V O L U M E 27, NO. 5, M A Y 1 9 5 5 battery with one of higher voltage. I t is desirable to be able to compensate completely the photosignal arising from the full light of the source using maximum slit width a t the wave length of greatest phototube sensitivity, if this photosignal is to be read, for then the broadest use of Methods I11 and IV is possible. CONCLUSIONS
The conclusions of the paper may be summarized as follows: The manner of using the slit-width, sensitivity, and dark-current controls does have an effect on the relative error of measurement of concentration unless the limiting precision of reading the dial is due to light source fluctuations. In the latter event the usual method of using a spectrophotometer-Le., using darkness and solvent to set the zero and 100 dial positions respectively-is recommended on account of its simplicity, as the precisions of all methods are then nearly or exactly equal. If the limiting precision of reading the dial is not due to light fluctuations, resort to other methods is determined by the need for precision-that is, the allowable error. The darkness-solvent references of Method I are convenient, rapid, and easily explained to untrained personnel, so that unless a real need for additional precision exists Method I is preferable. When very precise work is required, the type of work determines the method used. For very concentrated samples, Method I1 may be used, with the advantage that only one reference solution is needed. Still better precision may be had by diluting the sample to about 36.8% transmittancy and using Method IV,
725 but an additional reference solution is required. For trace analysis on very dilute solutions Method I11 is used, again requiring only one reference solution. If the precision of reading the dial is limited by the insensitivity of a null-point detector, the method should be used with low and constant sensitivity. It must be kept in mind that the over-all error will not be better than the care with which volumetric, chemical, and other noninstrumental processes are carried out, irrespective of the method used. When the new methods here proposed are used properly, a large increase in precision may be expected. ACKNOWLEDGMENT
This research was supported in part by the United States Air Force, through the Office of Scientific Research of the Air Research and Development Command. LITERATURE CITED
(1) Bastian. R.. ANAL.C H E M .21. . 973 (1949). (2j Gridgeman,’N. T., Ibid., 24, 445 (1952).’ (3) Hiskey, C. F., Ibid., 21, 1440 (1949). (4) Kimeroff, I . , Kational Bureau of Standards, Washington, D. C., private communication. ( 5 ) Ringbom, A , , and Osterholm, K., ANAL.CHEM.,25, 1798 (1953). ( 6 ) Steams, E. I., in “Analytical Absorption Spectroscopy,” M. G. hlellon, ed., Chap. 7 , p. 338, Wiley, New York, 1950. (7) Young, I. G., and Hiskeg, C. F., A x . 4 ~ .C H E M .23, , 506 (1951). RECEIVED for review November 9, 1953. Accepted December 28, 1954.
Automatic Photometric Titrations of Calcium and Magnesium in Carbonate Rocks LEONARD SHAPIRO and W. W. BRANNOCK
U. S.
Geological Survey, Agricultural Research Center, Beltrville,
Rapid nonsubjective methods have been developed for the determination of calcium and magnesium in carbonate rocks. From a single solution of the sample, calcium is titrated directly, and magnesium is titrated after a rapid removal of R20,$and precipitation of calcium as the tungstate. A concentrated and a dilute solution of disodium ethylenediamine tetraacetate are used as titrants. The concentrated solution is added almost to the end point, then the weak solution is added in an automatic titrator to determine the end point precisely.
T
HE determination of calcium and magnesium in carbonate rocks by titration with the sodium salt of ethylenediamine tetraacetate (Versene) has been reported by several workers (1, 3 ) . These procedures are more rapid than the conventional gravimetric methods, but it has been difficult for many workers to determine the exact end point in titrations by the usual visual means. Such procedures are suitable for work where high accuracy is not required and where results less accurate than those attained gravimetrically are acceptable. Higher accuracy can be attained by observing the end point as Versene is added to a solution placed in a photometer. This type of titration has been described recently ( 4 ) , but procedures of this kind, which involve incremental additions of Versene solution, are not suitable for routine analyses of large numbers of samples. The U. S. Geological Survey has had a need for rapid methods of analysis of limestones and dolomites. As a first approach a
Md.
method was devised to determine calcium and magnesium in which the titrations were done by allowing a solution of Versene to flow slo~vly,a t a constant rate, into a beaker placed between the light source and the photocell of a commercially available spectrophotometer. As the Versene reacted with the calcium or magnesium in the beaker the changing absorption of light was recorded continuously using a pen-and-ink recorder. It was found possible by this arrangement to achieve results within 1 to 2% of the amount present. These titrations required as much as 10 minutes for each titration. In place of the commercial spectrophotometer a titrator was designed for this specific application (Figure 2). This instrument is similar to a titrator described by Barredo and Taylor ( 2 ) . DISCUSSION OF PROCEDURES
The procedures now used in this laboratory for carbonate rocks are more accurate and more rapid than those obtained by the authors’ first approach. In place of the titration of an entire aliquot by the titrator, the procedure is as follows. A portion of the contents of the titration beaker containing the sample is temporarily withdrawn. The remaining solution is then titrated rapidly with a buret containing a relatively strong solution of Versene, to a point where the color change of the indicator has definitely occurred as observed visually. The volume delivered by the buret is read carefully. The portion of the sample previously removed is now replaced in the titration beaker, bringing the indicator back to its previous color. The titration beaker is now placed into the automatic titrator, and the
ANALYTICAL CHEMISTRY
726 small part of the calcium or magnesium not yet reacted is titrated with more dilute Versene. The sum of the results of the two titrations provides an accurate result for the sample. Both constituents are determined in aliquots from a single solution of the sample. Calcium is titrated directly without separations. Magnesium is determined after rapid separation of the R203 group with ammonium hydroxide and precipitation of calcium as the tungstate. If RpOa is not removed, results may be low by several tenths of a per cent. Calcium should be separated prior to the determination of magnesium, especially where the calcium-magnesium ratio is high, to avoid the errors arising in difference determinations. The use of tungstate as a precipitant for calcium is more satisfactory than oxalate, which is commonly used. INSTRUMENTATION
The titration assembly (Figure 1) consists of a Mariotte bottle, a titrator, and a recorder.
ESSURE-EOUALIZER TUBE
-
IR INLET TUBE
DOWNFLOW TUBE
Vacuum is applied until air starts to bubble through the Versene solution, and then the vacuum is turned off. The rubber stopper must, of course, provide an airtight seal to maintain the vacuum. Titrator. In Figure 1 the titrator is ready to use; in Figure 2 it is shown with the front panels removed. The wiring diagram is given in Figure 3. There are four simple circuits: the stirrer circuit, a, the titrator circuit, b, the light circuit, c, and the photocell circuit, d. The stirrer circuit consists merely of a magnetic stirrer with an on-off switch that is connected directly to the line current. The titrator circuit is a single-pole double-throw toggle switch connected in such a way that line current is allowed to go either to the electromagnetic hosecock or, when thrown to the opposite position, to the chart drive of the recorder. By these means coordination is achieved between the opening of the hoeecock, which starts the flow of Versene, and the start of movement of the chart. The light circuit is fed from the line, through a voltage stabilizer, to the primary of a 6- to %volt transformer. The secondary of the transformer is in series with a 50-cp. automobile lamp bulb and a 50-watt-5-ohm rheostat. The photocell circuit is a series circuit involving a barrierlayer selenium photocell, a 1.5-volt dry-cell battery, and the galvanometer of the recorder. In this application, advantage is taken of the decrease in resistance of the photocell as the light intensity is increased rather than the output of the photocell itself. The polarity of the units must be correct, as shown in the diagram, or the circuit will not function. A few minutes of experimenting a t this point will yield the proper arrangement. A carrier for two 2 X 2 inch filters is mounted in front of the photocell. An orange filter (Corning 3480) is used in titrating magnesium, and this plus a green filter (Corning 4015) is used in titrating calcium. Recorder. A pen-and-ink recorder that provides full scale deflection for l ma. is used. It should have an internal resistance not exceeding 2000 ohms. The chart drive is geared to a feed of 3 inches per minute. REAGENTS AND APPARATUS
ELECTRO. MAGNET1C HOSECOCK
Dilute Versene, 20 grams of disodium ethylenediamine tetraacetate dissolved in 20 liters of xater. Place in the Mariotte bottle. Concentrated Versene, 7.0 grams of disodium ethylenediamine tetraacetate dissolved in 4 liters of water. Hydrochloric acid, 1 3. in water. Sodium hydroxide, 15% in water. Buffer solution, 66 grams of ammonium chloride per liter of 1 1 ammonium hydroxide.
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ELECT RO HOSECOCK RECORDER
Figure 1.
TIT RATOR Titration assembly-
Mariotte Bottle. A Mariotte bottle provides a simple means for delivery of Versene solution t o a titration beaker a t a constant flow rate. A 20-liter bottle is mounted on a shelf or support about 4 feet above the work bench. It is fitted with a rubber stopper through which pass three glass tubes. The air-inlet tube starts a t a point near the bottom of the bottle, goes through the stopper, and opens into the air. The pressure-equalizer tube starts in the air space above the liquid, goes through the stopper, and is connected to a vacuum line. A rubber section with a pinchcock is placed somewhere along this tube, so that vacuum may be conveniently applied or shut off. The downflow tube starts close to the bottom of the bottle and passes through the stopper and down to the electromagnetic hosecock, which is part of the titrator. A few inches below the electromagnetic hosecock a capillary tube extends that has a tip so constricted that the f l o ~ rate is maintained a t 4 to 5 ml. per minute. By this arrangement when liquid is flowing through the downflow tube the hydrostatic head is from the bottom of the air-inlet tube (essentially the bottom of the bottle) to the top of the liquid in the beaker, which is in the titrator. This maintains a constant head as the liquid in the bottle is used up. To keep this hydrostatic head correct the air pressure within the bottle must be such that as liquid is removed, air is entering through the air inlet and bubbling through the Versene solution. As the temperature in the room changes from day t o day, it may be necessary to reduce the air pressure above the liquid before the titrator is used.
REfl
Figure 2.
Titrator
Front removed
V O L U M E 2 7 , N O . 5, M A Y 1 9 5 5
727
Ammonium chloride-methyl red solution. Add 10 ml. of 0.02% methyl red solution to 1 liter of 15% ammonium chloride. Sodium tungstate, 20% in water. Murexide indicator solution, approximately 0.2% in water. This solution should not be kept for more than 3 days. Eriochrome Black T indicator solution, approximately 0.2% solution in water. This solution should not be kept more than 3 days. Aluminum chloride solution. Dissolve 133 mg. of aluminum foil or ribbon in hydrochloric acid and dilute to 500 ml. A 1-ml. aliquot contains the equivalent of 0.5 mg. of aluminum oxide. Standard calcium oxide solution. Transfer 1.000 gram of Bureau of Standards standard sample No. 88 (dolomite) to a 250-ml. beaker. Add 20 ml. of hydrochloric acid (1 l), cover, and boil for 3 to 5 minutes. Cool the solution to room temperature and dilute to 1 liter in a volumetric flask. Ten milliliters of solution contain the equivalent of 3.05 mg. of calcium oxide. Titration beakers, 400-ml., tall-form beakers with a line marked off about 3.5 inches from the bottom of each beaker. Plastic-covered stirring bars. Several should be available.
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8. A4sthe titration proceeds the pen that is tracing out the curve moves to the left as the indicator changes color (Figure 1). When sufficient Versene has flowed into the beaker to react completely with the calcium, the pen no longer moves to the left but traces a line parallel with the chart movement. After about 1 inch of this straight portion is traced out, reset all switches to “Off.”
9. Repeat the procedure from 1 through 8, using a 10-ml. portion of the standard solution. 10. Use the curves to obtain the end point of the titrations as illustrated in Figure 4. Extend a ruled line from the last portion of the curve, the flat portion, and extend another ruled line along the portion of the curve immediately preceding the sudden change of direction. The intersection of these lines is the end point. The time required in seconds for the titration is measured from the starting point to the end point.
ST 4NDARDIZATION S
I t is necessary to standardize both the dilute Versene and the concentrated Versene. The dilute Versene is delivered in the titrator and is finally standardized in terms of milligrams of calcium oxide per second of flow or as milligrams of magnesium oxide per second flow. The concentrated Versene is delivered by an ordinary buret and is standardized in terms of milligrams of calcium oxide per milliliter of Versene or milligrams of magnesium oxide per milliliter of Versene.
ELECTROMAGNETIC HOSECOCK RECORDER CHART DRIVE
MAGNETIC STIRRER
Figure 4. Titration curve
‘’
-
a-STIRRER ClRCUlr’
1 I 5 V-AC.
/
1’
m50-cp
LAMP 6 - 8 V TRANS,
C-
b -TITRATE
\
11
VOLTAGE STABILIZER
LIGHT C I R C U I T
PHOTOCELL
kT-r] -
RECORDER PEN MOVEMENT
,I
+ -
d- PHOTOCELL CIRCUIT
Figure 3.
Circuitry of titrator
Standardization of Dilute Versene Solution. 1. Transfer 1 ml. of the calcium oxide standard solution to a 400-ml., tall-form titration beaker. 2. Add water to the mark on the beaker, then add 10 ml. of 15% sodium hydroxide. 3. Place a stirring magnet in the beaker and add 1 ml. of the murexide indicator solution. 4. Place the beaker in position within the titrator with both the green filter and the orange - filter in position in front of the photocell. 5. Insert the downflow tip into the liquid, and turn the stirrer switch to “on.” 6. Turn the light-control rheostat until the recorder Den indicates nearly maximum. 7. Throw the titrate switch to “titrate.” This de-energizes the electromagnetic hosecock, allowing Versene to flow, while a t the same time the recorder is energized and the chart proceeds to move a t a fixed rate.
11. Subtract a blank correction from each titration. It is obtained by dividing the total number of seconds for the 10-ml. aliquot by 10 and subtracting the result from the number of seconds for the 1-ml. aliquot. This procedure is better than the direct titration of a blank solution, as the curve for a blank solution is more poorly defined than one for a solution containing some calcium. The blank is a small number, and when obtained in this manner it differs from a theoretical blank by a negligible amount. It is essentially the same value for calcium oxide and magnesium oxide. 12. Complete the standardization of the dilute Versene with the following calculation:
3.05 (Seconds for 10-ml. standard)
mg. of CaO per - blank -
second
13. Then, hlg. of CaO per second X 0.719 = mg. of MgO per second The use of this factor has been confirmed with pure magnesium. Standardization of Concentrated Versene Solution. 1. Transfer 25 ml. of the standard solution to a 400-ml. titration beaker. 2. Add about 200 ml. of water, 10 ml. of 15% sodium hydroxide, and 1 ml. of the murexide indicator solution. 3. Pour 10 to 20 ml. of the solution into a small beaker and set aside. 4. By means of a buret add the concentrated Versene solution rapidly until the end point is definitely passed (the indicator changes from salmon to purple). 5. Read the buret carefully and record the reading. 6 . Replace the solution in the small beaker in the main portion of the Rolution; the color should change back to salmon. 7. Add water to the mark on the beaker, place it in the titrator, and complete the titration as in the standardization of dilute Versene solution above, steps 3 through 8. 8. Translate the curve obtained into seconds. 9. Subtract the “blank” correction. 10. Multiply the result by the value obtained for the standardization of dilute Versene. The answer is then milligrams of calcium oxide titrated by dilute versene.
728
ANALYTICAL CHEMISTRY
11. To complete the standardization of the concentrated Versene perform the calculation:
(7.63 mg.)
- (mg. titrated by dilute Versene) (step 10) -
Buret reading mg. of CaO per ml. concd. Versene
12. Then, Mg. of CaO per ml. X 0.719 = mg. of MgO per ml. concd. Versene PREPARATION OF SAMPLE SOLUTION
The choice of method of decomposing carbonate rock samples varies somewhat with the problem and the material to be analyzed. Generally, interest is centered upon the acid-soluble calcium and magnesium. The procedure for this decomposition is follows:
1. Transfer 0.500 gram of sample to a 250-ml. beaker. 2. Add 20 ml. of hydrochloric acid (1 3) and boil 3 to 5 minutes. 3. Cool to room temperature and make to volume in a 250-ml. volumetric flask. If desired, the calcium and magnesium in the insoluble portion may be recovered by filtering off the insoluble residue, burning the paper off in a platinum crucible, decomposing the residue with a few milliliters of hydrofluoric and sulfuric acids, fuming to dryness, and then taking the residue into solution with hydrochloric acid. The resulting solution is added to the filtrate from step 3, above, prior to dilution to 250 ml.
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DETERMINATION OF CALCIUM
The determination of calcium proceeds in a manner analogous to the standardization of concentrated Versene solution. Check to be sure that both color filters are in place. 1. Transfer 10 ml. of sample solution (equivalent to 20 mg. of sample) to a 400-ml. titration beaker. 2. Follow steps 2 through 10 in the section “Standardization of concentrated Versene solution,’’ This yields milligrams of calcium oxide in that part of the aliquot titrated by the automatic titrator. 3. Multiply the buret titration by the standardization value for the concentrated Versene to obtain milligrams of calcium oxide in that part of the aliquot titrated by the buret. 4. Add the value for milligrams of calcium oxide obtained by the automatic titrator (step 2) to the value for milligrams of calcium oxide obtained by buret titration to get total milligrams of calcium oxide in the aliquot. 5. Calculate per cent calcium oxide by use of the formula: Total mg. of CaO in the aliquot X 100 = per cent
cao
20
10. Add 1 mi. of Eriochrome Black T indicator solution, and proceed with the titration as in standardization of concentrated Versene solution, steps 3 through 10, which provides the value for milligrams of magnesium oxide in that part of the aliquot titrated with the automatic titrator. 11. Multiply the buret value by the value for the standardization of concentrated Versene. This provides milligrams of magnesium oxide in the part of the aliquot titrated by the buret. 12. Add the value for magnesium oxide (mg.) obtained by the automatic titrator to the value for magnesium oxide (mg.) obtained by buret titration to get magnesium oxide in the 200/ 250 part of the aliquot (this is equivalent to 40 mg. of original sample). 13. Calculate per cent of magnesium oxide by use of the formula : Total mg. of MgO X 100 - per cent MgO 40
DISCUSSION OF RESULTS
To provide information as to the accuracy and precision of the procedure, a set of mixtures was carefully prepared using National Bureau of Standards standard sample 88 (dolomite) and pure calcium carbonate to cover the range of concentrations of calcium and magnesium that may occur in limestones and dolomites. The range of calcium varied from 1.5 to over 100 times the magnesium content. The set was run through twice, on different days by different operators using different solutions of Versene and different breakups for each set. National Bureau of Standards standard samples 1A (limestone) and 88 (dolomite) were run with the set in the same manner except that No. 1.4 was treated with hydrofluoric and sulfuric acids to get the portion insoluble in dilute hydrochloric acid into solution. The results are shown in Table I. The value of statistical treatment of such limited data is dubious. The data indicate that there is no significant bias in either procedure as deviations occur in both positive and negative directions.
Table 1. Analyses of Known Mixtures Calcium Oxide, % Magnesium Oxide, % A
B C
D
E F
G H I J NBS 1A NBS 88
“True” 55.5 53.6 51.1 48.4 45.8 43.2 40.7 38.1 35.6 55.8 41.3 30.5
Run 1 55.8 53.9 51.2 48.3 45.8 43.3 40.7 38.0 35.4 55.6 40.9 30.4
Run 2 55.8 53.9 51.4 48.7 46.0 43.4 40.7 38.1 35.6 55.9 40.9 30.4
“True” 0.40 2.0 4.1 6.5 8.6 10.8 12.9 15.1
17.2 0.21 2.2 21.5
Run 1 0.42 2.1 4.2
6.6 8.4 10.7 13.0 15.1 17.0 0.23 2.3 21.7
Run 2 0.44 2.0 4.1 6.4 8.6 10.9 13.1 15.2 17.3 0.23 2.0 21.6
DETERMINATION OF MAGNESIUM
Check to be sure that only the orange filter is in place. 1. Transfer 25 ml. of sample solution (e uivalent to 50 mg. of original sample) to a 250-ml. volumetric fla& 2. Add about 200 ml. of water, 1 ml. of‘aluminum chloride solution, and 15 ml. of the ammonium chloride-methyl red indicator solution, then ammonium hydroxide (1 1) dropwise until the indicator turns yellow. 3. Make the solution to volume, mix, and let stand 15 minutes. 4 . Pour the solution through a dry filter paper in a dry funnel, catching 200 ml. of the filtrate in a dry 200-ml. volumetric flask. 5. Transfer the contents of the flask to a 400-ml. beaker and rinse the flask with distilled water. 6. Add 10 ml. of the buffer solution and 10 ml. of the 20% sodium tungstate solution to the solution in the beaker. 7. Cover the beaker and transfer to a hot plate. 8. Bring the solution to a boil, allow to boil for approximately 2 minutes, and then cool to room temperature in a water bath. 9. Decant the solution into a 400-ml. titration beaker without rinsing. The major portion of the precipitated calcium tungstate will adhere to the sides of the beaker. The amount of solution left on the walls is inconsequential, and the extensive transfer of the tungstate precipitate is undesirable because the resulting cloudy solution diminishes the quality of the titration. Small quantities of precipitate are of no consequence.
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The Versene method described in this paper has the advantage of greater speed over conventional gravimetric procedures. I n addition, the automatic titrator removes the subjective evaluation of end points, which often limits the desirability of Versene procedures. The titrator described is inexpensive and simple to construct, the major cost being the milliammeter recorder. If available, the titration can be made in any type of spectrophotometer for which a recorder can be adapted. LITERATURE CITED (1) Banewicr, J. J., and Kenner, C. T., A X A L . CHEM..24, 1186
(1952). (2) Barredo, J. M. G., and Taylor, J. K., Trans. Electrochem. SOC., 192, 437 (1947). (3) Cheng, K. L., Kurts, T., and Bray, R. H.. ANAL.CHEM.,24, 1640 (1952). (4) Sweetser. P. B., and Bricker, C. E., [bid.. 25, 253 (1953). RECEIVED for review November 17, 1954.
Accepted January 18, 1955. Publication of this paper is authorized by the director, U. 5. Geological Survey.
