HAROLD W . K N UDSON, C. JU D AY , a n d V. W . M E L O C H E U n iv e rsity o f Wise
T
H E presen t research represents an effort to stan d ard ize a colorim etric procedure for th e rap id d eterm in atio n of sm all q u an titie s and th e d etection of m inute changes in th e concentration of dissolved silica in n a tu ra l w aters of very low phosphate content.A prelim inary stu d y indicated th a t tw o general procedures were available. T h e m ethod of D ienert and W andenbulcke (4) takes ad v a n ta g e of th e yellow silicom olybdate color pro
duced w hen am m onium m olybdate reacts in acid m edium w ith dissolved silica. Isaacs’ m ethod (7) a tte m p ts to extend th e sensitivity of th is reaction b y reducing th e m olybdenum in th e silicom olybdate complex to th e relatively intense m olyb
denum blue color.
Fig u r e 1
P h o sp h ate interferes w ith b o th m ethods (12) b u t to a greater e x ten t in th e reduction m ethod. O ther serious ob
jections to th e reduction m ethod are interference due to iron or o th e r reducing agents an d th e in stab ility of th e blue color (3). A lthough m odifications of Isaa cs' procedure hav e been proposed to elim inate interference due to phosphates (1 ,5 ), th e o ther objections rem ain.
T h e procedure of D ien e rt and W andenbulcke w as se
lected for th is investigation because it w as sim ple an d rapid.
T he au th o rs hoped to increase its sensitivity a s well as its precision b y a critical exam ination of th e conditions su rro u n d ing th e developm ent of th e color and b y an im provem ent in th e m ethod of m easuring th e color.
E q u ip m e n t
T he following special equipm ent was used: a pH electrometer, a Duboscq colorimeter, m atched Nessler tubes, a
Cenco-Sanford-, M a d iso n Cenco-Sanford-, Wis.
Sheard photelometer, an Evelyn photoelectric colorimeter, and a specially designed therm opile-type spectrophotometer employing a continuous light source (8).
R e a c t io n
T h a t th e ra tio of silica to m olybdenum trioxide is 1 to 12 in th e sim ple reaction betw een am m onium m olybdate and so
dium silicate has been established for som e tim e. H ow ever, it w as necessary to show th a t th is relationship is co n stan t over th e entire range of concentrations studied. K now n am o u n ts of am m onium m olybdate were added to solutions containing an excess of dissolved silica and a fte r acidification and de
velopm ent of m axim um color, m easurem ent of th e in te n sity of th e color produced showed th e am o u n t of silica w hich re
acted. D a ta for several concentrations are p lo tted in Figure 1. A stra ig h t line is obtained w hich extrapolates to th e ori
gin. T h e slope of th e line is alm ost precisely l/ n , indicating th a t th e mole ratio is 1 SiC>2/12 M o 03 for th e concentrations studied.
E ffe c t o f M o ly b d a te C o n c e n t r a t io n , A c id ity , a n d T im e
M o l y b d a t e C o n c e n t r a t i o n . I t was recognized early in th is investigation th a t although am m onium m oly b d ate in m olecular concentration tw elve tim es th a t of th e silica would suffice for th e color reaction, a certain excess is desirable in order to reduce m aterially th e tim e required for m axim um color developm ent.
U sing th e photelom eter to d e te c t in te n sity of color and allowing no m ore th a n 10 m inutes for color developm ent, th e following s tu d y was m ade.
ML. 10^ AMMONIUM MOLYBDATE Fig u r e 2
MAY 15,1940 ANALYTICAL E D IT IO N 271 Six or more samples of the same concentration of dissolved
silica were poured into sample tubes and a known am ount of ex
actly 10 per cent ammonium molybdate solution was added to each. Ten m inutes after acidification and dilution to 100 ml. the intensity of color of each sample was measured with the photel
ometer and the minimum concentration of molybdate necessary for full color development within the 10-minute period was re
corded. This procedure was repeated for several different con
centrations of dissolved silica.
T h e d a ta are p lo tted in F igure 2, w here th e m inim um m o ly b d ate concentration required for full color developm ent w ithin 10 m inutes is p lotted against th e concentration of dis
solved silica. I t is a p p a re n t th a t th e am o u n t of m olybdate required is a straight-line function of th e silica concentration.
B y extrap o latin g th e curve to 0 p. p. m. of silica, one can de
term ine th e excess of m olybdate th a t is required for th e maxi
m um color developm ent w ithin 10 m inutes.
T h is excess is found to be 0.32 ml. of 10 per cent am m onium m o ly b d a te solution in 100 ml. of sample, or 0.0036 mole per liter of m olybdenum trioxide. A greater excess does n o t appreciably sh o rten th e tim e required for full color develop
m ent. A large excess is to be avoided because th e color is less stable. In ac tu a l practice, 2 ml. of 10 per cent am m onium m olybdate solution per 100 m l. of sam ple were found m ost satisfactory for concentrations of silica as high as 50 p. p. m.
A c i d i t y . P revious au th o rs have reported th a t acidity has a n im p o rta n t influence on color developm ent (6, 12, 15), n o t only w ith respect to th e in te n sity of th e color b u t also th e ra te of color developm ent an d color decay.
Three concentrations of dissolved silica solutions were selected for this s tu d y : 50, 20, and 5 p. p. m. A series of 100-ml. sample tubes was filled w ith silica samples, all of the same concentration, and treated w ith 2 ml. of 10 per cent molybdate solution. Vary
ing am ounts of acid were added to give a series of solutions of
decreasing pH. At 5 and 10 minutes after the acidification, the color development was noted on the photelometer. Such a series was repeated several times for each of the three concentrations of silica mentioned above.
T he results are illustrated graphically in F igures 3 an d 4, 5 and 10 m inutes, respectively, being allowed for color de
velopm ent. I n each figure th e per cent relative deflection of th e photelom eter is p lo tted against p H , w hich is eq uivalent to plo ttin g th e per cent of theoretical color developm ent against pH . Figures 3 and 4 indicate th a t th e p H lim it for m axim um color developm ent for sm all am o u n ts of silica is 1.6 to 2.0.
Fig u r e 4
T h e effect of increased tim e for color developm ent is to broaden th e p H range for higher concentrations of silica, b u t for 5 p. p. m . of silica th is effect is h ard ly noticed. I t is evident th a t g rea t precautions m u st be ta k e n to ensure proper ad ju stm e n t of p H for m easurem ents of sm all am o u n ts of silica.
W hereas th e ad ju stm e n t is less critical for higher concentra
tions of silica, th e color developm ent falls off rapidly on either side of th e optim um p H range an d care should be ta k en to operate w ithin th e proper lim its. Several buffer system s were investigated, b u t in general it w as found m ore convenient and reliable to add a predeterm ined am o u n t of free acid to th e sam ple in order to produce th e proper p H .
In stea d of using several drops of concentrated acid as rec- com m ended in m a n y procedures ( 4 , 9 , 1 0 ,1 1 , 1 3 , 1 5 ) , i t is sug
gested th a t a larger volum e of d ilu te acid b e used. One m illiliter of 4 N sulfuric acid gave th e desired p H for sam ples of n a tu ra l w aters in this investigation.
T i m e . U nder th e proposed procedure an d w ith in th e lim its of concentration used, it w as found t h a t th e m axim um color developed w ithin 10 m inutes an d rem ained unchanged for a t least 0.5 hour.
P r o c e d u r e
To a 100-ml. sample, add 2 ml. of a 10 per cent ammonium molybdate solution. Mix and immediately acidify to a pH of 1.6 to 2 .0 (the authors used 1 ml. of 4 N sulfuric acid). T he am ount of acid needed should be predetermined from pH measurements of the original sample. (It is im portant th a t the acid be added im
mediately following the addition of ammonium molybdate. If there is more th a n a m inute’s delay, the tim e required for full color development is appreciably increased.) After 10 minutes, compare w ith standards or read in a photom eter from which a use of buffered chrom ate according to th e ir procedure.
P h o t o e le c t r ic M e a s u r e m e n t s
tions of silicom olybdate absorb strongly only a t w ave lengths shorter th a n 4500 A. T he
MAY 15,1940 ANALYTICAL E D IT IO N 273 th e error is com paratively high. T able I shows typ ical results
using th e Cenco and E velyn photoelectric colorim eters. In concentrations as low as 1 p. p. m. using th e Cenco in stru m e n t th e per cent error is of th e order of 1 per cent. In concentra
tions less th a n 1 p. p. m., th e error increases, ranging to 10 per cent for concentrations as low as 0.25 p. p. m . U sing a thicker cell such as th e E v ely n provides, or a special 20-mm.
cell for th e Cenco, it is possible to m ake readings as low as 0.10 p. p. m. w ith an error of ab o u t 10 per cent.
Conclusion
M a n y n a tu ra l w aters in n o rthern W isconsin show very low silica concentrations. I t is im p o rta n t n o t only to m easure these low concentrations b u t also to follow sm all variations in silica concentration from tim e to tim e.
A m ore careful control of th e D ienert an d W andenbulcke procedure for th e colorim etric estim ation of silica is de
scribed. B y control of p H and use of photoelectric colorime
te rs for th e m easurem ent of color, th e procedure has been m ade m ore precise. T h e sensitivities of th e Cenco an d th e E v ely n photoelectric colorim eters were com pared for this system an d found to b e n early identical when cells of th e same thickness w ere used. T he careful selection of filters b y m eans of a spectrophotom eter is recommended.
A c k n o w le d g m e n t
T h e au th o rs wish to th a n k th e J . T . B aker Chem ical C om p an y for th e g ra n t w hich facilitated th is research a n d also express appreciation to th e J. T . B rittin g h am F u n d for th e special spectrophotom eter w hich w as used in th is w ork.
L it e r a t u r e C ite d (1) Alimarin and Zverov, Mikrochemie, 22, 89 (1937).
(2) Am. Pub. Health Assoc., New York, “Standard Methods for the Examination of Water and Sewage” , 8th ed., 1936.
(3) Bertrand, Bull. soc. chim. biol., 6, 157 (1924).
(4) Dienert and Wandenbulcke, Compt. rend., 176, 1478 (1923).
(5) Foulger, J. Am. Chem. Soc., 49, 429 (1927).
(6) Galakhov, Zavodskaya Lab., 6, 1011 (1937).
(7) Isaacs, Bull. soc. chim. biol., 6, 157 (1924).
(8) James and Birge, Trans. T7is. Acad. Sci., 31, 1 (1938).
(9) King, Contrib. Can. Biol. Fisheries, 7, Nos. 8-11, Series D, Nos.
1-1, 121-5 (1931).
(10) King and Lucas, J. Am. Chem. Soc., 50, 2395 (1928).
(11) Robinson and Kemmerer, Trans. Wis. Acad. Sci., 25, 129 (1930).
(12) Schwartz, M. C., Ind. Eng. Chem., Anal. Ed., 6, 364 (1934).
(13) Strohecker, R., Vaubel, R. and Breitwieser, K„ Z. anal. Chem., 103, 1 (1935).
(14) Swank and Mellon, Ind. Eno. Chem., Anal. Ed., 6, 348 (1934).
(15) Thayer, Ibid., 2, 276 (1930).
Pr e s e n t e d before th e D iv isio n of P h y sic a l a n d In o rg an io C h e m istry a t th e 9 0 th M eetin g of th e A m erican C hem ical Society, M ilw au k ee, W is.