AUGUST. 1953
CARBONATE ALKALINITY INDICATORS MAXEY BROOKE Old Ocean, Texas
IT
IS not known here just when volumetric methods were first introduced into the course of quantitative analysis. But since that time, most students have been introduced to the subject by acidmetry. And most students have standardized their acidagainst sodim carbonate using methyl orange as an indicator. Further, a large percentage of these students have complained about the difficulty which they experienced in observing the end point. If a survey were made i t would probably show t,hat there are more titrations made to the carbonate end point than any other type.
Although it has long been used and adopted as a standard (I) methyl orange leaves much to be desired as a carbonate end-point indicator. The color change from yelIow to light orange makes determinations difficult to reproduce. It has been shoivn to have a pH color response in disagreement with the equivalence point for complete neutralization. Water chemists in particular have done considerable work in attempting to obtain an indicator with a more distinct end point. I n general, three lines of attack have been made. Modified indicators in which other dyes such as bromocresol green, xylene cyanole FF, or
TABLE 1 No. 1
2 3
Dye
Methyl orange Bromocresol green Methyl orange Xylene cyanale FF Methyl orange Indigo carmine
g./liter
0.040 0.200 2.000 2.800 2.000 6.000
Solvent Water 50% Ethanol Water (make separate solutions)
Alkaline color
Acid color
Bluegreen Green .
Red-
Yellowgreen
Violet
violet Magenta
Ref. (5)
(6) (9)
-
420
JOURNAL OF CHEMICAL EDUCATION TABLE 2 No. 1
2 3 4 5 6 7 8
9
Due Methyl red Bromocresal green Methyl red Methylene blue Methyl red Guinea green Methyl red Xylene cyanole FF Methyl red Alphasurine Methyl red Indigo carmine Methyl red, sodium salt Alphiaurine Methyl yellow Methvlene blue Dimethyl yellow
dliter
Solvent
0.2M) 1.000 1.250 0.825 0.750 0.625 0.500 1.250 0.750 0.625 0.750 0.937 0.450 0.550 0.800
95% Ethanol
Alkaline wlor
10 11 12 13 14
Guinea green Sodium alksrin sulfonste Xylene cysnole FF Sodium alizarin sulfonate Alphasurine Sodium aliaarin sulfonate Indieo oarmine
indigo carmine are added to methyl orange to make the color change more distinct. Examples of this type are given in Table 1. Mixed indicators consist of two dyes, one of which undergoes a color change slightly below the desired pH and one which undergoes a color change slightly above. Approximate mixtures give sharp color changes a t the carbonate end point. Examples of this type are given in Table 2. Mixture No. 1is widely used in industry. The third method is substitution, a problem of finding an indicator which gives a color change at the same point as does methyl orange but has a sharper end point. Several of these have been reported in the literature and are shown in Table 3. Since none of these materials is available commercially, they have not been widely used. This presents a fertile field for an advanced research project. Having students prepare and compare these indicators serves a useful purpose. It will acquaint them with the fact that methyl orange is not the only indicator for carbonate titrations nor is i t the best one. It will stimulate their interest in the theory of indica tors. And in allowing them to exercise some judgment
A d color
Blue
Pink
Green
Violet
Green
Violet
Green
Violet
Green
Violet
Green
Violet
Green
Purple
Yellowgreen Blue
Pink
Red
Yellou-
Red
Yellow
Red
Yellow
Red
Yellow
Red
Yellow
Yellow
in the selection of the best indicator, it will make their introduction to volumetric analysis more interesting. LITERATURE CITED (1) A.Ph.A. Standard Methods for the Examination of \$-ater and Sewage, 9th ed., pp. 31-2. W. R., Ind. Eng. Chem.. Anal. Ed.. 17. 141-2 (d) CARMODY. ,
,
, .
,
,
FORTUNE, W. B., AND M. G. MELLON, J . Am. Chem. Soe., 60,2607-10 (1938). K., AND LINSTEAD, J . Chem. Soe. l d l . 2502-6 HICKMAN, (1922); Chem. Abstracts, 17,700. , HOPPNER, K., Deut. Zuckerind, 61, 361-2 (1936); Chem. Abstracts, 30, 5523. JOHNSON, A. H.,AND J. R. GREEN,I d . Eng. Chem., Anal. Ed., 2 , 2 (1930). MOERK.F. X.. J. Am. Pharm. Assoc.. 10. 7 4 3 4 119211:
TABLE 3 No.
Due
1
4,4'-his(pdimethy1amino phenylam)-2,2'-stilbene disulfonate 4.4'-bisi~tolv1triazeno)-2.2'stilheie dihfonate ' Hesaian purple N extra
2 3 4
-.
~
di-sodium 4,4'-bis(4-amino-lnmhthvlaso)2.2'-stilbenedisul-
g./liler
Solvent
Alkaline color
Acid color
Orange
Blue
Yellow
Muddy
Faint mauve Pink
Purple Blue
