I.VDUSTRIAL 9 N D ENGINEERI-VG CHEMISTRY
May. 1927
63 1
Some Sources of Error in the Colorimetric Determination of pH Values' By John W. Schlegel and Albert H. Stueber h7ATIOXAL :SUGAR
practically all the results here recorded were obtained with this indicator. Errors Due to Faulty
REFISERYCO., LOXGI S L A X D C I T Y , N. I'.
Various sources of error are found in the colorimetric determination of pH values of sugarhouse products, chiefly due t o faulty manipulative details and to the
DILuTIos-It is the almost universal practice in sugarhouse laboratories to dilute dark solutions with indeterminate amounts of distilled water, despite the fact that such dilution can lead to decided errors. A washed sugar liquor, originally testing 6.8 pH, gave the results shown in Table I o n progressive degrees of dilution with distilled water of p H 7.0 and 6.2. Table I--Effect
LIQL-OR
CC. 50 50 50 50 50 50 60 50
WATER
of Dilution PH Y4I.UE
Water ~ H 7 0
Water DH 6.2
Cr.
$!
6.8 6.9
10 30 75 250 1000 2500
7.0 7.1 7.2 7.1 7.0 7.0
6.8 6.9 6.95 7.0 7.05 7.0 6.8 6.6
These changes are probably due to alterations in the degree of dissociation of the complex mixture of salts and other 1 Presented before t h e Division of Sugar Chemistry zit t h e 72nd Meeting of t h e American Chemical Society, Philadelphia, P a . , September 5 t o 11, 1926.
Table 11-Results MATERIAL Distilled water Granulated sugar solution B'hite liquor
'
fered) were tested in dried tubes without rinsing, and in 'vashed tubes after rinsing 'vith the respective tions. The results are shown
with Dried a n d Rinsed Tubes DRIEDTUBE 7.2 6.4 7.6
RINSEDTUBE 7.0
5.p
1 . J
DIFFERENCE +0.2 10 2 c0.1
STORAGE OF DYESoLUTIosS-It is frequently recommended that reagent bottles for storage of buffer and other solutions be coated with paraffin or beeswax. If this is done great care must be exercised in the selection of a proper wax, as the ordinary paraffin has a distinctly acid reaction. Pyrex glass bottles have been found much better suited to this purpose. If used for storing indicator solutions, such as bromthymol blue, ordinary glass shows a decided alkaline reaction, particularly around the ground necks and stoppers. Upon prolonged storage the bromthymol blue becomes progressively more alkaline, so that pH determination, when made with an old indicator solution, may be much in error. Upon storage in Pyrex glass bottles, if' they remain unopened, bromthymol blue shows no change even after prolonged periods. If, on the contrary, the bottles are frequently opened, the reaction of the indicator dye becomes progres-
INDUSTRIAL 9 N D ENGINEERING CHEMISTRY
632
sively more acid, as evidenced by a gradual change from a greenish blue to a yellow-green. The change in reaction of the dye solution is therefore evidently the resultant of the extraction of alkali from the glass, and of absorption of carbon dioxide from the air, the former being the preponderating reaction in the case of ordinary glass, and the latter the only reaction in the case of Pyrex. It is therefore recommended that where weakly buffered solutions are to be tested, the dye solution be kept in a Pyrex glass buret supplied with a soda-lime tube, or else to be siphoned directly out of a Pyrex stock bottle protected by a soda-lime tube. FADING-It is advisable to make comparisons of tested solutions with the standard tubes promptly, as it has been observed that, particularly with dilute, slightly buffered solutions, the color produced begins to fade almost immediately after mixing the dye and solution. Errors Due t o Method of Preparation of Bromthymol Blue
I n preparing solutions of indicator dyes, it is frequently the custom to dissolve them directly, without neutralizing with sodium hydroxide. I n the case of acid dyes, such as bromthymol blue, the acidity precludes the possibility of obtaining reliable results. Where the proper procedure is adopted, that of neutralization with sodium hydroxide, care must be exercised to use the correct amount of NaOH for the dye in question. Even slight deviations will lead to serious errors in pH values, especially in the case of weakly buffered solutions. By using dye solutions made from the same lot of bromthymol blue, but with slightly varying amounts of sodium hydroxide, the results shown in Table I11 were obtained when 0.04 gram of bromthymol blue was dissolved in 100 cc. of water containing the indicated amounts of alkali. Tahle 111-Variations
D u e to A m o u n t of NaOH (0.02 N ) Added t o Neutralize
SOLUTION Distilled water Solution oi granulated sugar (practically unbuffered) White liquor from char filters (slightly buffered) White liquor from char Elters (moderately buffered)
Yol. 19, No. 5
It is evident that dyes from different sources. which give approximately identical results with buffered solutions, may give widely divergent results with those unbuffered or weakly buffered. The acidities of dyes No. 1 to No. 4 were determined by dissolving 0.1 gram of each in 95 per cent alcohol, and titrating with 0.05 N sodium hydroxide until the dye solution showed the first change of color. I n each case this acidity was equivalent to 3.2 cc. of the alkali, and it is significant that this figure corresponds exactly with that given by Clark as the amount necessary to prepare the dye solution. It is therefore evident that by preparing the dye solution with this amount of sodium hydroxide, decidedly incorrect results will be obtained with unbuffered or weakly buffered solutions. Successive lots of bromthymol blue solutions, even when purchased from the same source, have been found to give widely varying results. This was first brought forcibly to the writers’ attention by variations in their appearance, some lots having a greenish blue color by transmitted light, others varying from this to a deep purple. Two successive batches of this dye from the source gave the results shown in Table V. Table V-Variation
in Different Lots of B r o m t h y m o l Blue from S a m e Source
SOLLTIOS
FIRST LOT SECOKD LOT
DIFFERENCE ~~
6.8 pH buffer solution 7.2 p H buffer solution Washed sugar liouor No. 4 yellob liqior from char filters No. 3 liquor from char filters White liquor from char filters Extra white liquor from char filters Distilled water Double distilled water
6.8 7.2 7.0 6.4 6 2 6 0 6.8 6 6 6.9
6.8 7.2 7.0 6.4 6.2 6.2
7.0 6.9 7.2
0.0 0.0 0.0
0.0 0 0 +0 2 +0.2
+0.3 +0.3
Standardization of Bromthymol Blue
Considerable differences are shown in the p H values of those solutions which are weakly buffered, but no differences in the case of strongly buffered solutions. Such differences are probably caused either by variations in the amounts of sodium hydroxide used in the preparation of these indicator solutions, or by variations in the compositions of the dry indicators themselves before such preparation. Michaelis3 states:
4 . 0 ~ ~5 . . 0 ~ ~ 6. . 0 ~ ~ .7 . 0 ~ ~ .
6.8
7.1
7.6
6.2
6.4
6.6
6.8
6.4
6.5
6.6
6.7
7.6
7.6
7.6
7.6
>7.6
I n the preparation of solutions of bromthymol blue the method recommended by Clark2 is ordinarily employed. This consists in neutralizing 0.1 gram dye with 3.2 cc. 0.05 N sodium hydroxide and diluting to 250 cc. It was found that by using a solution so prepared incorrect results were obtained in the case of unbuffered solutions. I n order to overcome this difficulty a method has been developed in this laboratory (to be described later) which will give correct results with both buffered and unbuffered solutions. To demonstrate this point, the solutions as given were tested with bromthymol blue from four different sources, a solution of each having been prepared as recommended by Clark. I n addition, a solution of the dye purchased ready prepared and one prepared by the writers’ method were tested. The results shown in Table IV were obtained.
The indicator method can only give correct results if the pH of the fluid under investigation is not altered by the addition of the indicator. If, as is commonly the case, the indicator is either an acid or a base, this condition will only be satisfied if the nature of the fluid be such t h a t it is sufficiently well buffered to prevent such changes of reaction as the indicator might otherwise cause.
To demonstrate this fact two solutions of bromthymol blue were used, one prepared by Clark’s method, and the other containing approximately seven times as much sodium hydroxide. These two indicators were used in determining the p H value of a 7.2 standard buffer solution, and both dyes gave identical results of 7.2. When these two dyes were used for testing distilled water, however, the abnormal dye gave a p H value greater than 7.6, or beyond the range of the indicator. The normal dye gave pH 7.2. This shows forcibly that a very considerable variation in the alkali Table IV-Variations D u e t o Method of Preparing Bromthymol Blue content of the dye solution will have no influence upon tests DISTILLED GRANULATED C“UI;-E wAsHED made on buffered solutions, but that this difference becomes WATER SUGAR serious in the case of unbuffered or slightly buffered SoLUT1oN (DAWSON’S SOLUTION FILTER SUGAR METHOD) 58’ BRIX32O BRIX LIOUORLrouoR solutions. It is therefore evident that where dye solutions Clark’sMethod No. 1 6.3
