Carbonate alkalinity indicators

bonate using methyl orange as an indicator. Further, a large percentage of ... Blue-. Red-. Bromocresol green. 0.200 green violet. (5). 2. Methyl oran...
0 downloads 0 Views 1MB Size
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