ANALYTICAL EDITION
402 MATERIAL Flour 1
METHOD
2
Se (oxychloride) Flour 2
Ef
Se (oxychloride) Flour 3
Wheat 1 Wheat 2
8.34 8.28 8.35
70 55 12
Hg
14.05 14.01 14.18
135 120 23
Cracklings 2
cu Se (oxychloride)
F:
13.06 12.96 13.12
130 105 20
Cracklings 3
11.93 11.82 11.88
90 65 22
Se Wheat 3
Table I-Results of Analysis b y Modified Method PROTEINAMMONIA TIME MATERIAL METHOD % % Minutes 90 Cottonseed meal 1 Hg 15.35 16.19 70 cu 15.50 20 Se 14.25 75 Cottonseed meal 2 Hg 14.25 60 cu 14.39 18 Se
Hg
cu Se
2
Se
Vol. 3, No. 4
this laboratory the minimum time obtainable for flour digestion is about 1 hour, for wheat from 1to 1'1'2 hours, and for cracklings about 2 hours. Quite often, complete solution as evidenced by color and appearance may take from 2 to 4 hours. As the author happened to be working with selenium OXYchloride extractions of tantalum and columbian ores, he made use of that particular compound. Subsequently selenium, recovered from the wasteor spentacids from the above ess, was used. The results were &out the same, slightly below those obtained by the oxychloride.
Cracklings 1 (bones)
PROTEINAMMONIA TIME % % Minutes 42.19 8.21 125 41.44 8.06 100 42.06 8.18 35 46.63 9.07 130 45.75 8.90 110 46.44 9.03 40
cu Se (oxychloride)
Hg
44.19 43.69 44.60
8.60 8.50 8.66
180 150 40
2
45.44 44.81 45.25
8.84 8.72 8.80
165 140 55
2
34.44 34.00 34.25
6.70 6.62 6.66
140 120 45
Se Se
I n Table I is a list of products analyzed by the modification of the conventional method, all run at the same time and under the same heating conditions and with the same amounts of reagents (other t h a i the catalysts) as the process with copper sulfate and mercuric oxide-i. e., 10 grams of either sodium or potassium sulfate and 25 cc. of acid, and 0.1 to 0.2 gram of selenium. No sodium or potassium sulfide need be added before distillation. Literature Cited (1) Am. Assocn Cereal Chem., Methods for Analysis of Cereals and Cereal Products, 1928. (2) Am, Oil Chem. SOC.,Official Methods of Chemical Analysis, 1929. (3) Assocn. Official Agr. Chem., Methods, 2nd ed., 1925.
Continuous Measurement of pH with Quinhydrone Electrodes' C. C. Coons LEEDSh NORTHRUP COMPANY, 4901 STENTON AvE., PHILADELPHIA, PA.
The concentration of quinhydrone necessary for a pH measurement of an accuracy of *0.05 pH on most dilute water solutions is shown to be 7 mg. per 100 cc. of solution. A quantitative method for determining the concentration of quinhydrone dissolved in water is outlined and the solubility curve for quinhydrone is given from 0" to 50" C.
The practicability of various methods of adding quinhydrone to a flowing solution for the purpose of obtaining continuous pH measurements is discussed. A solution
of quinhydrone added continuously from an external source to the flowing test solution is considered the best method of introducing quinhydrone into the test solution. A noble metal electrode of platinum is shown to retain its power of reproducibility much better than a gold electrode during continuous pH measurements. A platinum electrode for continuous pH measurements is best cleaned by boiling it in a 5 to 10 per cent sodium bisulfite solution. If an acid wash is first necessary, it is followed by the sulfite treatment.
. . . . . . . . ....
T
H E quinhydrone electrode is an accurate and widely used method for measuring the pH of many types of solutions within the range of 0 to 8.5 pH, and sometimes 9 pH. The electrode may be applied either to individual pH measurements or to continuous measurements, the latter being employed for automatic recording and control. I n order to obtain reliable continuous measurements with the quinhydrone electrode, it is important to make sure that a proper amount of quinhydrone is supplied to the test solution and that the noble metal electrode at all times accurately indicates the quinhydrone potential. It is the purpose of this paper to discuss in some detail these features of the continuous pH electrode-namely, (1) the amount of quinhydrone necessary for a reliable pH measurement; (2) the solubility of quinhydrone under various conditions; and (3) the noble metal electrode. 1
Received April 24, 1931.
Quinhydrone Electrode
The quinhydrone electrode comprises a noble metal electrode immersed in a solution containing dissolved quinhydrone. The potential established on the metal electrode is compared with that of a reference electrode which is usually a saturated calomel cell. The voltage between these two cells is measured with a potentiometer in the usual way. To obtain continuous pH measurements of a flowing solution, it is most convenient to pass a portion of the solution through a flow channel in which the noble metal electrode and the reference electrode are suspended. The proper amount of quinhydrone must be dissolved in the solution prior to passing it through the flow channel. INTRODUCTION OF QUINHYDRONE IN TEST SOLUTIONThere are, in general, two types of methods by which quinhydrone may be introduced into the test solution prior to its passage through the flow channel. I n one type the quinhy-
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
403
drone is retained a t all times in the flowing test solution, quinhydrone electrode and a saturated calomel electrode, the and in the other quinhydrone is added to the test solution solution being saturated with quinhydrone for each measurefrom an external supply. I n the former type the crystals ment. The pH value of each buffer solution was then obof quinhydrone may be held in cloth sacks or pressed into tained in the same manner, except that definite small quanhard pellets, or the test solution may filter through the crystals tities of quinhydrone were added to 50 cc. of each buffer which are prevented by a filter screen from passing into the solution until the pH value was essentially that obtained by flow channel. In the second type the quinhydrone may be saturating the solution with quinhydrone. The data readded to the flowing test solution by a screw mechanism sulting from these experiments are given in Table I. similar to a lime feeder for water treatment, or the quinhydrone may be dissolved in a suitable solvent, and this solu- Table I-Results of Experiments t o Obtain pH Values of Buffer Solutions tion subsequently added to the test solution. BUFFERSATURATEDDROPSOF In comparing these two types it is apparent that there WITH QUINHYDRONE ACETONE VOLTAGEAT Voltage at SOLN. 21' C. WITH are certain inherent disadvantages involved in retaining 21° C. with ADDEDTOSATURATED saturated calo50 cc. OR CALOMEL crystals or pellets of quinhydrone in the flowing test soluCOMPOSITION OF BUFFER mel cell DH BUFFER CELL SOLNS. tion. First, these crystals or pellets have a filtering action I 4.2 grams citric acid, and, if solid particles are present in the solution, the flow is 1.6 crams sodium retarded or even stopped. Second, it is impossible to eliminate the formation of deposits over the surfaces of quinhydrone exposed to the test solution, and these films seriously retard the rate at which quinhydrone is dissolved. Third, crams citric acid. it is impossible to control the rate of solution of the quinhy- I1 42.0 16.6 grams sodium hydroxide, 2.0 liters drone which varies considerably with the temperature and distilled water -0,3415 1.94 4 -0.3415 the rate of flow of the test solution. I n addition to these 4 -0.3425 troubles the quinhydrone or cloth sacks absorb the solution I11 20.0 grams potassium acid ohthalate. 3.6 and may cause a serious lag in the ability of the electrode to cc. 36% hydrohcloric follow a changing pH. However, pellets are more satisfacacid, 2.0 liters distilled water -0,2798 3.12 4 -0.2810 tory than loose crystals or cloth sacks of crystals because -0,2808 10 they have less tendency to pack together. IV 20.0 grams potassium acid phthalate, 4.0 During the progress of this work the practicability of pellets cc. 0.2 N sodium hvused under ideal conditions was tested. These tests involved droxide, 2.0 li&s distilled water -0.2204 4.00 4 -0.2210 the determination of the solubility curve for quinhydrone i_ n -n 2210 _ -0.2210 20 and the variation in the rate of solubility of the pellets with a variation in rate of flow and temperature of the test solu- V 50.0 cc. N sodium hydroxide. 100.0 cc. N tion. acetic acid, 350.0 cc. distilled water -0.1845 4.62 -0.1855 The addition of quinhydrone to the test solution from an -n- . ____ ixm _i 4n_ external supply avoids all of the above difficulties. A solu20 - 0.1849 tion feeding device is considered superior to a dry feeding V I 20.0 grams potassium dihydrogen phosdevice for the following reasons: Not only does the quinhyphate, 56.4 cc. 0.2 N drone disperse rapidly in the test solution since it is already sodium hydroxide 1950.0 cc. distilled in solution, but the apparatus for introducing a solution of -0.1200 water -0.1105 5.72 4 10 -0.1193 quinhydrone is much simpler than that used for continuously 20 -0.1190 adding crystals of quinhydrone. V I 1 4.59 erams disodium AMOUNTOF QUINHYDRONE NECESSARY FOR PH MEASUREphoiphate, 7.06 grams potassium drM E N T - F O I making continuous pH measurements it is essenhydrogen phosphate, 2.0 liters distilled tial to know the minimum concentration of quinhydrone --0.0882 6 . 2 8 4 - 0,0880 water that will yield a sufficiently accurate pH measurement, not 10 - 0.0875 20 -0.0871 only from the standpoint of reliability but also for an economiVI11 6.2 grams boric acid, cal operation of the unit. 7.45 grams potassium chloride, 40.0 I n order to obtain a pH measurement of the highest accc. 0.2 N sodium hycuracy, it is customary t o saturate the solution with quinhydroxide, 1960.0 cc. distilled water +0.0085 7.85 4 +0.0060 drone. However, Biilman and Jensen ( 1 ) have shown that, 10 +0.0080 15 +0.0081 if an accuracy of only 1 millivolt is required, one-tenth the amount of quinhydrone required for saturation may be used at 18" C. It should follow that, if less accuracy is desired, The same procedure was repeated on the buffer solutions less quinhydrone may be used. However, these statements after they were diluted tenfold, and also on buffer solutions are true only in special cases, and to obtain a desired ac- ten times as concentrated. These data are given in Tables curacy the type of solution must be considered in limiting I1 and 111, respectively. the concentration of quinhydrone. I n order to avoid a It would be a difficult and tedious task to weigh out millilengthy discussion on this subject, it will suffice to say that grams of quinhydrone and add them successively to the the following procedure for quinhydrone pH measurements 50-cc. samples of buffer solutions. Therefore, instead of may be applied with sufficient accuracy (*0.05 pH) to most doing this, a concentrated solution of quinhydrone dissolved industrial solutions in which the quinhydrone electrode may in acetone was prepared. This solution was kept in a be expected to yield reliable results. glass-stoppered bottle, the glass stopper comprising a dropper. In order to determine the minimum concentration of Twenty drops of this acetone solution had a volume of 0.5 quinhydrone necessary for a sufficiently accurate pH measure cc., and each drop contained 0.0009 gram of quinhydrone. The results given in Tables I, 11, and I11 show that, rement, eight buffer solutions ranging from 1 to 8 pH were prepared according to Clarke ( 2 ) . The pH value of each gardless of the pH of the buffer solution (not above 8.0 pH), buffer solution was accurately determined with a dip-type four drops of the quinhydrone solution in 50 cc. of the buffer
ANALYTICAL EDITION
404
solution gave a pH measurement easily within an accuracy of *0.05 pH. The weight of quinhydrone in these four drops of acetone solution was 0.0036 gram. If less than four drops were used, the potential occasionally was unstable and a longer time was required for an equilibrium potential to be established. The concentration of acetone used had practically no effect on the pH measurement. Table 11-Results of Experiments with Same Buffer Solutions Diluted (60 cc. of buffer 4- 350 cc. of distilled water) BUFFER SATURATED WITH QUINHYDRONE DROPSOF BUFFER pH at ACETONE VOLTAGE SOLN. Voltage 21° C. SOLN. A T 21' c. I 0.3305 2.12 5 0.3300 10 0.3300 1 CC. -0.3310
-
I1 I11
--
-0.3014
-0.2690
2.61
4 10
-0.3010 -0.3010 -0.3018
1 cc. 4
3.17
1 cc.
10
-0.2894 -0.2690 -0.2680
V
-0.1820
4.66
4 10
-0.1822 -0.1819
VI
-0.1101
5.83
4 20 30
-0.1104 -0.1095 -0.1090
VI1
-0.0820
6.36
5 20 1 cc.
VI11
$0.0085
7.88
4 10 15
--0.0805 0.0318 -0.0784
$0.0092 +O.OlOO +o. 0100 +0.0115
1 CC. Table 111-Results
of Experiments with Concentration of Buffer Solutions Increased Tenfold
COMPOSITION OF BUFFER SOLNS. 12 cc. glacial acetic acid, 27.0 grams sodium acetate, 400 cc. distilled water
9.2 grams disodium phosphate 14.0 grams potaskurn dihydrogen phosphate, 400 cc. disiilled water
Vol. 3, No. 4
permanganate solution was standardized by the procedure given below against aliquot parts of (10 cc.) standard solutions of quinone, hydroquinone, and quinhydrone, each standardization yielding the same result within the experimental error. These standard solutions of quinone, etc., contained 2 to 4 grams of pure compound in a liter of distilled water. The procedure for determining the concentration of quinhydrone solutions is as follows: The sample was diluted if necessary to 250 cc. with water, 25 cc. of 95 per cent sulfuric acid added, and the solution heated to 80" to 85" C. Standard permanganate solution was added rapidly, with constant stirring, 2 or 3 cc. past the end point (the first pink color). The solution was boiled for a minute to insure the complete oxidation of the quinhydrone, after which the excess permanganate was titrated with the reducing solution until the solution became clear. Then the standard permanganate solution was added slowly until the first pink tinge of color appeared. The volume of quinhydrone solution used for a determination depended upon the quinhydrone concentration. If too concentrated a sample was employed, an unnecessarily large amount of permanganate was required. In some cases the quinhydrone solution was so dilute that the sample employed had a volume of 250 cc., and a t other times a volume of only 10 cc. was used, which was diluted to 250 cc. SOLUBILITY OF QUINHYDRONE I N DISTILLEDWATER-IIl order to calculate the per cent concentration of quinhydrone in the test solution after it had passed through the tube containing pellets of quinhydrone, it was necessary to determine the solubility curve for quinhydrone. This was accomplished by saturating distilled water at various temperatures and determining the amount of quinhydrone dissolved in a definite volume of solution. The saturated solu-
DROPSO F ACETONE BUFFERSATURATEDSOLUTION WITH QUINHYDRONE ADDEDTO pH at 50cc. OF Voltage 21" C. BUFFER VOLTAGE
-0,1396
4.52
2 4 10 20
-0.1910 -0.1890 - 0.1890 -0.1890
-0,1086
5.92
4 10 20
- 0.1094 -0.1086 - 0.1036
Although these data were obtained at 21" C., essentially the same results were found a t other temperatures and on different types of solutions, whether buffered or unbuffered. Hence, it may be concluded that for an accuracy of *0.05 pH, 7 mg. of quinhydrone per 100 cc. of test solutions are sufficient in the temperature range of 2" to 40" C. for a large number of industrial solutions. In a few special cases it has been necessary to increase this concentration of quinhydrone, the requisite concentration being determined as above by a dip-type pH apparatus. QUANTITATIVE DETERMINATION OF QUINHYDRONE DISSOLVED IN WATER-The method developed for the quantitative determination of quinhydrone utilized the oxidizing power of potassium permanganate. The following solutions were prepared: A potassium permanganate solution containing 15 grams per liter of solution, and a reducing solution containing per liter of solution 100 grams of ferrous ammonium sulfate, 20 grams of ammonium sulfate, and 25 cc. of 95 per cent sulfuric acid. The equivalent of reducing solution to the permanganate solution was obtained. The
Tamperatwo, 'c.
Figure 1-Solubility Curve for Quinhydrone Dissolved in Distilled Water
tions were prepared in a thermostat with frequent stirring. Two hours were allowed at each temperature (with the exception of 50" C.) for saturation, after which period of time check results were obtained. At 60" C. the oxidation of quinhydrone became appreciable, causing results that were too high. Definite volumes of these saturated solutions were removed with a pipet, the tip of which was covered with a glass tube filled with cotton to filter out solid particles of quinhydrone. The data for the solubility of quinhydrone up to 50" C.
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
are given in Table IV and the solubility curve in Figure 1. Luther and Leubner (3) give the solubility of quinhydrone 10
I'.
PI. rni"YtD
0
Figure 2-Variation of Quinhydrone Concentration i n City Water Passed a t Different Flow Rates and Temperatures through Tube Packed with Pellets of Quinhydrone
at 25" C., determined by an iodine titration, as 0.018 mole per liter. The present author obtained at the same temperature slightly less than 0.019 mole per liter. Table IV-Solubility
of Quinhydrone Dissolved i n Distilled Water QUINHYDRONE IN TEMPERATURE 100 cc. SOLN. Gram O c. 0.3 0.116 0.122 6.0 0.156 8.4 0.154 10.0 15.0 0.245 20.1 0.320 21.0 0.321 30.2 0.530 30.9 0.542 40.5 0.754 50.0 1.035
RATEOF SOLUBILITY OF PELLETS O F QUINHYDRONE-The pellets of quinhydrone were inch (1.42 cm.) in diameter and '/q inch (0.31 cm.) thick on the edge with convex faces making them l/d inch (0.63 cm.) thick at the center. Each pellet had a weight of approximately 1 gram. They were manufactured in a tablet machine which insured uniformity. Before the pellets were used in the following tests they were retained in flowing water for 24 hours so that under test they would yield a more uniform rate of solubility. Forty of these pellets were retained in a tube 1 1 / ~inch (2.85 cm.) in internal diameter and 3 inches (7.62 cm.) long. City water was allowed to percolate a t various controlled rates and temperatures through them. The quinhydrone concentration in the effluent was determined by the above method. The curves in Figures 2 and 3 depict the variation in concentration of quinhydrone dissolved from the pellets with a variation in the rate of flow and temperature of the water, and the per cent saturation of the resulting quinhydrone solutions a t each condition. Several interesting facts, some of which are apparent and not unexpected, are observed. The rate a t which quinhydrone dissolved increased rapidly with a rise in temperature. Also the concentration of quinhydrone increased rapidly with a decrease in rate of flow. The per cent saturation of the solution with quinhydrone a t each rate of flow decreased with an increase in temperature up to 21" C., from which temperature it increased at about the same rate it had decreased, the rate in each case being greater the smaller the rate of flow. However, the most noteworthy fact is that
405
at approximately 100 cc. a minute and below 20" C. the concentration of quinhydrone was on the border line as to its ability to yield a reliable pH measurement, the minimum concentration as determined above being 7 mg. per 100 cc. Again it is emphasized that these pellets were operated under ideal conditions. As the pellets dissolve, their surface decreases, and solids in industrial solutions will cover their surfaces, both conditions requiring frequent service to maintain the minimum operating concentration of quinhydrone in the test solution. It is also apparent that quinhydrone may easily be wasted. In other words, none of the above-mentioned methods for the introduction of quinhydrone into the test solution which involves retaining the quinhydrone in the flowing stream is industrially satisfactory. It is also apparent that an apparatus which will add continuously to the flowing test solution the minimum amount of quinhydrone requisite for a pH measurement is the ideal method for introducing quinhydrone into the test solution. Such an apparatus will be described in a later communication. Noble Metal Electrode
The function of the noble metal electrode in making a pH measurement with a quinhydrone electrode is to acquire accurately the potential of the oxidation-reduction equilibrium established in the solution by the dissolved quinhydrone, this potential being indicative of the pH of the solution. For an individual pH measurement where it is possible to clean the electrode before making a determination and to add quinhydrone much in excess of the above-determined minimum concentration, it matters little whether 8 gold, platinum, or a gold-plated platinum electrode is employed. However, in continuous pH work it is desirable to use an electrode which does not require frequent cleaning.
8
18
I8
w
b4
28
aa
Tamperatwe, 'C
Figure 3-Variation i n Saturation of Water Solution of Quinhydrone with City Water Passed a t *DlffePent Flow Rates and Temperatures through Tube Packed with Pellets of Quinhydrone
During the progress of this work it was discovered that a gold electrode, after operating a few hours in some types of solutions, failed to acquire accurately the potential of the quinhydrone equilibrium reaction existing in the solution. This inaccuracy was detected by making individual pH measurements on the effluent from the flow channel in which was suspended the gold electrode. Subsequent experiments were conducted in order to correct this difficulty. These experiments were made by using the continuous pH apparatus, a detailed discussion of which will be given in a later communication. Briefly, the apparatus consists
of a flow channel in which are suspended the noble metal electrode and a saturated calomel electrode. The test solution was passed through this flow channel after the requisite amount of quinhydrone had been added to it. The potential existing between the electrodes was automatically recorded by a Leeds and Northrup pH recorder having a paper speed of 4 inches (10.16 cm.) per hour. The potential of all dip-type pH measurements was obtained by a Leeds and Northrup portable potentiometer accurate to 0.5 millivolt.
I 4
5 ~ H
I
4
Figure 4-Increasing Unreliability of Gold Electrode for Continuous pH Measurements o n City Water Curve a obtained during first day of operation. Curves b and c obtained after 2 and 3 days of continuous operation.
.
Vol. 3, No. 4
ANALYTICAL EDITION
406
The test solutions that were passed through the flow channel for this experiment were either city water as it comes from the tap, or city water whose pH had been lowered by the addition of an acid, usually sulfuric acid. The latter test solution was made up and contained in a 30-gallon (113.5liter) reservoir. During the early stages in the operation of this continuous pH measuring apparatus, accurate results were obtained no matter how frequently the test solution in the flow channel was changed. Thus, curve a, Figure 4, is typical of these results. (The curves in Figures 4 and 5 were taken directly from the recorder paper.) The curve a, Figure 4,will be explained. The pH of the tap water (6.6 pH) was recorded from m to n, at which point the tap water was turned off and the acid tap water turned into the flow channel. The pH fell rapidly to 3.24 pH and was accurately recorded as such. At T the acid tap water was turned off and the original tap water turned into the flow channel. T h t pH changed to its original value of 6.6 DH and was recorded as such.
and dilute ammonium hydroxide. Upon replacing the electrode, the correct pH value of 3.24 pH was almost immediately recorded. The cleaned electrode then operated satisfactorily for several hours, but eventually became increasingly insensitive for accurate pH measurements, yielding successively more inaccurate results similar to curves b and e, Figure 4. The cause of a gold electrode’s becoming increasingly insensitive is not known. It is thought to be connected with the presence of chlorine in the water. However, in this particular problem it was not so much the cause as the result which was of greater interest, since to eliminate the cause would probably require some type of purifying apparatus for the test solution. It was much simpler to change the gold electrode for one of platinum. The cause of a gold electrode’s increasing insensitiveness within a few hours’ operation fortunately has no effect upon a platinum electrode. Similar tests were made with a platinum electrode as were conducted with the gold electrode. The results of these tests are given in curves a, b, and c, Figure 5. The platinum electrode retained its power of reproducibility after being operated for 6 days under the same conditions as caused the gold electrode to become increasingly insensitive after a few hours’ operation. These tests were extended over periods of several months in an industrial installation on the same type of solution (Dalecarlia Filter Plant, Washington, D. C . ) . Although the platinum electrode lost its luster, its sensitivity was not impaired. Upon cleaning the electrode the indicated potential essentially remained unchanged. Cleaning of Platinum Electrode
The usual methods of cleaning a gold electrode are not applicable to a platinum electrode to be used for continuous pH measurements in solutions which contain little more than the minimum concentration of quinhydrone to yield a sufficiently accurate pH measurement. The feasibility of a cleaning method was tested by checking the pH of the test solution in the flow channel, as indicated by the poten-
\
-_
9 D
It
B
I
8:.
/
i
Q
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
A cleaning method was finally discovered which permits the platinum electrode to indicate accurately the correct potential within 2 to 5 minutes. (In any electrometric pH measurement a variable amount of time is always necessary for the establishment of equilibria conditions.) The method of cleaning the platinum electrode consists in boiling the electrode in a 5 to 10 per cent sodium bisulfite solution for at least 3 minutes. The electrode is removed, washed in water, and inserted into the flow channel. Care must be taken not to touch the electrode or contaminate it in any way. Obviously such a cleaning will not remove many things that deposit on the electrode from industrial solutions. Therefore, in cleaning an electrode that has been contaminated, it is better to boil the electrode until it becomes bright in 50 per cent nitric acid, followed by a water wash. But it is absolutely essential to follow this acid treatment by
407
boiling the electrode in the 5 to 10 per cent sodium bisulfite solution. The thoroughness of terminating the cleaning of a platinum electrode with the sodium bisulfite treatment was further demonstrated with a battery of five flow channels operating in parallel on the same solution. The five platinum electrodes were cleaned by this new method and each inserted into a flow channel. Within a few minutes every electrode was indicating the correct pH within 2 millivolts. This experiment was repeated several times to show that the results were not accidental. Literature Cited (1) Biilman and Jensen, Bull. SOC. chzm., 41, 151 (1927). (2) Clarke, "The Determination of Hydrogen Ions," 3rd ed., p. 205, Williams & Wilkins, 1928. (3) Luther and Leuhner, J . $yak#. Chem., 96, 314 (1912): 2. onorg. Chem., 74, 389 (1912).
Ignition Losses in Potash Analyses of Triple Superphosphate Mixtures' L. B. Lockhart TENNESSEE COPPER & CHEMICAL CORP.,ATLANTA, GA.
OR several years it has been known that fertilizer mix-
F
tures containing potash salts and superphosphate may give erratic results, usually low, upon analysis for potash content by the Official Method. A number of papers have appeared on this subject from 1925 to date, the difficulties being usually ascribed to the presence of phosphoric acid or its salts which are present in the solution used for evaporation and ignition. Various methods for removal of phosphates have been proposed (1). Kerr (6) found that where a small quantity of phosphoric acid was present in the potash solution, the potash results were from 0.2 to 0.3 per cent too low. Bible (2) found low results with silica dishes, especially with "treble" superphosphate mixtures. Some of his results in porcelain were 30 per cent too low, with low results also in platinum. Haigh (4)' using the Official Method without removing phosphates, obtained losses as high as 36 per cent of the true potash content with fertilizer mixtures containing up to 16 per cent of phosphoric acid and 4 to 5 per cent of potash. Satisfactory checks were not obtained. I n a special study of the effect of silica dishes, Haigh (3, 6) found low results as compared with ignition in platinum dishes. The present paper deals primarily with the temperature of ignition. None of the papers referred to states the time, temperature, or method of ignition. The Official Method (1) states: "Maintain a full red heat until the residue is perfectly white.'! This temperature may be between 600" and 900" C. With no precautions indicated as to the effect of excessive temperatures, the higher temperatures are often used. The samples used were commercial mixtures of superphosphates, potash salts, and in most cases mineral and organic ammoniates. The procedure was to prepare the solution by the Official Method, using 2.425 grams of the fertilizer, making to 250 cc., and taking a 40-cc. aliquot for evaporation. A smaller aliquot was used for samples of high potash content. Thus the size of the aliquot represented a somewhat smaller sample than specified in the Official Method. Silica 'Received September 26, 1930. Presented before the Division of Fertilizer Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12,1930.
dishes 4 inches (10.16 cm.) in diameter and with flat bottoms were used, except where platinum is designated. These dishes were in regular use and in good condition. For each comparison, aliquots were taken from the same flask so that variations in making up the solution would not affect the reTable I-Comparative
Potash Results under Different Ignition Conditions POTASSIUM
KINDOF PaOa IN CHLOROPLATINATE FERTILIZER ALIQUOT 4-24-12
Gram
Gram
0.0438
0.2750 0.2764 0.2752a 0.2724
4-24-12
0.0232
4-24-12
....
0-24-24
0.0242
0.1200 0.1178 0.1180 0.1170 0.1092 0.2344 0.2350 0.2282
0. 2326a 0.2326 0.2338 0.1175 0-24-24 n iim 0.1167 4-24-4 0.0511 0.0922 0.0904 0 . 089Za 2-14-4 0.0050 0.0810 0.0772 2-12-6 0.0064 0.1222 O.ll60 0.0852 2-10-4 0.0802 Laboratory 0.2640 0.1246 sample: b.0 KC1 85% HsPO4 0.1306 0.1084 0.0203 a 0.1287
....
IGNITION PERIOD Min. Veryfaintred, Cone022 (590OC.) 3 Dark cherry red, Cone 014 (830' 5 C.) Dark cherry red 5 Very bright cherry, Cone 012 (890O C.) 6 Low red 3 Bright cherry red 3 Lowest red 2 HEATOF IGNITION
Full red Lowest red Dark cherry red Bright cherry red, Cone 011 (920' C.) Bright cherry red Full cherry red no HnSO4 added Lowest red, no 'H&Ok added Lowest red
10 3
4
a
4 4
~
4
2
Bright red Lowest red Bright cherry red Bright cherry red Lowest red Bright cherry red Lowest red Bright cherry red Lowest red Full red Just below white (above 900' C.)
lo
Just below white (above 900' C.)
8
0.1114 Just below white (above 900' C.)
13
. . ..
3 4 4 4 10 4 10 2 10 2
+
O>b
0.1316 Ignited in platinum. b Additional potash found in silica dishes by boiling with hydrochloric acid. Dishes used were badly damaged, and from 25 to 29 mg. of silicon dioxide were present with the potassium chloroplatinate precipitate. a
