sodium silicate glass, + 70% Si0 . (Reproduced, with permission, from Ref. 18. Copyright 1979, North-Holland Publishing Co.) 2


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s h i f t i s l e s s pronounced at higher sodium c o n c e n t r a t i o n , i n d i c a t i n g a trend toward energy equivalence of the non-bridging and b r i d g i n g oxygens (19). These ESCA data are complemented by high r e s o l u t i o n X-ray fluorescence spectroscopic r e s u l t s which show a decrease i n the average strength of s i l i c o n - o x y g e n bonds (20) and a r e l a t i v e decrease of p o s i t i v e change on s i l i c o n atoms (21) with i n c r e a s i n g sodium c o n c e n t r a t i o n . These trends were monitored by observing chemical s h i f t s i n s i l i c o n Κ X-ray l i n e s . The e f f e c t o f h y d r a t i o n on hydrogen and sodium d i s t r i b u t i o n i n a l k a l i s i l i c a t e g l a s s e s has been studied by sputter induced photon spectroscopy (SIPS) and by i n f r a r e d r e f l e c t i o n and t r a n s m i s s i o n spectroscopy (22, 23). SIPS i s a r e l a t i v e l y uncommon but powerful technique which i n v o l v e s measuring the i n t e n s i t y o f c h a r a c t e r i s t i c emission l i n e s o f molecular and atomic fragments sputtered from the surface of materials. I t s advantages as a surface technique l i e i n the a b i l i t y to detect hydrogen ( u n l i k e ESCA or Auger spectroscopy) and n e u t r a l species ( u n l i k e SIMS) . Using t h i s technique Houser, et a l . , were able to determine that i n s i l i c a t e g l a s s hydrated at 30° for one hour hydrogen had d i f f u s e d inward from the surface f o r a d i s t a n c e of 2 urn, with accompanying d e p l e t i o n of sodium i n t h i s l a y e r . Figure 3 shows the depth p r o f i l e o f hydrogen and sodium i n a Na20*3Si02 g l a s s under these c o n d i t i o n s (22). The presence o f a broad band i n a t h i n f i l m of hydrated " s i l i c a t e at 3360cm" was i n t e r p r e t e d by Doremus (23) as i n d i c a t i n g the presence o f hydronium i o n s . He a l s o observed i n r e f l e c t i o n s p e c t r a o f hydrated s i l i c a g l a s s a decrease o f the 950 cm" band i n t e n s i t y , assigned to the Si-0 M s t r e t c h i n g v i b r a t i o n and a major increase i n the Si-O-Si s t r e t c h at 1050-1100 cm" . He a t t r i b u t e d these changes to the formation o f a porous g e l l a y e r produced by h y d r o l y s i s of the surface l a y e r . It i s l i k e l y that f u r t h e r a p p l i c a t i o n s of s o p h i s t i c a t e d instrumentation to a n a l y s i s of s i l i c a t e s w i l l appear i n future literature. In a d d i t i o n to ESCA, SIPS, X-ray spectroscopy, l a s e r Raman and d i s p e r s i v e i n f r a r e d spectroscopy, newer techniques such as F o u r i e r transform i n f r a r e d and photoacoustic spectroscopy may be used as t o o l s to characterize s i l i c a t e structure. 1

1

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1



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Analysis

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4.00

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Figure 3. Depth profiles of H and Na in a Na O · 3Si0 glass after hydration of 1 hat 30° C. The intensities of both H and Na are expressed in photon counts/s. (Reproduced, with permission, from Ref. 22. Copyright 1980, North-Holland Publishing Co.) t

9


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Literature Cited 1. Chemical and Engineering News, 54, May 4, 1981. 2. J . G. Vail, "Soluble S i l i c a t e s " , V o l s . 1 and 2, Reinhold, New York (1952). 3. N. A. Tananaeff and A. K. Babko, Ζ. a n a l . Chem. 82, 145 (1930). 4. I. A. V o i n o v i t c h , J . Debras-Guedon and J . L o u v r i e r ; "The A n a l y s i s of Silicates"; Herman; P a r i s (1965). 5. J . D. H. S t r i c k l a n d , JACS 74, 862 (1952). 6. S. W. Babulak and L. Gildenberg, JAOCS 50, 296 (1973). 7. R. K. Iler, "The Chemistry o f Silica", John Wiley; New York (1979). 8. C. Manoliu, B. Tomi, A. Daescu and T. Petrue, Rev. Chim, 24, 639 (1973). 9. K. Govindaraju, G. Mevelle and C. Chouard, A n a l . Chem. 48, 1325 (1976). 10. W. W. F l e t c h e r , Glass Technology, 17, 226 (1976). 11. H. Strauss and R. Rutkowski, S i l i k a t t e c h n i k , 29, 339 (1978). 12. E. Van Dalen and L. G. Ward, A n a l . Chem. 45, 2248 (1973). 13. L. Kovar, p r i v a t e communication (1979). 14. H. Small, T. S. Stevens and W. C. Bauman, A n a l . Chem. 47, 1801 (1975). 15. H. V e r w e i j , H. Van den Boom and R. E. Breemer, J . Am. Cer. Soc., 60, 529 (1977). 16. A. M a r i n a n g e l i , M. A. M o r e l l i , R. Simoni and A. B e r t o l u z z a , Can. J . Spectroscopy 23, 173 (1978). 17. W. C. Beard, 3rd I n t e r n a t i o n a l Sumposium on Molecular Sieves, 162 (1973). 18. J . S. Jen and M. R. K a l i n o w s k i , J. Non Cryst S o l i d s , 38, 21 (1979). 19. R. Bruckner, H. W. Chun, H. G o r e t z k i and M. Sammet, J . Non C r y s t . S o l i d s , 42, 49 (1980). 20. S. Sakka and A. Senga, J . Mat. S c i . , 13, 505 (1978). 21. T. Maekawa, N. K i k u c h i , S. Sumita and T. Yokokawa, B u l l . Chem. Soc. Japan, 51, 780 (1978). 22. C. A. Houser, J . S. Herman, I. S. T. Tsong and W. B. White, J . Non C r y s t . S o l i d s , 41, 89 (1980). 23. R. H. Doremus, J . Non C r y s t . S o l i d s , 41, 145 (1980). RECEIVED

March

2,

1982.


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