n of Total Hardness in Water Ernployin etric Titration Procedures
. P. Sickafoose, and M. A. Schmittl Institute for Atomic Research and Department of Chemistry, Iowa State Udversity, Ames, Iowa 50010
ater hardness is determined by titration with EDTA ixed indicator containing Arsenazo I and a taining THAM. The end point is sharp and uick; iron, copper, aluminum, and common anions do ot interfere in the amounts present in most water samples. A rapid and accurate spectrophotometric titration procedure is also reported. Formation constants for the cakiurn(ll) and magnesium(l1) complexes with Arsenazo I at pH I10 have also been determined.
FEWanalytical methods compare in importance with the EDTA titration of total hardness in water. Despite this, the existing procedures for water hardness (1-5) have several unfortunate drawbacks. Eriochrome Black T, Calmagite, and other indicators (which are all of similar general structure) change color slowly at the end point and thus require that the end point be approached slowly and carefully. Further, traces of iron, copper, and certain other metal ions dissolved in the water block these indicators and either prevent an end point or seriously reduce its sharpness. Cyanide and other masking agents may be employed, but cyanide is a potential safety hazard. In the present work Arsenazo 1is proposed as an indicator for the determination of total hardness in water by EDTA titration. Arsenazo I has previously been used as an indicator for EDTA titration of rare earths, thorium, and other metal ions (6). Arsenazo I changes color rapidly at the end point and is not blocked by small amounts of iron, copper, or aluminum in hard water. The one disadvantage of Arsenazo I is its relatively poor color contrast between the metal-complexed indicator (violet) and the free indicator (orange). However, this problem is solved by the use of color-screening dyes so that a sharp color change of gray-violet to yellow is observed at the end point. Arsenazo 1is also well adapted to the use of a spectrophotometric end point. Methods are given for the visual and spectrophotometric titration of total hardness with EDTA, and for spectrophotometric titration of barium(I1). EXPERIMENTAL
Apparatus. A Beckman Model B spectrophotometer, modified for spectrophotometric titrations as described previously (7) was used. Titrant was delivered from a 5-ml micro buret whose tip extended through the cover and 1
Present address, Sweet Briar College, Sweet Briar, Va.
(1) American Public Health Association, Inc., “Standard Methods
for the Examination of Water and Wastewater,” 12th Ed., 1965, pp 146-152.
(2) H.Diehl, C. A, Goetz, and C. C. Hach, J. Am. Wurer Works Assoc., 42, 40 (1950). (3) W. Biederrnann and G. Schwarzenbach, Chimia, 2, 1 (1948). (4) J. Patton and W. Reeder, ANAL.CHEM., 28, 1026 (1956). (5) J. D. Betz and C. A. Noll, J. Am. Wurer Works Assoc., 42, 49 (1950). (6) J. S . Fritz, M. J. Richard, and W. J. Lane, ANAL.CHEM.,30, 1776 (1958). (7) J. S. Fritz and D. J. Pietryzk, ibid., 31, 1157 (1959).
1954
e
beneath the solution surface. The titration cell was a 180-ml tall-form beaker with 4.5-cm path length. Reagents. Arsenazo I, 0-(1,8-dihydroxy-3,6-disulf0-2naphthy1azo)-benzenearsonic acid, Aldrich Chemical Co. No. 10,798-0, was used as received for indicator solutions. For formation constant determination Arsenazo I was reprecipitated as described previously (6), passed through a hydrogen-form cation exchange column, and again reprecipitated. The purified indicator was filtered, dried for 2.5 hours at 110 “ C and stored over anhydrous magnesium perchlorate in a vacuum desiccator. This material is hygroscopic and difficult to weigh accurately, so a portion was equilibrated overnight with the atmosphere to constant weight. A thermogram of the Arsenazo I on a DuBont 950 Thermogravimetric Analyzer showed a weight loss of 11.6 % at 40 “C corresponding to absorbed surface moisture, and a further weight loss complete at 122 “ C that is equivalent to the loss of two waters of hydration. Thus the atmosphere equilibrated indicator has a purity of 88.4% based on the dihydrate. This is in good agreement with the purity factors, fA, obtained in our formation constant determinations. A comparison of the purified Arsenazo I with the commercial material (Aldrich Chemical Co. No. 10798-0) showed that the two have identical spectra from 220 to 700 mp. This demonstrates that there are no appreciable colored impurities in the commercial material. Xylene Cyanole FF, Eastman No. T1579, was used as received. Martius Yellow, and Eriochrome Black T, Hartman-Leddon Co., were used as received. Calmagite, G. Frederick Smith Chemical Co., was used as received. Disodium dihydrogen ethylenediaminetetraacetate, Eastman No. 6354, was used as received. Primary standard calcium carbonate, G . Frederick Smith Chemical Co., was dried overnight at 110 OC and stored in a desiccator until used. Tris-(hydroxymethy1)-aminomethane or THAM, J. T. Baker Chemical Co. No. A820, was used as received. Analytical reagent nitrate, chloride, or sulfate salts of the metal ions were used for interference studies. Solutions. PH 10 BUFFER. Dissolve 13.1 grams of THAM in 100 ml of water; add 4.0 ml of concentrated hydrochloric acid, then 30 ml of concentrated ammonium hydroxide. ARSENAZO I MKXED INDICATOR.Dissolve 100 mg of Arsenazo I, 100 rng of Martius Yellow, 52.5 mg of Xylene Cyanole FF, and 1.0 gram of THAM in 10 ml of 2-propanol. Add slowly 20 ml of water, then transfer to a 100-ml volumetric flask and dilute to volume. This solution is stable for more than seven months when stored in the polyethylene container. Procedures. The water sample taken should require less than 15 ml of 0.01M EDTA for titration, and the titration should be completed within 5 minutes from the time buffer is added (I). For visual titrations add about 25 ml of distilled water to a 25-ml water sample. Add 2 ml of THAM buffer solution and stir; then add 2 drops of Arsenazo I mixed indicator solution. Titrate immediately with 0.01M EDTA, stirring continuously. The end point is when the solution flashes from light violet-gray (almost colorless) to a permanent yellow.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
Table I. Data for Stopped-Flow Experiment Arsenazo I Calmagite 9 x 10-6M 9 x 10-6M THAM, ammonia Ammonia 1.99 x 1 0 - 3 ~ 1.99 x 1 0 - 3 ~ 2.00 x 1 0 - 3 ~ 2.00 x 10-SM
Indicator Buffer Magnesium(I1) EDTA titrant Ionic strength PH Wavelength Oscilloscope sweep Time of change, total
Arsenazo I 9 x 10-SM ammonia 1.99 X 10-8M 2.00 x 1 0 - 3 ~ 0.10
0.10 10.1
0.10 10.1
10.1
560 mp 50 msec/cm 0 . 4 sec
620 mp 1000 msecjcm 8.5 sec
I
I
I
I
I
I
0
2
4
6
8
IO
I
crn Figure 1. Time of color change A. Calmagite-ammonia buffer, 1000 msecicm B. Arsenazo I-THAM buffer, 50 msec/cm C. Arsenazo I-ammonia buffer, 100 msec/cm
For titrations where cyanide is required to mask metal ions, add about 250 mg of sodium cyanide after the buffer but before the indicator is added. For spectrophotometric titrations add about 85 ml of distilled water to a 10-ml water sample in a 180-ml tall-form beaker containing a magnetic stirring bar. Set the spectrophotometer wavelength at 560 mp and adjust the absorbance to zero with the sample in the light path. Add 2 ml of THAM buffer and 0.3 ml of Arsenazo I mixed indicator. With magnetic stirring titrate with 0.01M EDTA. A plot of absorbance US. milliliters of EDTA may be made and the point where the slope is greatest taken as the end point. For routine titrations, however, a “one-point” titration is recommended. With a sample containing buffer and indicator in place in the spectrophotometer, adjust the slit width so that the absorbance reads 0.440 plus of the total absorbance change previously determined for EDTA titration of a water sample. Titrate to an absorbance of 0.440 as the end point. Most titrations were repeated three or more times using a “bias elevator.” This is a short paper cylinder around the buret graduations to prevent observation until the end point is decided. Barium(I1) was titrated spectrophotometrically at 555 mp. Dilute a 5.0-ml sample with 82 ml of distilled water in a 180-ml tall-form beaker and add 10.0 ml of 1.0 X 10-4M Arsenazo I indicator solution. Use 3 ml of THAM buffer and titrate with 0.01M EDTA. Plot absorbance us. milliliters of EDTA as the indicator begins to change. Extrapolate the straight line portions of the curve; the intersection of the extrapolated lines is the end point.
560 mp 100 msecjcm
0.5 sec
rate of Arsenazo I color change was compared with the rate for Calmagite indicator using a Durrum stopped-flow spectrophotometer. A solution of magnesium(I1) containing pH 10 buffer, indicator, and a solution containing a 0.5% stoichiometric excess of EDTA were rapidly mixed. Stock solution aliquots were diluted to ensure that conditions were identical except for the indicators and buffers. The results in Figure 1 and Table 1 show that the total color change with Arsenazo requires 0.4 or 0.5 sec as compared with 8.5 sec for Calmagite. The one disadvantage of Arsenazo I is the relatively poor visual color contrast between the metal-complexed indicator (violet) and the free indicator (orange). This problem was solved by the use of color-screening dyes. From a chromaticity diagram it was determined that a yellow-green dye (absorption maximum around 430 mp) would screen the uncomplexed color. No single dye stable at pH 10 was found, so a mixture of blue Xylene Cyanole FF and Martius Yellow was used. The color contrast of these dyes in various proportions for water hardness titrations was judged by a panel of chemists. The mixture selected was Arsenazo I, Martius Yellow, and Xylene Cyanole FF in the weight ratio 1.O :1.O : 0.525, which gives a sharp color change at the end point from pale gray-violet to yellow. The end point is best viewed against a white background under fluorescent light ( I ) . Although the conventional ammonia-ammonium chloride pH 10 buffer works reasonably well, we found that a buffer containing THAM [tris(hydroxymethyl)aminomethane] gives much sharper end points with the Arsenazo I mixed indicator. THAM is a weak chelating agent and appears to be helpful in masking aluminum(II1).
11.2
v t
1
/6
P
10.6
I R ’10.2 OS41
’“’“t 9.8
!i
RESULTS AND DISCUSSION
Visual Titrations. Unlike Eriochrome Black T, Calmagite, and other indicators of this general type, Arsenazo I changes color quickly at the stoichiometric point in a water hardness titration. This was proven by an experiment in which the
3.920
3.960
4.000 4.040 m l EDTA Figure 2. Effect of pH
4.080
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
1955
Table 11. Determination of Total Hardness in Water Samples Source Ames, Iowa Municipal Iowa State University
End point detection Visual
Replicates
Hardness, ppm CaCOa
15
83.24
0.085
Relative standard deviation 1.02 ppt
Spectrophotometric, “one point” Visual
5
83.33
0.071
0.85
Spectrophotometric, “one point” Visual
Kelley, Iowa Well
Spectrophotometric, “one point”
15
351.1
0.63
1.79
5
350.6
0.22
0.63
15
148.0
0.29
1.96
5
148.3
0.23
1.55
Table 111. Comparison of Indicators Indicator Arsenazo I mixed Calmagite Eriochrome Black T
Rep- Hardness, Standard licates ppm CaCOs deviation 5 5
387.4 387.2
0.43 0.45
5
387.6
0.46
Relative standard deviation l.llppt 1.16 1.19
Table IV. Effect of Interfering Ions Maximum interference concentration, pprn Interfering THAM substance THAM buffer buffer-NaCN Aluminum(II1) Over 50 Barium(I1) Titrates as hardness Cadmium(I1) Titrates as hardness Cobalt(I1) 3 50 Copper(I1) Titrates as hardness Over 250 Iron(I1) and (111) 20 50 Lead(11) Titrates as hardness Manganese(I1) Titrates as hardness Nickel(I1) 3 Over 250 Strontium(I1) Titrates as hardness Zinc(I1) Titrates as hardness Fluoride Over 20 Perchlorate 10,000 Phosphate Over 20 Sulfate Over 40
The effect of pH was studied by a series of carefully performed spectrophotometric titrations with mixed Arsenazo indicator at different pH values. The results are shown in Figure 2. A pH of 10.0 to 10.2 is recommended, but a pH as low as 9.7 or as high as 10.5 causes an error of less than 0.02 ml in a 4.00-ml titration. Data for repetitive analyses of three different water samples for total hardness are presented in Table 11. These were done as carefully as possible with a 5-ml micro buret. The results indicate that this method is capable of excellent precision. Comparison with titrations performed according to the APHA procedure using Calmagite and Eriochrome Black T showed excellent agreement between the Arsenazo I and APHA procedures (see Table 111). In this comparison all titrations were performed with a 10-ml buret. Ions likely to be present in hard water were added in varying amounts to determine what effect they might have on the 1956
Standard deviation
new method for total hardness. Small amounts of either ferrous or ferric ion cause no interference in the Arsenazo I procedure; the end point is still sharp and the results are accurate. When the added iron reaches about 20 ppm, there is some slowness around the end point owing to absorption by iron hydroxide, but the titration is still satisfactory. Cyanide may be used in the Arsenazo I method to mask larger amounts of iron, but for most water samples the use of cyanide is not necessary. Aluminum(II1) up to at least 50 ppm neither titrates nor reduces the sharpness of the end point. A small amount of copper(I1) does not interfere with the end point as it does in other hardness methods, but it does titrate as hardness. Small m o u n t s of cadmium(I1) and zinc also titrate as hardness. Cyanide effectively masks these metal ions. Manganese present in 11-ppm concentration (20 ppm as CaCQl) partially titrates and causes some fading of the end point; smaller amounts of manganese cause little trouble. The most serious metal interferences encountered were cobalt(I1) and nickei(I1) which prevent an end point when present in 3 ppm. However, cyanide effectively masks cobalt and nickel. Data for the interference study are given in Table IV. Fluoride, sulfate, phosphate, and other common anions cause no interference in the amounts reported in Table IV. Only polyphosphate, which has complexing properties, causes any detrimental effects. Spectrophotometric Titrations. Visible spectra of uncomplexed Arsenazo I and magnesium(IT), calcium(II), strontium(II), and barium(I1) complexes of Arsenazo were determined at p H 10 on a recording spectrophotometer. From these spectra, a wavelength of 560 to 570 mp appears to be optimum for a spectrophotometric titration with EDTA using Arsenazo I as indicator. At 560 mp at pH 10 Arsenazo I has a molar absorptivity of 9400, magnesium-Arsenazo ]I is 30,800 and calcium-Arsenazo I is 31,000. Formation constants for calcium(l1) and magnesium(I1) complexes were determined at pH 10.0 by a modified mole ratio method using a new linear plotting method (8). The graphical plots and results obtained are summarized in Figure 3, where K is the degree of complex formation and X’ is the molar ratio of metal to ligand. The slope and intercept of the curves were determined by the method of least squares using only data having three significant figures. For a 1 to 1 complex the (8) K. Momoki, J. Sekino, H. Sato, and N. Yamaguchi, ANAL.
CHEM.,41, 1286 (1969).
ANALYTICAL CHEMISTRY, VQL. 41, NO. 14, DECEMBER 1969
I
I
I
I
I
I .6
1.4 X’ 1.2 K
L
0.400 I ~
I .o
0.360
r
0.320 0.280
0
4
2
1 I-K
6
IO
8
0.24 0 3,600
Conditions and results: pH 10.0 buffer; ionic strength 0.10. Absorbance measurements made at 560 mp in 1-cm cell. C’L = 2.71 0 0
10-5
Ca(I1)-Arsenazo I: f L found = 0.87, K found = 106.B8 Mg(I1)-Arsenazo I: fL found = 0.87, K found = lo6. 0.440 0.400
t
z 0.360 a
t
0.240
-
1
’8
1
I
0.320
0 200
ARSENAZO I
I
I
I
I
I
Y
4.200 ml EDTA
4,000
4.400
4.600
4.800
Figure 4. Spectrophotometric titration of magnesium(I1) and calcium(1I)
Figure 3. New linear plotting method
x
3.800
n
T
I
-
MIXED -
Y
P
ml EDTA
Figure 5. Spectrophotometric titration of hardness in Ames, Iowa, municipal water with Arsenazo I mixed indicator at 560 mp slope is related to the conditional formation constant by the expression 1 K = (slopeXC’L) CIL is the constant concentration of the indicator species and the intercept of the line gives the purity of the experimentally weighed indicator. Curves for titration of magnesium(I1) and calcium(l1) with 0.01M EDTA using Arsenazo I mixed indicator are shown in Figure 4; the absorbance is measured at 560 mp. In both cases there is a sharp change in absorbance around the stoichiometric point, even though the horizontal scale is greatly spread out. The theoretical ratio of metal-Arsenazo I to free Arsenazo I at the stoichiometric point may be calculated from the formation constants of magnesium- and calcium-EDTA complexes at pH 10 and from the magnesium- and calciumArsenazo I complex constants. Following the method developed by Ringbom (9) the absorbance at the equivalence point in the titration of magnesium in Figure 4 is calculated as 0.47, and the absorbance at the equivalence point for calcium in the (9) A. Ringbom, “Complexation in Analytical Chemistry,” Interscience, New York, 1963, pp 108-114.
same figure is 0.30. These calculations are macle by substituting our values for the formation constants (along with known metal-EDTA formation constants) into Equation 51 of reference (9). An examination of Figure 4 shows that the maximum slope in the titration curves occurs close to the calculated absorbance values. A curve for spectrophotometric titration of an actual hard water sample is shown in Figure 5 using mixed Arsenazo I indicator. The change in absorbance around the stoichiometric point is considerably sharper for Arsenazo I at 560 mp than is a similar titration using Calmagite at 530 mp (where the magnesium complex absorbs) and is slightly sharper than with Calmagite at 620 mp (where the free indicator absorbs). More important from a practical standpoint, however, is the greater speed at which the absorbance attains a steady value when Arsenazo I is the indicator. The curve for spectrophotometric titration of hard water is more like the curve for magnesium than for calcium (see Figure 4), indicating that the more stable magnesium-Arsenazo I complex predominates over the calcium-Arsenazo complex. Titrations in which a curve must be plotted are time-consuming. However, the spectrophotometric curves with Arsenazo I are sufficiently reproducible that hardness in water may be determined accurately and simply by titrating with EDTA to a predetermined absorbance. To do this the
-
0.6001
I
1
I
I
L-----l
I
I
I
I
I
1
,
I 1
1
0.560
8a
0,3601 0.320
t
o~ 0,240 3.400
3.600
3.800
4.000
4.200
4.400
4.600
ml EDTA
Figure 6. Spectrophotometric titration of strontium(I1) and barium(I1)
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
1957
justing the spectrophotometer. The data in Table I1 show higher precision for titration of hardness spectrophotometrically than by visual observation of the end point. Spectrophotometric titration curves for strontium(I1) and barium(I1) are shown in Figure 6. Special attention was given to barium(I1) which is difficult to titrate accurately by existing EDTA methods. Around the stoichiometric point the curve is nearly linear. If the concentration of Arsenazo I is increased, this linear region is further extended. To utilize this, barium may be titrated with Q.01MEDTA taking the end point as the extrapolation of two linear portions of the titration curve (Figure 7). This end point is probably slightly higher than the true end point, but this may be compensated for by standardizing the EDTA by a similar linear extrapolation method. Results for three titrations averaged 0.051 1 mmole of barium compared with 0.0510 mmole obtained by passing through a hydrogen-form cation exchanger and titrating the liberated acid with standard sodium hydroxide. 0.70 -
ACKNOWLEDGMENT
amount of indicator added must be carefully controlled, and the correct absorbance setting for the instrument before any titrant is added must be ascertained. Once this is done a large number of samples may be titrated without further ad-
We express appreciation to A. Tateda who first determined formation constants for calcium- and magnesium-Arsenazo I. The assistance of James Espenson with the stopped-flow experiment is also gratefully acknowledged. RECEIVED for review March 12, 1969. Accepted September 12, 1969. Work performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2516.
Mass Spectral Metastable Transitions Determined by Electric Sector Variation Robert W. Kiser, Richard E. Sullivan, and Michael S. Lupin Department of Chemistry, University of Kentucky, Lexington, K y . 40506 A technique is presented that enables unique determinations of metastable transitions to be made. The dynamic range is greater than 10,000 and the method is equally applicable to positive and negative ions. Additionally, consecutive fragmentation processes may be studied. Theoretical and experimental details are given for this method, first suggested by Major, that employs a variation of the electric sector voltage in double-focusing mass spectrometers. The application of the technique to hexacarbonylchromium(0) Is presented. Advantages and limitations of the approaches employed are evaluated.
METASTABLE TRANSITIONS observed in the mass spectrometer provide significant information about fragmentation processes and ionic mechanisms in the gas phase. In double-focusing mass spectrometers usually only a fraction of the metastable transitions that occur is recorded, because the electric sector does not transmit ions without corresponding kinetic energy and therefore the only metastable transitions observed are those that occur in the region between the electric sector and the magnetic field. Thus only a fraction of the data is commonly obtained. In 1964 Barber and Elliott ( I ) and Beynon, Saunders, and (1) M. Barber and R. M. Elliott, “Comparison of Metastable Spectra from Single- and Double-Focusing Mass Spectrometers,” 12th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Montreal, Canada, June 7-14, 1964. 1958
*
Williams (2) reported means of overcoming this loss of available metastable information for double-focusing mass spectrometers by increasing the accelerating voltage. Important theoretical and experimental implications of this technique have been discussed in subsequent work (3-15). An alter-
(2) J. H. Beynon, R. A. Saunders, and A. E. Williams, Nature, 204, 67 (1964). (3) 0. Osberghaus and Ch. Ottinger, P h y ~Letters, . 16, 121 (1965). (4) K. R. Jennings, Chem. Commun., 1966, 283. (.5 ,) M. Barber. K. R. Jennings. - . and R. Rhodes. 2.Nufurforsch., 22a, 15 (1967). (6) J. H. Beynon and A. E. Fontaine, ibid., 22a, 334 (1967) and references therein. (7) K. R. Jennings and A. F. Whiting, Chem. Commun., 1967, 820. (8) L. A. Shadoff, ANAL.CHEM.,39,1902 (1967). R. Dalv. A. McCormick, and R.E. Powell, Rev. Sci. Instr., (9) . ,39,N.1163 (1968). (10) P. Schulze and A. L. Burlingame, J. Chem. Phys., 49, 4870 ’ (1968) and references therein. (11) J. H. Beynon, J. A. Hopkinson, and G. R. Lester, Intern. J . Muss Spectry. Ion Phys., 2,291 (1969). (12) L. P. Hills, J. €3. Futrell, and A. L. Wahrhaftig, “Experimental and Calculated Metastable Peaks in Toluene,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 18-23,1969. (13) V. Lohle and Ch. Ottinger, “Consecutive Metastable Decompositions,” 17th Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, Dallas, Tex., May 19-23, 1969.
ANALYTICAL CHEMISTRY, VQL. 41, NO. 14, DECEMBER 1969
