INDUSTRIAL A N D ENGINEERING CILEMISTRY
252
Vol. 17, S o . 3
A Study of the A. L. C. A. Official Method for the Determination of the Acidity of a Tan Liquor’ By H. T. Beans and Ernest Little COLUMBIA UNIVERSITY. N s w YORK. N. Y
H E determination of the acidity of a tan l i q u o r has been a favorite problem of investigation among leather chemists during the past forty years. It is a question of very great importance inasmuch as the acidity of the liquor is one of the factors governing the rate of tanning, which, in turn, determines to a very great extent the quality of leather produced.
T
Previous Work
*
A study of the official method of the A. L. C. A. for the determination of the acidity of a tan liquor has revealed the following facts: 1-The addition of 15 grams of kaolin will, according to the writers’ results, introduce a n error of about 3.50 per cent, owing to adsorption of the acids present. 2-The removal of tans with gelatin is subject to the following objections: (a)The precipitation is incomplete and the tannins remaining in solution give rise t o a color change during titration which is quite likely to be confused with t h e color change of hematin. (b) The solution resulting from t h e precipitation of the tannins is of such a nature t h a t when it is titrated with 0.1 N alkali, the decrease in the hydrogen-ion concentration per cubic centimeter of alkali added is not great enough t o allow of a satisfactory titration. No sharp color change of t h e indicator could be expected. ( c ) The large amount of gelatin in solution a t t h e time of titration will interfere with a proper color change of hematin, owing t o what Sorensen has called “the protein error.” 3-Hematin is not a satisfactory indicator. Its color change is very vague and extends over a considerable range of hydrogen-ion concentrations. Furthermore, it is not sufficiently free from the protein error to be successfully used in this titration.
Procter? has presented a thorough review of the articles bearing on this subject previous to 1910. Since this time there have been few published articles of importance. Sand and Law3published an electrometric method for the estimation of the acidity of tan liquors. The apparatus used in their work is practically the same as the well-known millivoltmeter set-up emPloYed by Hi1debrand*4 A voltage Of 0.69, Obtained ning 0.1 N sodium hydroxide into 0.1 N acetic acid until the faintest pink color of phenolphthalein appears, was chosen as the end point. Bennett divides the acids present in tan liquors into three groups: (a) the tannic acids, (b) the volatile acids, and ( c ) the nontan and nonvolatile acids. He outlines a method which, when used in conjunction with the analysis of the liquors for tans by his own modification of the hide powder method, gives the above information without complicated apparatus or manipulation. Atkin and Thompson8 have published a very ingenious method in which they make use of a comparator designed by Cole and Onslow’ to determine the hydrogen-ion concentration of the liquor and the amount of standard alkali or acid necessary to bring the liquor to the definite hydrogen-ion concentration that may be desired. They set the range of acidity as from pH 2.8, the highest acidity likely to be found in tan liquors, to pH 4.6, the isoelectric point of gelatin. A. L. C. A. Method Without further discussion of the previously published articles on this subject, it seems a warranted assumption that the American Leather Chemists have incorporated in their 1
Received November 18, 1924.
* J . SOC.Chem. Ind., 19, 1354 (1910).
s Ibid., 6,52, 428 (1911). 4 J . A m . Chem. Soc., 96,847 (1913).
official method what they consider to be the strong points in these earlier articles. It is the purpose of the writers, therefore, to present a critical examination and discussion of the official method of the A. L. C. A., pointing out any defects which may be found there. T h e p r e s e n t official method of the A. 1,. C. A.8 for the determination of the acidity of tan liquors is as follows: To 25 cc. of liquor in a cylinder that can be stoppered add 50 cc. of the gelatin solution, dilute with water to 250 cc., add 15 grams of kaolin, and shake vigorously. Allow t o s e t t l e for a t least 15 minutes, remove 30 cc. of the s u p e r n a t a n t liquid, dilute with 50 cc. of water, and titrate with 0.1 N sodium hydroxide, using hematin as an indicator. The gelatin is added to precipitate the tannins as their color interferes seriously with an indicator change, The kaolin is added to assist in clarifying the solution and enable the tanno-gelatin precipitate to settle out quickly. Action of Kaolin
Fifteen grams of kaolin possess a very large surface, and suggest a t once the probability of adsorption of a portion of the acids present, thus reducing their concentration in the liquor and giving low results. In order to determine whether or not this adsorption actually takes place, experiments were carried out with different normalities of sulfuric, hydrochloric, formic, acetic, lactic, and butyric acids. These acids were chosen in view of the fact that the work of A. Seymour-Joness on the natural acids of tan liquors shows that acetic, propionic, butyric, and lactic acids constitute the effective portion of the natural acids found in a normal tan liquor. In some tanneries small amounts of sulfuric acid are added to “sharpen” the tan liquor. Fifteen grams of kaolin were added to 100 cc. of the standard acid in a 250-cc. glass-stoppered flask and put in a constant temperature bath at 18” C. The flask was shaken a t short intervals during the first 4 hours and then allowed to remain in the bath for 20 hours. The clear supernatant solution was then decanted through a small filter paper into a buret, which was thoroughly rinsed with the first portion of the filtrate. Twenty-five cubic centimeters were run from this buret and titrated against standard sodium hydroxide, using phenolphthalein as an indicator, and the difference in the
1
Collegium (London), 1916, 106. J . SOC.Leather Trades Chem., 4, 143 (1920).
8
7
“Practical Physiological Chemistry.”
9
6
Bull Oj5cial Methods .1. L . C. A., 1911. Tanner’s Year Book, 1911.
INDUSTRIAL A N D ENGINEERIXG CHEMISTRY
March, 1925
concentration of the solution and its original concentration was determined. I n every case considerable quantities of acid were removed from solution by the kaolin. For hydrochloric acid the amount removed varied from 1.66 per cent for 0.1863 N to 14.29 per cent for 0.0091 N . For acetic acid the range was from 0.88 per cent to 5.94 per cent, while for lactic acid for normalities ranging from 0.1820 to 0.0090 the per cent of acid adsorbed was from 1.09 to 11.11. No special precautions were taken in the foregoing work to insure uniformity in the samples of kaolin used. The kaolin was weighed out in the condition received, just as it is used in the laboratory from which it was obtained. With sulfuric, butyric, and formic acids special precautions were observed to obtain as accurate results as possible. The kaolin was run through a 200-mesh sieve and then shaken for one-half hour in a large wide-mouth bottle to insure uniformity of the sample. Exactly 15-gram samples were weighed out and the temperature was maintained constant to within 0.05’ C. The three acids so treated were chosen as they seemed to be especially representative acids; formic acid being the strongest organic acid, butyric acid an organic acid of ordinary strength, while sulfuric acid is the mineral acid always used. The results obtained with these acids are given in Table I. The results for the three acids given above show that for a 0.01 N solution an average of 10.03 per cent of the total amount of acids present was taken out of solution by the kaolin and for a 0.02 N solution, as would usually be present during a titration according to the official method, the error
A blank was run on the acid, with no kaolin present, for 24 hours, and no decrease in the strength of the acid resulted. A blank was also run on a 15-gram sample of kaolin in 100 cc. of dist.illed water for the same length of time, and the resultant solution showed the pink color of phenolphthalein when one drop of 0.1 N sodium hydroxide was added. Table I
,9m.of
nqo, 30
/ w i n , 50cc. x iqo 40
50
I
60
io
ACID
Normality
Formic
0.09620 0.04850 0.02420 0.01110 0.17680 0.09620 0 04640 0.00940 0.18450 0.09860 0.04630 0.01080
Butyric
Sulfuric
Acid adsorbed Per cent
1.98 3.30 4.55 7.21 1.02 1.46 2.20 5.32 1.73 2.73 4.97 15.74
The equilibrium concentrations of sulfuric, formic, and butyric acids remaining in the solution were plotted against the mols of acid adsorbed by the 15 grams of kaolin for each concentration, as were also the logarithms of these values. I n each case the first curve approximated a parabola and the second a straight line. The values used were in some cases the average of a number of determinations. The results obtained with sulfuric and butyric acids are given in Figures 1 and 2. A portion of kaolin was then washed with normal sulfuric and another with normal acetic acid as follows: The sample
20
20
253
/O
40
30
20
50
-
I
.5
.6 .7
/q9m bufyric o r i d l e f t i n SOc:.
x100 1
1
1
,
1.6
1.7
1.8
19
I
J
.8
.9
1
1
1
1
1
18
If
12
13
1.4
1
1.5
/3 20
Figure 1
Figure 2
due to the kaolin would amount to about 3.5 per cent. Inasmuch as when the official method is used the kaolin usually stands in contact with the liquor for about 10 minutes, the error resulting from adsorption will be somewhat smaller than the value here given. When it is considered, however, that the adsorption equilibrium is very quickly established, i t is evident that the error will be considerable.
was shaken frequently with 2 liters of the acid and allowed to stand overnight. The clear solution was then siphoned off, 2 liters of distilled water were added, and the solution was again allowed to stand overnight. This washing by decantation was repeated twenty times. This kaolin was spread out on unbleached muslin and allowed to dry a t room temperature. The dry kaolin waa then ground very fine and experi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
254
ments were run with the 0.1 N acids as had been done with the non-acid-washed kaolin. The results for the sulfuric acid-washed kaolin are given in Table 11. Table I1 -----NORMALITY--
Per cent adsorbed
At eauilibrium Or0961 0.0854 0.0947 0,0956 0.0950 0.1069
Original
ACID Sulfuric Hydrochloric Formic Acetic Butyric Lactic
0.0986 0.0878 0.0962
0.0965 0.0961
0.1083
2.54 2.73 1 .if3
0.93 1.14 1.29
With the acetic acid-washed kaolin the results agreed, within experimental error, with those obtained with the nonacid-washed kaolin.
"
I
Vol. 17, No. 3
Precipitation of Tannins with Gelatin
Since the only specification of the official method is that the gelatin solution shall be neutral to hematin, isoelectric gelatin was not used, but a large sample of gelatin powder. A 1 per cent solution of this gelatin containing 25 cc. of ethyl alcohol per liter was made up, and with it the official method for the determination of the acidity of a tan liquor was carried out, except that the total volume was made up to 300 cc. instead of 250 cc., and 50-cc. aliquots were titrated. The titration was carried out in a solution containing a Hildebrand type of hydrogen electrode, which was coated with platinum black and bathed with hydrogen. The hydrogen had been purified by being passed through alkaline permanganate, alkaline pyrogallate, water, glass wool, and finally through a diluted tan liquor. A type K potentiometer was used together with a high-sensitivity galvanometer, a standard Weston cell, and a calomel cell with appropriate salt bridge containing a saturated solution of potassium chloride. The mercury used in the calomel cell was specially purified, haring been distilled under reduced pressure, according to Hulet. The potassium chloride was carefully recrystallized and the calomel madeiby Tin Ppted wdh Gelatine, Curres
4
t
w 4
3 0
1.
l
" 8
2
3 4
.
5
'.
0 . 1
.I
'
Cc NaOti 1'
8
.
0
.I
'
1.
'
1 . a
' . A " . '
6 7 8 9 IO I/ I2 Id IC IS W I? /6 /Y V . PI 12 23 ZI
25
Figure 3
The kaolin used in this work was obtained from one of the largest and most reliable leather laboratories in the country. It contained practically no water-soluble material and was neutral to phenolphthalein. Freyln as a result of study of the action of six samples of kaolin toward 0.02 N acetic acid, gives data showing that in each case the acid-washed kaolin takes up as rpuch acid as the unwashed kaolin.' Frey added 15 grams of kaolin to 250 cc. of 0.02 N acetic acid, shook the solution well, then allowed it to stand for 1 hour, when 50 cc. were pipetted
' 0
i
2
3
4
5
6
7
8
9
IO
!I
It
13
N
IS
10
17
id
j9
20
21
27 IJ 74 I5
Figure 4
off and titrated. For the six samples that he used, he found an average of 5.80 per cent of the total amount of acid present taken up by the kaolin. 10
J . A m . Lsother Chem., 14,393 (1919).
Figure 5
adding an excess of purified mercury to slightly diluted, redistilled nitric acid. After the action had continued for some time the nitric acid and remaining mercury were poured into about 300 cc. of distilled water. A 20 per cent solution of hydrochloric acid was then distilled and the middle portion of the distillate was added to the acid mercurous nitrate solution until precipitation was complete. The precipitated mercurous chloride was washed repeatedly by decantation with very pure distilled water. This specially prepared calomel was kept suspended in pure distilled water in the presence of some mercury. The titrations were run a t 25' C., the temperature remaining constant to within 0.05O C. The titration beaker, as well as the lower part of the calomel cell and salt bridge, was immersed in an electrically heated and controlled water bath which was thoroughly stirred. With this set-up electrometric titrations were run. The voltages corresponding to certain additions of alkali were recorded and the corresponding pH values calculated by use of the Kernst formula. These p H values were then plotted against the corresponding cubic centimeters of alkali added, and the titration curves shown in Figures 3 to 5 developed. These curves show in practically every case a peculiar flattening from a p H value of about 7 to 10. It was a t first thought that this was due to the presence of gelatin salts of the various organic acids and their titration by sodium hydroxide to sodium gelatinate, but subsequent work showed
I,VDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1925
that such was not thecase. Another supposition was that the flattening of the curve and hence the absence of a definite break was due to the presence of unprecipitated tannins. The fact that those solutions which were the yellowest and darkened most when titrated gave curves more nearly approximating a straight line made this a reasonable assumption. In order to get data on this point titration curves for (a) gallic acid, ( b ) gallic acid in the presence of gelatin, ( c ) pyN(
I
1
I
1
I
I
I
I
I
I
I
I
I
acid. An indication of a break occurs when about 8 cc. of sodium hydroxide have been added and then a more gradual decrease due to the neutralization of weaker acid substances. The very close similarity between the curves for the untreated tan liquors and those from which tannins had been precipitated with gelatin is quite surprising. It shows that very few of the weakly acid substances present in the tan liquor are removed by gelatin, and it seems, therefore, to have
I
CURVE I.
,2
I
255
IOcc. acid solutionplus5Occ. water Cc.NoOH. V. PH,
-
0.4260
-
0.4600 0.4846 0.5055 0.6310
2 4 6
8 Jf-
10-
-
3.05 3.65 4.10 4.40 4.86
:1
:::E
12 14 16 18 20
0.7060 0.7400 0.7540 0.7700 0.7816
25 30
0.8200 0.8616
GALLIC ACID
CWPM I-Gofliracidalone C U ~ V F ~ I - G O I Iacidplusqclah IC
5.40
7.03 7.80 8.40 8.65
8.90 9.06 4.75 13.40
0.05399NNo OH used
9-
-2
* -4
0
2
-2
k
4 6 8 9 10 12 14 16 18 20
7-
6-
-
0.4716 0.4920 0.5076 0.5281
4.86
0.5546
6.25
5.76 7.10 8.16 8.53 8.80 9.10 9.60 lO.eO 11.05
0.5850 0.6653
0.7236 0.7446 0.7645
0.7826 0.6080 0.8450 0.8975
25
30
no pnciPitate
roused by
0.05399N NaOH used
CC.Ma Off 2
I
I
I
I
I
I
I
l
I
4
6
8
10
f2
I4
I6
I8
20
I
I
I
I
22 24 26 28 30
Figure 7
Figure 6
rogallic acid, (d) tannic acid, ( e ) tan liquors with the tannins unprecipitated, and (f) these same tan liquors with tannins precipitated with gelatin, were developed. The results are shown in Figures 6 to 10. In developing these curves difficulty was experienced in obtaining permanent voltages between pH values 8 to 10. At this point the solution darkens greatly owing to the condensation of the phenols. The procedure here was to use the lowest concentration of hydrogen ion resulting from each addition of the alkali. It is quite evident on the gallic acid curve (Figure 6) that the carboxyl group was Completely neutralized when 9.5 cc. of sodium hydroxide had been added, resulting in a rather sharp decrease in the hydrogen-ion concentration, and that the flattening of the curve from there up is due to the reaction of the sodium hydroxide with the phenolic hydroxyl groups. The pyrogallic acid curve (Figure 7 ) is, as would be expected, more nearly a straight line, the sodium hydroxide reacting only with the weakly acid phenolic groups. The curve, however, would indicate that the sample still contained a little gallic acid as about 1.5 cc. of 0.05 N sodium hydroxide seems to have been used up in neutralizing the carboxyl group before the more gradual neutralization of the hydroxyl groups sets in. The tannic acid curve (Figure 7) gives a much poorer break, indicating that the properties of the carboxyl groups are not so dominant as in the case of the gallic acid. The curves for the untreated tan liauors (Figures 8 to 101 are quite similar to that of tannic acid, there beiig, however, a less pronounced break in the case of either gallic or tannic
been definitely established that the flattening of the gelatinprecipitated tan liquor curve is due to the neutralization of weak acid substances which remain unprecipitated by the gelatin, rather than to any action of gelatin or gelatin salts with sodium hydroxide. This is also the reason why the acidity of this solution cannot be quantitatively determined by the use of indicators. The tan liquor curves also show quite conclusively that when tannins are precipitated with gelatin and the resultant solution is titrated with 0.1 N alkali, the decrease in the concentration of hydrogen ion per unit of alkali added is in most cases too small to allow of an accurate titration by use of an indicator. The acidity decreases so gradually that a sharp color change is not expected. It was also noticed that when the tannins were precipitated with gelatin the precipitation was very incomplete, the resultant solution being frequently quite yellow in color. This error might be avoided to some extent by carrying out the precipitation in the presence of lower concentrations of gelatin. When weak liquors are analyzed by the official method a very large excess of gelatin is used. Procter” points out that the tannins, even in very dilute solutions, are all precipitated by gelatin, but that thia precipitate dissolves in the presence of a large excess of gelatin which evidently acts as a protective colloid. A better procedure when using gelatin as a precipitant would be that suggested by SmootJ1* who recommends that the amount of gelatin 1’ 12
“Principles of Leather Manufacture,” 1922, p . 151. J . A m . Leather Chem., 6, 585 (1911).
INDUSTRIAL A N D EN(XNEERING CHEMISTRY
256
added should vary with the percentage of tannin present. It is, however, difficult to determine when enough gelatin has been added to precipitate the tannins completely; also, when an excess of gelatin is not present the precipitate settles out much more slowly. In this connection good use could be made of the quantitative results furnished by Wood13 relative to the precipitation of tannincby gelatin.
Most indicators combine with natural protein substances and as a result, even if the combinations are not indicated by a visible deposit, the indicator does not change color a t all in most cases and hence becomes useless. In the presence of large amounts of natural proteins most of the indicators are useless.
In view of the titration curves presented herewith (Figures 6 to 10) and the fact that in the official method the final I
I
J
curve
V.
4.86
0.6140
4.60 6.00 6.60
0.6380 0.6800
6.40
0.6486 0.6680
6.80 7.80
0.6800
7.36 7.66
0.6930
CcNoOH
-
I
,
I
V
Tan Liquor
I
,
.
- second layer
pH
8.TO
0.8135
8
I
a m
0.9516 0.7636
6
,
7-16
0.7025
4
,
s.w
0.4060 0.6746
2
I
Tan 1iquor ftrsf layer
pH.
0.4770
"
I
I
liquor diluted to 100 cc. tltrotedwith0.05399N NoOH
CURVE I.
Cc. NaOH.
Vol. 17, No. 3
10
12
14
16
18
20
22
4
9.65
24
26
2.9
30
Figure 8
The failure on the part of gelatin to precipitate the tannins completely has been recognized by various investigators. Reedll points out that it is not necessary that tannins be entirely absent from the solution when titrated, as they will not interfere when hematin is used as an indicator. It has been the writers' experience that the unprecipitated tans do interfere most seriously, because when a p H value of about 6.7 is reached the tannins turn dark brown, just as does pyrogallic acid when its solutions are made alkaline. The color change that takes place when only small amounts of tans are present is very similar to the color change of hematin a t this pH value, and the writers venture the opinion that in control work many analysts have recorded data based upon the color change of the unprecipitated tans rather than on any color change of the indicator itself. Solutions containing an amount of tans so small that they were practically colorless gave this pronounced color change when titrated electrometrically with 0.1 N sodium hydroxide, no indicator being present and a constant flow of hydrogen gas through the solution being maintained throughout the titration. Effect of Concentration of Gelatin
Another serious defect in the official method, due to the method of detannizing, is that in many cases the final titration is carried out in the presence of a very high concentration of gelatin. Sorensenls long ago pointed out this protein error when he said: 1) 14 16
J . SOC.Chem. Ind., 27, 384 (1908), Collegzum (London), 1908, p. 494. J . A m . Lealhw Chem., 8, 87 (1908). Compt.-rend. Ira8 lab. Carlsberg, 8, 59 (1909).
titration is frequently carried out in the presence of high concentrations of gelatin, it is readily understood why various investigators have been unable to obtain an indicator that would give a satisfactory color change. P r o c t e P used the gelatin precipitation method as outlined by Koch, using azolitmin as an indicator. He states that his results were very unsatisfactory and further adds: "The color change is gradual and the end point difficult to determine, so that the persona1 equation enters largely and concordance between observers is not usually good." Attention is called to this statement in particular, as the writers claim to have shown that this defect is characteristic of any method that uses gelatin as a detannizing medium, and is quite independent of the indicator used. It seems quite clear, therefore, that some other method must be employed for detannizing the solution. Hematin as a n Indicator
Hematin is a very unusual indicator obtained by the extraction of logwood and subsequent oxidation and purification. Freylo in relating his experience with this indicator says : It should be mentioned that none of the end points were sharp, this being true to some extent for even the blank solutions, but because of lack of experience with the indicator it cannot be stated whether or not the bewildering sequence of colors was characteristic. Practically all of the solutions darkened as the titrations progressed, but it is believed that they were not carried beyond a point which might be mistaken for the end point, 1'
J . A m . Lcdker Chem., 6, 59 (1911).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1925
257
Although the writers have never been able to use hematin with any degree of certainty, the results in Table 111 are representative of data obtained in attempting to determine the pH values of the more pronounced color changes observed during a titration. Twenty-five cubic centimeters of 0.1234 N acetic acid were added to 100 cc. of distilled water and titrated electrometrically with 0.08955 N sodium hydroxide in the presence of hematin as an indicator. Three color changes were observed which seemed more distinct than the others and although duplicate analyses frequently failed to check within 0.6 cc., these values are fair averages of the results obtained: Table I l l E N DPOINT First (worst) Second (best) Third (intermediate)
CUnVE
I.
IOcc. liquor diluted t o IOOCG 12 -5Occ fitrated with 005399"NaO~ Cc NaOH. V pH. 0
-6 1
;
11-
4 6
6 7
; 10
IO-
-
11 12
la
0.4016 0.4066 0.4095 0.4980
0.W56
-%,a
0.6146
4.60 4,76 4.90
0.6236
0.6366 0.6496 0.6696 0.6975 0.6330 0.6660 0.6720 a.6070
9-
16 4
-
0.7000 0.7120
16
OaT240
17
0.7860 0.7680 0.8210 0.8720
En
a-2 Z
B -2
7
-
---__
NaOH 33 40
6.20 6.00 6.60 7.00 7.20 7.46 7.70 7.90 0.15 8.30 8.90 9.76 10.60
5v
COLOR Faint violet Lilac Deep violet
PH 2.92 6.68 9.47 9.90
E N DPOINT------Volts 0 61j5
PH 6 27
From the foregoing facts it seems quite clear that hematin is an impossible indicator. Kot only is it constantly changing color with changing acidities, but in some cases the colors are not permanent, even when the pH value is held constant. Cc. NaOH. 0
1 2 4 6 8
6-
Volts 0.4177 0,6404 0.8043 0.8302
The best end point of the three appeared a t a pH value of 9.47, but as this is far too alkaline the very poor end point a t 6.68 must be used. I n order to determine whether or not hematin was subject to a protein error, the following work was done: Twentyfive cubic centimeters of 0.12342 N acetic acid were added to mixture of 25 cc. of neutral 1 per cent gelatin solution and 75 cc. of distilled water. This solution was hhen titrated with 0.08955 N sodium hydroxide, using hematin as an indicator. If the solution was allowed to stand for 5 minutes after the addition of the acid the yellow color of the hematin disappeared and no color developed when sodium hydroxide was added. If titrated immediately after the addition of acid the only color change was a faint lilac when 33.40 cc. sodium hydroxide had been added. Upon further addition of sodium hydroxide no further color changes were observed.
6.60
-t
4
Tan Llquor-headrocker
4O :O 4.10 4.16 4,30
NaOH cc. 0 3 4 . M) 34.65 35.00
10 12 14 16
20 26
SO
V. 0.4745 0.4826 0.4900 0.5070 0.6800 0.6686 0.6466
0.6880 0 . 7 0 ~ 0.7310 0.7790 0.7760 0.9226
pH. 3.90 4.00 4.15 4.48 4.86 6.50 6.80
Preparation of o-Tolidine Solution for Estimation of Chlorine'
7.60
By Charles E. Roake
7.90
8.20 9.00 10.70
FLEISCHMANN Co., PEEKSKILL, N. Y .
11.46
%
different proportions which give a range of pH values from 4.5 to 8.3, the colors given above were checked up. Kot all of the colors mentioned by Salm could be distinguished without the colorimeter, a greenish yellow a t pH values of 2 and 3 could easily be seen. This color changed gradually into a reddish brown a t a pH of 6 and with decreasing acidity this color gradually changed to a violet which slowly darkened in color.
T H F,c1'irections found in the texts for the preparation of o-tolidine solution for estimation of chlorine in chlorinated waters2state that 1 gram of o-tolidine, purified by recrystallization from alcohol, is dissolved in 1 liter of 10 per cent hydrochloric acid. For a long time the writer had great difficulty in preparing this solution. The o-tolidine would not dissolve completely, and he filtered off the undissolved part, using a weaker solution than that called for. He has now found a way to get the o-tolidine completely into solution. To 1 gram of o-tolidine add the calculated amount of concentrated hydrochloric acid (about 236 cc.), stir well, dilute to about 500 cc., and filter. The residue left on the filter paper will be found to be soluble in distilled water. Make up to 1 liter.
1'
J . Am. Lcarhcr Chem., 3, 86 (1908).
1
18
Z. physrk. Chem., 67, 471 (1906).
2
Received January 16, 1925. Race, "Chlorination of Water," p . 147.
