INDUSTRIAL AND ENGINEERING CHEMISTRY
456
The particle s h e of the solute is of considerable importance. The material must be fine enough to give a compact, reproducible residue in the capillary in which it is measured, and to have a rapid rate of solution. However, it must be coarse enough to be readily precipitated into the capillary of a Goetz tube with a moderate centrifugal force. A material screened to pass an 80-mesh sieve is usually satisfactory. A centrifuge is used to pack the residues in the capillaries of Goetz tubes for volume measurement. While its use is not vitally necessary, the centrifuge facilitates the experimental procedure by greatly reducing the time required to settle out the residues and by yielding well packed uniform columns. It has been the authors’ practice to centrifuge for 5 minutes at 2000 r. p. m. with a maximum radius of approximately 18.75 cm. (7.5 inches), using an International centrifuge, Size 1, Type SB. There is no need to centrifuge under these particular conditions, and even considerable variation from them on duplicate determinations has not been observed to affect the results. However, it is best to standardize the time and force used during the experiments for any one solubility determination in order to minimize variations due to the packing of the residues. The top of the residue column in the capillary is usually not level when removed from the centrifuge. This condition makes accurate reading of the residue volume difficult and may be overcome by holding the tube vertically and gently tapping the capillary until the residue levels itself. Occasionally some liquid is entrapped in the bottom of the capillary, in which case it is best to loosen the residue and recentrifuge. Mechanical difficulties of this sort are minor and can be easily overcome.
Vol. 14, No. 6
Temperature control is difficult. Centrifuging causes a small rise in temperature of the solutions above the air temperature, and i t is not easy to control the resulting solution temperatures a t any predetermined level. The purposes of the writers have so far been served by starting with the solutions a t room temperature and measuring the final temperature of the solutions as they are removed from the centrifuge, regarding the latter as the temperature a t which the determination was made.
Conclusions The residue volume method of determining solubilities was originally designed for commercial mixtures but its scope is considerably broader. The accuracy obtainable is sufficient for the majority of commercial applications, and it can serve usefully for obtaining quickly a preliminary view of solubility limits in many systems. The solubility is found from a number of individual determinations which operate to check one another and to define the precision of the determination as a whole. The fact that no analyses need be made is an important feature from the standpoint of speed, cost, and often the time lost in development of suitable analytical methods. Other features are the ability to give good results on substances containing insoluble impurities and in cases where attack on container vessels interferes. Solubility data obtained may be corrected for insoluble impurities if necessary. Difficulties with supersaturation are not likely to be met. The method is not applicable to solutes of density lower than the solvent, nor effectively to solutes having very slow rates of solution. P R E ~ E N Tbefore E D t h e Division of Analytical and Micro Chemistry a t the C H E h f I c a L SOCIETY, Memphis, Tenn. 103rd Meeting of t h e AMERICAN
Determination of Tannin Substances in Boiler Waters A. A. BERK AND W. C. SCHROEDER, Eastern Experiment Station, Bureau of Mines, College Park, Md.
T
ANNIN and similar complex organic materials are finding increased use in boiler feed-water treatment. For example, the phlobatannin quebracho will frequently reduce dissolved oxygen concentration, prevent hard scale, retard clogging of feed lines and injectors, and inhibit caustic embrittlement. Both operating experience (2, 7, 9) and laboratory observation (4, 6, 11-13) substantiate the value of such substances. A correlation of the concentration of tannin with the effect it produces is obviously of importance for its efficient and economical use as well as to ensure reproducible results. Analytical procedures must therefore be available for determining dissolved tannin. Color produced by these materials when they are in mater solution, loss during ignition of dried solids, and oxygen consumed from per manganate have all been found unreliable and inadequate for estimating concentrations.
water analysis. The reagent is reduced by the tannin or lignin to give a blue solution of a color density that varies with the concentration of the reducing agent. Procedures for Nessler tubes and photoelectric cell colorimetry have been developed. The method is so rapid and simple that it may be used as a routine procedure’.
Method
REAGENTS.The following solutions are required : Tyrosine reagent, made from phosphotungstic-phosphomolybdic acid, by dissolving 100 grams of sodium tungstate, 20 grams of phosphomolybdic acid, and 50 ml. of 85 per cent phosphoric acid in 750 ml. of water. The liquid is boiled under reflux for 2 hours, cooled, and made up to 1 liter. Saturated sodium carbonate. Enough sodium carbonate (250 grams) is added to 1 liter of water t o supersaturate the solution with respect to sodium carbonate decahydrate and the excess is allowed t o crystallize at 20’ C. The clear supernatant liquor is the sodium carbonate reagent and is decanted and stored in a rubber-stoppered bottle. Comparison solution. This solution represents the %tandard” for the method and should be made up of exactly the same material that is fed to the boiler. Five grams of a representative
The reagent developed in 1912 by Folin and Denis (6) to differentiate tyrosine from uric acid was adapted by Mehta ( 8 ) to estimate lignin extracted from plant tissues. The solution of phosphotungstic and phosphomolybdic acids has been found to be sensitive to small amounts of tannins and similar substances, indicating a suitability for boiler and feed-
1 After the type had been set for this publication, the authors became aware of a parallel study of the photocolorimetric determination of tannln in connection with the analysis of whisky, by Rosenblatt and Pelnso (10). However, preliminary announcement of the authors’ procedure for the determination of lignin and tannin in boiler water wae made several years ngo (3)
June 15, 1942
ANALYTICAL EDITION
457
TABLE I. TRANSMISSION CHARACTERISTICS OF BLUESOLUTION“ 60
5300 4050 67.5 52 :o 4350 67.0 5450 52.0 4600 63.0 5600 48.5 4850 59.5 5760 49.0 5900 49.0 5100 57.0 I, Thickness of solution 915 mm. Color developed procedure) by 0.1 mg. of quebracho tannin.
50
40
g
/’
h
6050 46.5 6250 45.0 6400 45 0 6600 45.0 6850 45 .-n (regular phototester
202 ties of the tannin complex to the above procedure and plotting the readin s of the microammeter scale against the amount of tannin use!. Figure 1 shows a typical calibration curve for quebracho tannin. This curve is steepest and the accuracy highest for the first two thirds of the scale.
IO
QUEBRACHO TANNIN
0.A25
0.05
- milliprornr
0.075
0.10
0.!25
0.!5
0.17;
FIGURE1 sample of the solid or liquid extract used for the boiler feed are weighed out and dissolved in a liter of water, 10 ml. of this solution are pipetted out and diluted to 1 liter. This latter solution is the comparison or standard solution and contains 50 p. p. m. of the original material, but does not represent 50 p. p. m. of pure tannin. Alkaline contamination of the comparison solution results in rapid deterioration due to air oxidation of the tannin and should be avoided. For most tannins, the standard will remain sufficiently accurate for 2 months from the date of preparation. Master standard solution. To determine the rate of deterioration of the standard solution and to check the uniformity of the extract used, comparison with chemically pure tannic acid is recommended. One gram of tannic acid is weighed out and dissolved in 1 liter of water, and 25 ml. of this solution are pipetted out and diluted to 1 liter for a master standard containing 25 . m. of tannic acid. p. $ROCEDURE FOR NESSLERTUBES.A 50-ml. filtered, clear, and cooled sample of boiler or feed water containing less than 3 p. p. m. of tannin or 20 p. p. m. of lignin is put into a 100-ml. Sessler tube. (Generally this will require diluting a smaller sample to 50 ml.) Two or three similar samples are made up from the comparison solution to contain lower and higher concentrations of tannin or lignin than the unknown sample. These reference solutions are also made up to 50 ml. with distilled water, and therefore contain 1 p. p. m. of tannin for every milliliter of comparison solution used. Each known and unknown is treated with 2 ml. of the tyrosine reagent, stirred and allowed to stand 5 minutes. Ten milliliters of saturated sodium carbonate are added to each sample, stirred, and allowed to stand 10 minutes. The blue color of the unknown is matched with the knowns to estimate the amount of tannin or lignin.
No light filter is necessary for the phototester. Table I shows t h a t the blue color which is developed will absorb wave lengths over a large part of the spectrum. Partial correction for a n y color which may be present in the boiler water may be made in the null-point bottle used to set the scale at zero light absorption.
Effect of Variations in Procedure The effect of variations in procedure and conditions during the development of this color can best be discussed in terms of a typical rate of color-development curve as measured with the photoelectric cell. The curves shown in Figures 3 and 4 are identical and were developed by 0.1 gram of quebracho tannin, using the standard phototester procedure. Figure 2 shows that change in the tannin content results in a series of similar curves. The color develops very rapidly during the first minute after the carbonate is added, and while it is not a maximum at 10 minutes, the curves have flattened. The 10-minute interval is therefore empirical and may be modified to the requirements of the individual laboratory. Once the interval is selected, however, it must be rigidly adhered to, both for the development of the calibration curve and the analysis of unknowns. For Nessler-tube work this is not so important, since the color of the unknown and of the standards is developed simultaneously.
PROCEDURE FOR PHOTOELECTRIC CELL COLORIMETRY. This procedure was developed for a phototester with a 15-ml. (0.5-ounce) test bottle (light path 1.83 cm., 0.75 inch). The ratio of the reagents to the volume of solution in the cell should be maintained for other instruments, but the size of the sample may require modification by a factor depending on the dimensions of the test cell. A shorter light path would require a larger sample of a given concentration of reducing agent for a similar effect on the photoelectric cell. Exactly 1.0 ml. of the sample is pipetted into the test bottle and diluted to half the bottle volume with distilled water. The tyrosine reagent (0.5 ml.) is added with a calibrated “medicine dropper” pipet. The contents are mixed and allowed to stand 5 minutes. Using a larger calibrated dro per pipet, 2 ml. of the saturated sodium carbonate are added. %he bottle is now filled to the base of the neck with distilled water, the screw cap is replaced, and the contents are mixed. Exactly 10 minutes from the time the carbonate is added, the effect of the blue color developed is measured in the phototester. The reading is converted t o milligrams of organic matter, accordin t o a calibration curve for the particular tannin present. h e calibration curve is obtained by subjecting known quanti-
FIGURE2 Figure 3 (upper) shows that the time that the tyrosine reagent is permitted to react with the tannin before the addition of the sodium carbonate is not critical. The result was the same whether this interval was 2, 5, or 8 minutes. A longer reduction interval of 12 minutes does result in somewh‘at higher readings. The quantity of tyrosine reagent used does not materially affect the color development after the first few minutes (Figure 3, center). Excess reagent must be avoided, how-
450
INDUSTRIAL AND ENGINEERING CHEMISTRY 6C
SAMPLING.If much time elapses between the sampling of the water and its analysis, appreciable oxidation of the tannin can occur. This action is best prevented by acidifying the boiler water slightly. About 1 ml. of concentrated hydrochloric acid should be added to 1 liter of cooled and filtered water for each 300 p. p. m. of caustic alkalinity. This treatment may cause some of the tannin to be precipitated, so that the sample should be thoroughly shaken before a portion is taken for analysis.
TIM~E FACTOR
5c i
REDUCTION
-
INTERVAL
2-8 MINUTES 0 I2 MINUTES
40
RE AG EN T C 0 N C E N T RAT ION K
QUANTITY
W
t-ul
OF TYROSINE REAGENT
-
,W 40 0 0
0
2 30 6C
0
0 5 ml. 0 . 7 ml.
5
IO
TIME
-I5MINUTES2 0
25
1 P. P. M.
O F C . P.
TANNIC ACID Permanganate Number ( 1 ) P. p . m .
Quebracho tannin 1.4 0.9 Cutch tannin 1.9 1.1 Chestnut tannin 1.8 ... Pure lignin 3.3 0.9 7 Sodium lignin sulfonate 1.3 16 Waste sulfite liauorb 2.8 Ferrous sulfateJ 18 Sodium sulfite Over 100 7.3 0 Quantity uf each sabstance required t u produce 3an1ere3u.t :n each sta:idard test procedure as 1 p. p rn. of c. P. tsnnic acid. b Solution contAining 50: solds.
CON CE N T RAT I ON
40
0
OF
Tyrosine Test P. p . m.
50
--
TABLE 11. EQUIVALENCE'
~
1
RE AGE NT
Vol. 14, No. 6
30
Significance of Results
FIGURE 3. EFFECTOF VARIATIONS IN PROCEDURE
ever, since the tungstate and molybdate formed upon addition of the sodium carbonate have a limited solubility. The quantity of saturated sodium carbonate used is more important, as shown in Figure 3 (lower). Care should be taken to add approximately the same quantity of carbonate for each determination. EFFECTOF TEMPERATURE. The importance of a controlled room temperature is shown in Figure 4 (upper). Analyses made a t 18.5" C. (65" F.) could be referred to a calibration curve obtained at 24" C. (75" F.) without serious error. If the determinations were made a t 32" C. (90" F.), however, the results referred to the same curve would be about 10 per cent too high. Where there are pronounced variations in laboratory temperature, it may be necessary to make winter and summer calibration curves. Kessler-tube comparison, however, is not affected by the room temperature. INTERFEREWE. Figure 4 (center) shows that considerable alkalinity or acidity in the wat'er sample analyzed does not affect the rate of color development for a given amount of tannin. The solution to which sodium hydroxide rt-as added would represent a boiler water containing 4000 p. p. m. of caustic soda for every 100 p. p. m. of quebracho tannin. Khere the alkalinity is very high, however, and the tannin concentration low, poor results may be obtained owing to excessive neutralization of the acid tyrosine reagent'. For Nessler-tube comparison under such conditions, 1 drop of dilute sulfuric acid (1 t,o 5 ) should be added to every 25 ml. of alkaline boiler water tested, before the tyrosine reagent is added. Inorganic substances ordinarily present in water do not affect the rate of color development. Figure 4 (lower) shows that a ratio of sodium chloride to tannin of 100 to 1 caused no departure from bhe normal curve. The effect of sodium sulfite is almost, negligible, since 1 mg. of this compound when added to the 0.1 mg. of quebracho caused variations of about 10 per cent. Ferrous iron will interfere, but in alkaline boiler waters the concentration is usually too low to cause difficulty. Phenolic compounds which are present in some feed waters will interfere. A blank should be run if their presence is suspected.
The tyrosine reagent does not evaluate a particular boiler water in terms of its effect on boiler operation. Yeither does it determine the oxidizable matter in solution. Table I1 lists the equivalence to tannic acid for several substances used for boiler-water conditioning. The tyrosine equivalence is the amount of the substance required to develop the same color in this test procedure as 1 p. p. m. of c. P. tannic acid. The permanganate equivalence is similarly the amount of the substance which will consume as much oxygen from permanganate as 1 p. p. m. of c. P. tannic acid. The tyrosine test was developed, and is used, to determine concentrations of substances which have aromatic hydroxyl groups. It makes no distinction between such substances, so that the simultaneous use of lignins and tannins is obviously undesirable from the standpoint of analytical control. It has been found that concentrations as measured by the tyrosine test can be correlated in some degree with quanti-
0
0
5
10 TIME
5mg.H2S0.
-I5MINUTES20
FIGURE 4. EFFECTO F VARIATIONS IN
25
ADDED
30
PROCEDURE
