Determination of Phosphate, Silicate, and Sulfate in Natural and Waste Water by Atomic Absorption Inhibition Titration C. I. Lin and C. 0.Huber Department of Chemistry and Centerfor Great Lakes Studies, University of Wisconsin-Milwaukee, Milwaukee, Wis. 53201 The atomic absorption inhibition titration (AAIT) method was employed to study the simultaneous determination of silicate, phosphate, and sulfate with a single titration. A titration curve with three very distinct shifts in slope of linear segments is obtained. A mechanism in terms of rate processes in the droplets and particles is proposed to interpret the titration curve observed. Mathematical treatment on aspirating loss is developed and shows that such errors are limited to about 1% or less. A set of linear equations allows evaluation of the anion concentrations. The method was successfully applied to the determination of tap water and lake water for silicate and sulfate, to simulated waste water for all three anions, and to commercial detergent products for phosphate.
SILICATE,PHOSPHATE, AND SULFATE ANIONS are commonly present in most drinking, surface, and waste waters. With increasing concern for public health and environmental quality, determination of low level concentrations of these species is of considerable importance. Conventional methods (1-3) for quantitative analysis of each of these anions are somewhat lengthy and empirical, requiring a separation step. Indirect atomic absorption measurement of phosphate has been performed by either extracting phosphomolybdic acid ( 4 ) or dissolving the phosphomolybdate precipitate (5) and then determining the molybdenum content. Sulfur was determined in biological materials by determining the barium in the barium sulfate precipitate (6). In spite of the large literature on chemical inhibition effects in atomic absorption and emission spectrometry, little use has been made of this effect for analysis of the inhibiting anions. In 1968, Bond and O’Donnell demonstrated the indirect determination of fluoride based on the atomic absorption inhibition effect (7). Singhal explored a titration of cations with anions using a double capillary approach (8). The effects of several experimentalvariables o n chemical inhibitionat anionto-metal mole ratios less than one were examined in this laboratory and resulted in application to silicate (9), phosphate (IO), and sulfate (11) determination. The method in~~
(1) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chem-
istry,” Part 11, Vol. 2, Interscience Publishers, New York, N.Y., 1962. (2) F. D. Snell and C. T. Snell, “Colorimetric Methods and Analysis,” D. Van Nostrand Co., New York, N.Y., 1959. (3) “Standard Methods for the Examination of Water and Waste
Water,” American Public Health Association, 13th ed., New York, N.Y., 1971. (4) W. S . Zaugg and R. J. Knox, ANAL.CHEM., 38, 1759 (1966). (5) T. V. Ramakrishna, J. W. Robinson, and P. W. West, Anal. Chim. Acra., 45, 43 (1969). (6) D. A. Rose, P. S. Miller, and L. Lutwak, Anal. Biochem., 15, 313 (1966). (7) A. M. Bond and T. A. O’Donnell, ANAL.CHEM.,40, 560 (1968). (8) K. C. Singhal, R. C. P. Sinha, and B. K. Banerjee, Technology (lndiati), 6 , 219 (1969). (9) R. W. Looyenga and C. 0. Huber, ANAL.CHEM.,43, 498 (1971). (IO) C. 0. Huber and W. C. Crawford, Abstracts, 160th National
Meeting of the American Chemical Society, Chicago, Ill., September 1970, No. A71. 2200
volves titrating the anion solution with a metal cation solution while monitoring the atomic absorption signal for the metal. The semiautomatic atomic absorption inhibition titration (AAIT) proves to be relatively rapid, sensitive, and accurate for each of these anions. No method for simultaneous titration of silicate, phosphate, and sulfate or any combination of them in the same portion of solution has come to the authors’ attention. Kirkbright, Smith, and West (12) did sequential determination of phosphorous and silicon by indirect atomic absorption spectrophotometry. I n this report, simultaneous determination of silicate, phosphate, and sulfate anions in a single titration is introduced and evaluated. The shape of the titration curve obtained is interpreted. EXPERIMENTAL
Apparatus. Ordinary atomic absorption instrumentation with an infusion pump for titrant insertion is used. This apparatus has been described previously (9). Metallic cations, which would otherwise interfere, are removed by shaking the sample for 1-2 minutes with a hydrogen-form cation exchange resin (Dowex 50-X8, 20-50 mesh). This ion-exchange procedure is performed conveniently using a plastic separatory funnel with a glass wool plug and exposing the resin t o acid solution when not in use. Glassware used was acid hardened (1 :I H2S04-HN03 overnight) t o minimize errors due to leaching and solubility. Reagents. All solutions were prepared from reagent grade materials using deionized distilled water. The standard solutions of sulfate and phosphate were prepared from the sodium salts. Silicate solutions were prepared by dissolving silica using a minimum amount of sodium hydroxide. Solutions of these anions were passed through a hydrogen form cation exchanger to remove sodium ions. Magnesium solutions were prepared from dried MgClz. 6Hz0 and their concentrations confirmed using EDTA titration. Simulated drinking water and waste water (secondary sewage treatment effluent) conforming to an average composition (13) were prepared according to Table I. The silicate, phosphate, and sulfate anions were added later for convenience in varying the amount of these anions in test solutions. Procedure. I n order t o remove metallic cations, the sample containing silicate, phosphate, and sulfate is treated with H-form cation-exchange resin after which the p H is typically 3-4. The volume is adjusted t o 50 ml. The flame gas flow rates are adjusted to 18 ft3/hr (30 psi at flowmeter input) air and 25 ft3/hr (30 psi at flowmeter input) hydrogen. (11) R. W. Looyenaa and C. 0. Huber, Anal. Chim. Acra., 55, . i79-83 (1971): (12) G. F. Kirkbright, A. M. Smith, and T. S . West, Analyst (London), 92, 41 1 (1967).
(13) “Cleaning Our Environment, The Chemical Basis for Action,”
American Chemical Society, Washington, D.C., 1969, p 109.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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Table I. Composition of Simulated Drinking and Waste Waters Concentration, pg/ml Component Drinking Waste 65 113 Naf 5 9.8 K+ 45 60 Gag+ Mgz+ FHCOaNos-
c1NHi+
18
;:% la
io’l w a 0.20
25
1 61 62 220
300 15 150 20
- 0
10
20
30
4 0
TIME, min
Figure 1. Experimentally obtained titration curve A 100- or 150-ml beaker serves as titration vessel which is then placed o n the magnetic stirrer. Injection and aspiration tubes are positioned and stirring is begun. As soon as possible after aspiration has begun, titrant flow and recording are initiated simultaneously via a common switch. Titrant flow rate is 2.03 ml per minute and magnesium titrant concentration is 50 pg per milliliter. Aspiration rate is 2.0 ml per minute. Titration is terminated when the final rise in the titration curve (Figure 1) is fully developed.
Solution contained 1.0 pg/ml S O ? , 4.0 Mg/ml PO,, 20 pg/ml SO*. Titrant: 50 pg/ml Mg as MgCI2. Chart speed: 2.0 in./min
90r
RESULTS AND DISCUSSION Titration Curve Shapes. The use of chemical inhibition effects for analysis requires a relatively cool flame with readily adjustable flame temperature. In addition, AAIT requires that the range of droplet sizes be small so that dehydration and chemical reaction times are well defined. The use of a hydrogen-air flame and a chamber-nebulizer burner fulfills these requirements. The shape of the titration curve obtained experimentally is shown by Figure 1. The titration curve can be discussed in terms of three lines of evidence. First, the shape of the curve is dependent upon flame temperature. Data illustrating this dependence are illustrated for solutions containing silicate and sulfate in Figure 2 . As observed, there is a large dependence of curve shape upon flame temperature. A more stoichiometric (i.e., higher temperature) flame results in little inhibition by sulfate, whereas a cool flame (R = 0.3-0.5 in Figure 2) results in total inhibition by the silicate and sulfate and thus n o occurrence of points A and B of the titration curve. The positions of end points A and C o n the titrant axis show a limited dependence o n flame temperature. Such behavior was also observed for solutions containing a single inhibiting anion ( 9 , l i ) . Second, measurements show that the end points in AAIT titrations are independent of height of the beam in the flame. Earlier data by Alkemade (14) showed that the occurrence of a limiting inhibition for phosphate on calcium is independent of height in the flame. Third, observations of titration curves (Figure 1) as a function of varying concentrations of silicate, phosphate, and sulfate shows that end point A is a linear function of silicate concentration, end point B of silicate and phosphate concentrations, and end point C of silicate, phosphate, and sulfate concentrations. These relationships have been used for analytical measurements as further described later. The three lines of evidence described above provide bases for interpreting the remarkable and useful titration curve shapes. (14) C. T. J. 91 (1958).
Alkemade and M. H. Voorhuis, Z . Anal. Chem., 163,
/
0‘
I
7=075
2
3
TIME, min
Figure 2. Effect of air : hydrogen flow ratio on titration curve R: Air : hydrogen flow ratio H? flow rate: 10 ft3/hr (30 psi)
The first two lines of evidence show that the rate processes which establish the stoichiometry of the end points occur in the evaporating droplet or the solid particle rather than in the vapor phase. This is supported by reports that when magnesium and interfering anion were introduced into the flame by separate nebulizers, such inhibition effects were not observed (14, 15). The droplets are originally of the order of 0.1-1 X mm in diameter with concentrations of the reacting species a t the parts per million level. Conversion through concentrated solution, dehydration, pyrochemistry, and vaporization stages apparently occurs in less than 1 millisecond (16). The third line of evidence suggests that specific rate-controlled reactions occur, some of which lead to formation of oxides and other highly refractory species. Such inhibition reactions depend not only upon flame temperature, but also (15) W. T. Elwell and J. A. F. Gidley, “Atomic Absorption Spectrophotometry,” 2nd ed., Pergamon Press, London, 1966. (16) C. T. J. Alkemade, “Flame Emission and Atomic Absorption Spectrometry,” J. A. Deans and T. C. Rains, Ed., Marcel Dekker, New York, N.Y., 1969, Vol. I, Chap. 4.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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Table 11. Simultaneous Determination of Silicate, Phosphate, and Sulfate in Simulated Waste Water Anions added, pg/ml Anions found, pg/ml Si02 POi SO, Si02 PO4 so4 1.00 0.00 30.0 0.92 0.1 29.4 1.50 3.00 20.0 1.44 3.24 20.5 1.50 2.00 20.0 1.60 1.56 20.3 2.00 1.00 25.0 2.10 1.50 24.3 20.0 20.3 4.00 2.00 3.48 2.38 10.0 5.00 20.0 7.30 9.56 20.0 20.0 1.06f 4.04i: 21.3 j= 1.00 4.00 0.19 O.lOa 0.8” Mean and estimated standard deviations for five replicate titrations. Table 111. Linear Regression Analysis for Standardization Series (Titrant concentration, 50 pg/ml Mg; titration volume, 50 ml; titrant flow rate, 2.03 ml/min) Linear Concentration of Slope, Intercept, correlation coefficient min/ppm minutes standard, ppm Si02 0.50, 1.00, 2.00, 3.00, 4.00 0.523 0.008 0.997 (20 pg/ml Sod
so1 0.5.00, 10.00, 12 00, 20.00, 25.00. 30.00
0.096
0.337
0.984
upon metal to concomitant ion ratio and upon the relative concentrations of concomitant ions in the sample solution. The interpretation is summarized by a series of rate processes, Reactions 1 to 6, where m, n, p , and q are stoichiometric factors, not necessarily integers. Before A : SiOs2-
+ Mg2+ -%
MgO.SiOz +N.R.
(1)
A to B:
+ Mgz+ k2_ (1 + ni)Mg0.Si02 -% Mg P o 4 + -+ nMgz+ A nMgO.P2O6-% Mg
SO3*-
(2)
(3)
After B (negative slope)
+ (1 + m)MgO.SiOz -% p M g 0 . S i 0 2 -% Mg Mg2+ + nMgO.P*O5-% qMg0.P20S-% Mg
Mg2+
(4) (5)
Before C (plateau) Mg2+
+ S042--% M g 0 . S 0 8 +N.R.
(6)
Referring to Figure 1, up to point A, reaction of magnesium with silicate in the dehydrating droplet and resulting particle completely eliminates formation of magnesium atoms, Reaction 1. It apparently occurs a t considerably greater rates than inhibiting reactions with phosphate or sulfate. The stoichiometry of this reaction corresponds to M g 0 . S i 0 2 as reported earlier for solutions containing silicate only (9). The region A to B represents reactions with phosphate in addition to 2202
those with silicate, which are partly decomposed to release atomic magnesium, Reactions 2 and 3. The reversal at point B is a remarkable aspect of the titration curve, indicating a region of concentration where increasing magnesium content results in decreasing magnesium atomic absorption. This negative slope suggests an inhibiting reaction with a net rate order in magnesium greater than unity. Formation of a more stable silicate and/or phosphate species by reaction of the inhibiting species of Reactions 2 and 3 with added magnesium as shown in Reactions 4 and 5 fulfills these criteria. This sequence requires k q fand k s fto be greater than kqf and ks’. It should be noted that the pair of processes Reactions 2 and 3, as well as the pair 4 and 5 may be single mixed salt formations. After the region of negative slope, a horizontal portion of the curve occurs. When phosphate concentrations are relatively low, virtually total inhibition is observed as shown in Figure 2. With larger phosphate concentrations, this portion of the curve remains horizontal even though elevated above the base line suggesting that the degree of inhibition is independent of increasing total magnesium concentration. The magnesium sulfate species produced apparently contributes negligible magnesium atoms to the flame. Beginning a t point C, magnesium in excess to that forming refractory species appears. The limited variations in endpoint positions for A and C described earlier (9) can be attributed to partial loss of the inhibiting anion as its refractory oxide without interaction with magnesium, or alternatively to temperature-dependent variations in the net stoichiometry represented by m and n in Reactions 2 and 3. Such variations are known under the pyrochemical conditions existing during the life of the droplet particle. Stability of a variety of such species is well known (17). It is important to note that the end points designated mark sharp changes in efficiency of generating free magnesium atoms in the flame as a function of anion/Mg atom ratio in the titration solution. Thus it is not the extent of inhibition which is measured, but rather points at which shifts in the net inhibiting processes occur. The measurement is therefore properly regarded as a direct titration rather than as a n indirect inhibition technique. The chemical inhibition processes observed and discussed here are to be distinguished from occlusion or matrix effects which are observed at much higher concomitant : analyte mole ratios. The titration curves provide a new useful technique for obtaining additional information on the chemistry of the atomization processes. Concomitant metal as well as anion effects can be studied so that the large number of chemical inhibition and enhancement effects ( e . g . , 6 , 18) are open to further investigation. The chemical effects used in generating these titration signals involve reactions in the evaporating droplet and the resulting particle. Thus, the effects are independent of the mode of observation of the flame species. The titration method is therefore applicable to atomic emission and atomic fluorescence flame spectrometry as well as to absorption. Concentration Measurements. Titration of standard solutions containing silicate, phosphate, and sulfate showed that the following set of linear relationships apply for end points A, B, and C: (17) H. E. Schwiete and R. W. Hechler, “Proceedings of the 8th Conference on the Silicate Industry,’’ F. Tamas, Ed., Akademiai Kaids, Budapest, 1966, pp 93-132. (18) P. E. Thomas and W. E. Pickering, Tnlmtu, 18, 127 (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
A B C
=
=
=
+b
(7)
+ d(P04) + h
(8)
@io2)
c(Si02)
+ g(S04) + b
e(SiO2) +f(POJ
(9)
where A, B and C are the respective end points (Figures 1 and 2) and b is the titration blank associated with time required for titration solution to be transferred from the titration vessel t o the aspirator. Rearrangements of Equations 7-9 provides the following set of equations:
- K2 K4A - K5
(SO2) = KIA (PO*) = K3B (SOJ = K6c
- K;B
(10) (11)
+ KsA + Ks
(12)
Data for 9 titrations of standard solutions with concentrations 1-3 pg/ml Sios, 0.5-3.0 pg/ml POq, and 15-25 pg/ml SO4were used to generate values of the constants in Equations 9-1 1 by a computerized multiple linear regression technique. The values obtained were: KI
=
0.666
Kq
=
1.584
K;
1,000
K?
=
0.406
Kj
=
1,137
Ks = 1.675
K.,
=
1.704
K6
=
1.626
KQ = 4 . 3 9
Application of the technique to actual determinations in real samples was performed using prepared solutions simulating drinking and waste waters and real samples of drinking water. The contents of the simulated waters conformed to average values for the nation (13)and are shown in Table I. By applying Equations 10-12 to data for simulated waste water containing various amounts of silicate, phosphate, and sulfate, the data of Table I1 were obtained. The results in line 6 show that when total pg/ml SiO, and PO4 exceeds one half of Mg/ml SOr, the error for silicate and phosphate determination becomes excessive. This is a result of merging of points B and C o n the titration curve. Virtually all surface waters and most water supply and waste waters (13, 19, 20) however, are accommodated within the ranges for which excellent results are obtained. Many potable water supplies and their sources contain very low concentrations of phosphate. For such samples, titration of silicate and sulfate by this technique can be performed without the set of calibration Equations 10-12, but with simple linear calibration plots for pure solutions. The data for such calibrations are given in Table 111. Application to real samples yields data shown in Table IV. The satisfactory results shown for both initial values and known additions in Table IV indicate no interference by the other constituents of such water (see Table I). It is interesting that for the flame used here, the fluoride present in the treated water does not interfere (cf. Ref. 7). Accuracy and precision compare well with existing slower methods for these determinations. Table V shows results for titration of silicate only in treated drinking water. The technique was also applied to the determination of phosphate content of commercial detergent products. A sample equivalent to 20-40 mg orthophosphate was dissolved in water to 100.0 ml. One-tenth aliquots were used for each titration. Sulfuric acid was added to each titration solution (19) “Handbook of Chemistry,” N. A. Lange, Ed., Handbook Publisher, Sandusky, Ohio, 1956, p 804. (20) “FWPCA Methods for Chemical Analysis of Water and Wastes,” 1969, U.S. Dept. of the Interior.
Table IV. Simultaneous Determination of Silicate and Sulfate in Raw and Treated Municipal Water (Titration variables same as for Table 111) Concentration of Concentration of SO43 pg/ml Si02, pg/ml GraviGraviAAIT metric AAIT metric Sample method“ method* methoda methodb Raw HzO 21.5 & 0 . 1 20.7 1.39 i 0.04 1.2 Raw HnO 1.00 pg/ml Si02 21.7 f 0 . 5 ... 2.41 + 0.05 Treated H20 24.8 =t 0 . 1 26.0 0.95 i.0.01 1 .O Treated HzO 1.00 pg/ml ... 2.03 f 0.03 ... Si02 25.3 =t 0 . 2 Deviations shown indicate range for two or three determinations. * As reported by City of Milwaukee Water Purification Plant.
+
+
Table V. Determination of Silica in Simulated Drinking Water Number of Si02 added, Si02 determined, !Jg/ml titrations pg/ml 0.50 0.54 2 1 .OO 1 .OO 2 2.00 2.06 1 3.00 3.09 1 4.00 3.77 1 2 (mean pg Sios foundipg Si02 present) = 1.02; s = 5.1 %. Table VI. Per Cent Phosphate (POq) Determination in Commercial Detergent Products Brand AAIT Standar& No. method method Label 1 36.2 Z?Z 0.1 36.2 37.8 2 16.8 5 0 . 6 17.4 26.7 3 36.4 + 0 . 7 36.2 37.8 4 37.6 =t 0 . 3 36.4 Not available 5 28.0 i 1 . 0 26.6 Not available 6 0 0 0 a Molybdate-heteropoly blue method using ascorbic acid for reduction (Ref. 3, p 518).
to bring the sulfate concentration to approximately 20 pg/ml. A titration curve similar to that of Figure 1 was observed. Phosphate concentration was determined by Equation 11 with K3 reevaluated to 0.978 using the standard heteropoly method results for Brand No. 1 as the basis of standardization. The results are shown in Table VI. Titration Accuracy. Loss of analyte by aspiration into the flame introduces errors which are small because such losses are compensated in the standardization process--i.e., they also occur when calibration standards are being titrated. Titration solution volume changes are eliminated by adjusting titrant flow rate equal to that of aspiration rate. The double capillary method of Singhal et ai. (8) could readily be applied if necessary. Analytical treatment of titration accuracy as developed below shows that such errors are quite small. The rate of removal of titratable anion from the titration solution is given by: - -d= S M + y RS dt
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2203
where M is ,ug/min equivalent anion titration rate due to addition of titrant, S is pg titratable anion present at time t , V is titration solution volume in milliliters, and R is the aspirating rate in mllmin. The solution to this equation obtained by multiplying each term by exp(Rt/V), is:
where C , an integration constant, can be obtained from the initial condition at t = 0, S = So; So is the initial amount of anion in the titration. Thus C = So
+ MVIR
(1 5 )
At the end-point time, T , S = 0. Then with rearrangement Equation 14 becomes : exp(RT/V) =
S*R MV
+1
and RTjV
=
ln(S,R/MV
+ 1)
(17)
The right side of Equation 17 can be evaluated by a series. The quantity S,R/MV is distinctly less than unity (see data for Figures 1 and 2) so that the series evaluation of Equaation 17 rapidly converges and the first two terms suffice. This gives: RTjV
=
SoR/MV - 0.5(S0R/MV)'
(18)
Solving for T, the end-point time, and adding a term B, for the titration blank gives:
The term Q in Equation 19 is a correction factor which can easily be maintained at values greater than 0.90 by suitable adjustment of the experimental parameters, R (aspiration rate), M (anion titration rate), and V (titration volume). Measurements of T must always be made by comparison to standards. The titrant titer must always be observed at the same instrument setting because of sensitivity of the titration stoichiometry to flame temperature. Therefore, variations in Q in practice ordinarily lead to errors less than 1 %. Applicability of the Method. The sensitivity of the method is sufficient for the concentrations of all three anions in most waste waters and that for silicate and sulfate for surface and drinking waters as well. The concentrations of phosphate in the latter waters are of the order 0.01 pg/ml and not directly attainable by this technique. For those applications where lower detection limits are required, preconcentration by vaporization is viable since the three ions under consideration are not volatilized, although formation of insoluble silica and
2204
other species must be avoided. Preliminary studies show that pre-separation of these anions followed by titration using cooler flames than reported here should lower the determination limits to less than 0.1 pg/nll. It can be noted that the technique allows titration at concentrations several orders of magnitude less than is common for other titration methods. This advantage results because in this technique, the desolvation and pyrochemistry of discrete portions of the titration solution are observed. Although determinations based on the anion inhibition effects would initially appear to be non-specific, the results shown in Tables 11-VI with the variety of possible interferents present (Table I) indicate rather remarkable specificity. This specificity can be attributed to proper selection of flame temperature, use of a well-defined range of droplet sizes (i.e., chamber-nebulizer burner), and most importantly on use of a titration technique with its dependence on anion/Mg atom ratios in the aspirated solution. The cation exchange process which must precede the determination step need not impede the overall speed of the method in routine applications because it may be performed in a batch rather than a column ion-exchange procedure. For the low concentrations of cations in the samples considered here, kinetic processes involving exchange sites near the resin surface predominate during the ion-exchange step. Results using a batch procedure were indistinguishable from those using the slower columns. The technique described exposes a range of new titration reactions. Observation of the atomization of droplets in the flame results not only in extremely low concentration titrations but in signals related to reactions not otherwise observed by titration--i.e., desolvation and pyrochemical processes. As discussed in the previous section, such reactions are intricate. They include enhancement as well as inhibition effects and can involve cation-cation competitive effects as well as cation-anion interactions. Thus, new determination methods as well as new information on the pyrochemical and desolvation inorganic processes discussed in the previous section should result. ACKNOWLEDGMENT
The authors acknowledge the assistance and cooperation of Tom Dolan and the City of Milwaukee Water Purification Plant. Thomas Chojnacki contributed helpful discussion on the mechanism. RECEIVED for review March 30, 1972. Accepted July 21, 1972. Part of the paper was presented at the 163rd Meeting, American Chemical Society, Boston, Mass., April 1972. Financial support was provided by the Federal Water Quality Administration, Environmental Protection Agency, Grant No. 16020DHD.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
