B y m eans o f the recording photoelec
tric spectrophotom eter, which provides a m ethod o f color m easurem ent entirely thiosulfate, bisulfite, and fluoride interfere, while Sagai- dachnuii and R avich (8) say th a t citrate, ta rtra te , and oxalate m u st be absent.
T he purpose of th e w ork described in this paper was to m ake a stu d y of this m ethod b y m eans of th e photoelectric recording spectrophotom eter (i?),‘w ith p articular atten tio n to the effect of o ther elem ents and ions upon th e color system . Sim ilar studies of other colorimetric m ethods have recently been m ade by Swank and Mellon (12) and by W right and M ellon (14). If two solutions have th e sam e color under an y condition of illum ination and to an y observer, th e y will give identical spectral transm ission curves. By such curves very sm all differences in color in ten sity and in hue can th e re
fore be detected.
A pparatus and Solutions
All spectrophotometric measurements in the present work were made with the instrument built for the Departm ent of Chemistry of Purdue University by th e General Electric Co. (5). All determinations of pH values were made with the glass electrode setup described by Mellon (4).
Standard solutions of iron, each milliliter of which contained
0 . 1 mg. of iron, were prepared by dissolving weighed amounts of ferrous ammonium sulfate of known iron content in water con
taining dilute sulfuric acid and by dissolving weighed amounts of iron wire of known purity in dilute hydrochloric acid and in perchloric acid, oxidizing the iron in the first two cases with a few milliliters of 3 per cent hydrogen peroxide and boiling out the excess. The hydrocliloric acid solution was used for the tests involving calcium, strontium, and barium ions, the perchloric acid solution for the lead-ion tests, and the oxidized ferrous ammonium sulfate solution for all other tests. Standard solu
tions of the metals were prepared from the chloride, nitrate, glacial acetic acid and 15 M ammonium hydroxide, respectively.
Redistilled w ater was used for all solutions.
For producing the color system Snell’s directions (10) were followed. Five milliliters of the standard iron solution, repre
senting 0 . 5 mg. of iron, were nearly neutralized by adding 1 to 1 the rear beam of light a similar cell filled with distilled water.
Conform ity to Beer’ s L aw
T h a t th e color system follows B eer’s law, a t least up to a concentration of 2 0 mg. of iron per liter, is a p p a ren t from T able I, in which are shown th e observed transm ittancies a t 520 m u, th e wave length of m axim um absorption, for ten iron solutions of different concentrations, together w ith the transm ittancies calculated from th e tran sm itta n cy of the
M ARCH 15, 1938 ANALYTICAL ED ITIO N 137 tions of p H value in order to bring th e la tte r w ithin th e proper
limits.
T able I I shows th e effect on th e color in te n sity and on th e p H value w hen th e volum e of acetic acid is varied from 10 ml. T he calculations are based upon the solutions contain
ing 1 0 ml. of acetic acid.
T a b l e II. E f f e c t ' o f V a r y i n g V o l u m e s o f A c e t i c A c i d (0 .5 m g . of ir o n p e r 100 m l.; 1 .9 6 1 -cm . cell)
V o lu m e of T r a n s m i t t a n c y
A p p a r e n t C h a n g e in
A c e tic A cid p H a t 5 20 tnn I r o n C o n c n .
M l. % %
1 5 .0 2 . 4 5 3 .1 - 4 . 4
1 2 .5 2 . 5 5 2 .4 -2 . 2
1 0 . 0 2 . 6 5 1 .6
7 . 5 2 .7 5 1 .0 + ['.8
5 . 0 2 . 8 5 0 .7 +2 . 8
T hese results indicate th a t th e volum e of acetic acid m ay v a ry from ab o u t 8 to 1 2 m l. if a 2 per cent error in th e iron concentration is allowed as th e lim it. This lim it was chosen in accordance w ith th e procedure of Swank and Mellon (12). T he presence of 2 drops of concentrated hydrochloric acid in addition to 1 0 ml. of acetic acid gave a decrease in color intensity corresponding to an error of 13.2 per cent.
Mg. Iron p e r L iter
Fi g u r e 1 . Co n f o r m i t y t o Be e r’s La w
Small am ounts of n itric and sulfuric acids caused sim ilar de
creases in inten sity . In actual practice, however, since the solution is alm ost neutralized w ith am m onium hydroxide before th e salicylate reagent is added, none of these acids will be present in objectionable am ounts.
T able I I I shows th e effect when volumes of sodium salicyl
a te solution greater th a n th e specified 1 ml. are used.
T a b l e III. E f f e c t o f V a k y i n g V o l u m e s o f S o d i u m S a l i c y l a t e
(0 .5 m g . o f ir o n p e r 100 m l .; 1 .9 6 1 -c m . cell)
A p p a r e n t C h a n g e in
V o lu m e of T r a n s m itt a n c y
S a lic y la te p H a t 5 2 0 m/i I r o n C o n c n .
M l. % %
1 2 . 6 5 2 . 0
2 3 . 2 5 0 .2 + 5 . 0
3 3 . 4 4 9 .4 + 8 . 0
4 3 . 5 4 7 .7 + 1 3 .2
As th e volum e of salicylate increases a noticeable brownish tin t develops. I t is evident th a t n o t more th a n 1 ml. should be used unless th e volum e of acetic acid is increased in order to lower th e p H value. T he solution of sodium salicylate gives a tran sm itta n c y of practically 1 0 0 per cent throughout th e range of 400 to 700 m u, thus showing no absorption.
S tab ility o f the Color
T here is some difference of opinion as to th e sta b ility of th e color. Yoe (15) sta te s th a t i t fades fairly rapidly in th e light, while Snell (10) says it is stable for 48 hours, b u t fades rapidly in sunlight. Curves were m ade for four solutions, containing 0.2, 0.5, 0.7, and 1.0 mg. of iron per 100 ml., a t intervals after th e solutions had stood in glass-stoppered Pyrex bottles in diffuse light under ordinary lab o rato ry con
ditions. T he color was stable w ithin th e 2 per cen t lim it for 6 6 hours. E xposure to direct sunlight for 6 hours greatly accelerated fading.
Effect o f Anions
In th e interference studies th e curve produced by th e sta n d ard iron solution containing 0.5 mg. of iron per 100 ml. w a3 com pared w ith th e curve produced by a sim ilar iron solution containing a known am ount of th e added ion. F rom the transm ittancies a t 520 m/x of th e two solutions and th e known concentration of th e sta n d ard iron solution, by use of th e form ula used above in th e B eer’s law calculations, th e ap p are n t concentration of iron in th e solution containing th e added ion was calculated. T he difference between this value and th e actual concentration m ultiplied b y 1 0 0 and divided by th e actual concentration gave th e percentage error. As ex
plained above, a 2 per cent error was a rb itra rily se t as the m axim um allowable for no interference.
A num ber of th e common anions cause a decrease in th e color intensity, either through their a b ility to form colorless complexes w ith ferric ion which are m ore stable th a n th e sali
cylate ferric complex or because th e y reduce th e iron to the ferrous condition. In th e form er class are ta rtra te , oxalate, citrate, orthophosphate, pyrophosphate, arsenate, cyanide, tungstate, and fluoride, while in th e la tte r class are sulfite, thiosulfate, and iodide. In Figure 2, curve 3 shows th e effect of 1 mg. of fluoride per 100 ml., curve 4 th e effect of 10 mg. of phosphorus pentoxide (present as orthophosphate) per 1 0 0 ml., and curve 5 the effect of 1 mg. of pyrophosphate per 100 ml. A lthough n itrite acts as a reducing agent, i t also causes a change in hue and th e solution becomes yellowish green, probably because of form ation of colored products from sec
ondary reactions. Because it forms a reddish complex with ferric ion, thiocyanate causes a change in hue, as shown by a change in th e general shape of th e curve (Figure 2, curve 2).
Iodide n o t only tends to decrease th e in te n sity of th e color because of th e rem oval of ferric ion b y reduction, b u t also to increase th e intensity because of th e red color of th e liberated iodine. Since a num ber of factors, such as acidity, tim e, and
Fi g u r e 2 . Ef f e c t o f An i o n s
0 .5 m g . o f i r o n , d iv e r s e io n , a n d 1 m l. of 10 p e r c e n t s a li c y la te in 100. m L o f s o lu ti o n . 1 .9 6 1 -cm . cell
138 IND U STR IA L AND E N G IN E E R IN G CHEM ISTRY VOL. 10, NO. 3 tem perature, affect th e ra te of reduction, it is safest to rem ove
all iodide before m aking th e determ ination of iron.
T he effects of th e com mon anions an d th eir approxim ate degree of interference, since a cation m ay form a colorless com
plex w hich is m ore stable th a n th e desired colored complex and in th e absence of sufficient reagent a decrease in in te n sity will result (Figure 3, curve 2). Sagaidachnuii an d R avich (8) m ethod for th e determ ination of iron colorim etrically is sa tis
factory, provided experim ental conditions are carefully con
trolled.
in-M ARCH 15, 1938 ANALYTICAL E D IT IO N 139 vestigation was carried on, for his in terest and helpful sugges
tions and to th a n k him for th e privilege of using th e P u rd u e carbon residue rem aining after carbon monoxide absorption and passing th e gas over platinized silica a t 100° C. T he
The apparatus was th a t used in the previous work, except th a t the copper oxide tube was replaced by a similar Pyrex tube con
taining 1 gram of the commercial platinized silica gel containing 0.075 per cent of platinum produced by the Silica Gel Corpora
tion. The limiting tem perature of the standard heater is 400° C., at which tem perature m ethane is only partially oxidized. The heating element was removed from the steel shell and replaced
by a heating element for a coal volatile-matter furnace. This gave a heating chamber 15 X 3.75 cm. ( 6 X 1-5 inches) and could attain a tem perature of 950° C. The heater was connected in series with a variable-plate resistance by which the temperature could be controlled. Tem perature was measured with a mer and higher hydrocarbons began a t tem peratures considerably below th a t for m ethane. If com plete m ethane oxidation can be secured, it m ay be concluded th a t oxidation of th e higher hydrocarbons will also be com plete.
A stock m ixture of m ethane or ethane, oxygen, an d n itro