ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
1041
Semiautomated Procedure for the Determination of Low Levels of Total Manganese Joan Crowther Ontario Ministry of the Environment, Laboratory Services Branch, Water Quality Section, Box 2 13, Rexdale, Ontario, Canada
Total manganese is determined In water samples using batch digestion by autoclavlng in acid medium followed by automated colorimetry based upon the formation of the manganeseformaldoxime complex. Catlon interference is suppressed by complexing agents while color interference is eliminated by using a blanklng stream which is synchronized with the color stream. Both streams receive identical quantities of reagents, but the order of addition is altered to prevent the formatlon of the manganese-formaldoxime complex in the blank stream. I n the range of application (0.006-0.200 mg/L Mn), the accuracy and precision of the method are comparable to those of flame atomic absorption spectrophotometry (AAS).
Pretreatment to solubilize manganese is unnecessary for some samples acidified at the time of collection ( I ) , but a more rigorous digestion is advisable when a laboratory is analyzing process drinking waters, natural surface and well waters collected from a broad geographical area-especially if they contain particulates. The subject digestion procedure, which requires autoclaving in an acid medium for 1 hour, is similar to that described in Standard Methods ( 2 ) . The selected colorimetric procedure for manganese is based on the formation of its formaldoxime complex (1,3-5). The pink color produced has a constant intensity over the pH range 9.0 to 10.5, and the absorbance is measured a t 480 nm. The molar absorptivity is 1.1 X lo4 (1). Water Research Association Digest, 85 (1) contains an excellent account of the development of this colorimetry including the use of complexing agents to suppress cation interference. The automated procedure presented herein is similar in principle to Abdullah's (4),but it includes a reference or blanking stream synchronized with the color stream. The reference stream includes the digested sample and all the reagents a t the same concentrations as the color stream, but the order of reagent addition is changed. This technique was suggested by the work of Goto et al. (3) who noted that the formation of the manganeseformaldoxime complex was suppressed if ethylenediaminetetraacetic acid was added before formaldoxime.
EXPERIMENTAL Apparatus. An autoclave equipped with automatic slow exhaust and capable of being operated at 121 "C was used. Culture tubes (25 X 150 mm) with Teflon-lined screw caps were the sample containers and racks for these tubes were required for the autoclave step. Digestant was added via an Oxford Pipettor. The colorimetry utilized a Technicon AutoAnalyzer system (AAII Colorimeter) and a single pen chart recorder. The base plate height of the drum of the Technicon sampler (large industrial model) was altered to accept the tall culture tubes. A Pye Unicam SP1900 spectrophotometer and single pen chart recorder were used to provide control data. Reagents. All reagents were prepared with deionized distilled water. The digestion acid contained 640 mL concentrated hydrochloric acid plus 30 g hydroxylamine hydrochloride/L. Hydrochloric acid was the active or solubilizing agent. Although hydroxylamine was unnecessary for the manganese procedure, 0003-2700/78/0350-1041$0i .OO/O
it was included to prevent loss of iron as the same aliquot of digested sample is normally used in this laboratory for both iron and manganese determinations. An acid-wash is required for the AutoAnalyzer system to maintain an acceptable baseline. The concentration of hydrochloric acid (4% by volume) in this wash equaled that in the digested sample. The formaldoxime solution was prepared by dissolving 250 g hydroxylamine hydrochloride in 700 mL of water, adding 100 mL of 37% formaldehyde, and diluting to 1L. The ascorbic acid solution contained 84 g/L. The buffer was prepared by dissolving 267 g ammonium chloride in 867 mL concentrated ammonium hydroxide, and diluting with 133 mL of water. The final reagent contained 100 g of hydroxylamine hydrochloride and 37.2 g of the disodium salt of ethylenediaminetetraacetic acid (EDTA) per liter. Procedure. Using a wide mouth pipet, a 30.0-mL aliquot of sample was transferred to a culture tube and 2.0 mL of the digestion acid was added via an Oxford Pipettor. After tightly screwing on the cap, the vial was placed on a rack, and the rack was set on an enamel tray to protect the autoclave in case of breakage or leaking. Culture tubes (as many as 250) were autoclaved at 121 "C for 60 min. After the pressure in the autoclave had been discharged, the door was left ajar for 15 min as a safeguard against an exploding tube. To date approximately 15000 tubes have been autoclaved without breakage. Moreover losses by leakage have been minimal: after 200 tubes had been autoclaved, the trace of condensate in the enamel trays had a pH value of 6.8. The samples were allowed to cool to room temperature before proceeding with the colorimetric phase. After removing the caps, the culture tubes were presented to the Technicon AutoAnalyzer system. The flow diagram is schematically illustrated in Figure 1. The reference and color stream had to be synchronized so that both reached the respective flow cells of the double beam colorimeter at the same time. This was accomplished by introducing a phenolphthalein solution as a sample and adjusting the length of the appropriate sample line. Manganese determinations were completed in accordance with accepted technique for AutoAnalyzers (6).
RESULTS AND DISCUSSION Simply acidifying 30.0-mL aliquots of sample with 2.0 mL acid digestant, i.e. no autoclaving step, was sufficient pretreatment for approximately 75% of 200 surface and ground water samples submitted for total manganese determinations. In the remaining cases, manganese recoveries varied from 40 to 90%. These samples, which were collected over a wide geographical area, frequently contained particulates, and visual observation indicated that incomplete recovery was associated with the presence of particles. The autoclave-digestion pretreatment proved, therefore, to be necessary and is an integral part of the subject method. Furthermore, errors associated with sample transfer were significantly reduced because the culture tubes used as sample containers for the digestion step were subsequently presented to the Technicon sampler of the AutoAnalyzer system. For the colorimetric step, the initial experimental phase was designed to confirm the data in the literature (1, 3-5), and adapt the formaldoxime method to the AutoAnalyzer system (AAII colorimeter) for the range 0.006-0.200 mg/L Mn. As reported ( I ) , the ammonia buffer was required to maintain pH control and any delay between the addition of the buffer and formaldoxime resulted in loss of sensitivity. 0 1978
American Chemical Society
1042
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978 FLOW RATES (ml / m i d
I
BUFFER
0.42
I AIR
I
1
I 0.32
SOLOR STREAM SAMPLE
1-40
I ASCORB I C AC I O
20 TURN C O I L
I
1 0. 16
FORMALOOXIME EDTA-HYDROXYLAMINE
1 0.16
AC I D WASH
-
3.90 RESERVOIR
c TO
3.90
SAMPLE ASS IS T
I.oo
P U L L THROUGH
0.80
P U L L THROUGH
0.80
BUFFER
0.42
AIR
0.32
SAMPLE
1-40
F L O W CELLS: 5 0 mm
SAMPLE PROBE RESERVO I R
T O WASTE A T O WASTE
F 1 LTERS: 480 nm
- -rn WASTE * TO WASTE L
I "
BLANK STREAM
1 I
FORMALDOX I YE
2 0 TURN C O I L
1
20 TURN C O I L
I
0. I 6
ASCORBIC A C I D EDTA-HYDROXYLAMINE
-i
I
0.16
I
A C I D WASH-FROM
2 0 TURN C O I L
I
0.16
I
I
0.16
I
I
Figure 1. AutoAnalyzer I1 manifold for manganese
Several other cations react with formaldoxime to give a pink color, and these reactions must be suppressed. Ferrous ion was selected as the test cation for evaluating the effectiveness of the system in controlling cation interference because iron is invariably found with manganese in natural waters and usually at a higher concentration. Ascorbic acid eliminated approximately 80% of the ferrous ion interference at the 5 mg/L Fe level, but three other complexing agents were required to reduce the ferrous ion interference to an acceptable level: ammonia, EDTA, and hydroxylamine. It proved necessary to develop a blanking system to compensate for the natural color of samples and to complete the suppression of ferrous ion interference. Surface and ground waters are often colored due to the presence of humates, lignins, tannins, iron and/or manganese, and these color components contribute to absorbance values at 480 nm, the wavelength selected for the manganese-formaldoxime color measurements. When actual samples were adjusted to pH 10.0 (the pH of the system) their absorbance values at 480 nm ranged from nil to an equivalent of 0.050 mg/L Mn. As full scale for the designed system was 0.200 mg/L Mn, the need for a blanking system was evident. Goto et al. (3) noted that manganese reacted with EDTA instead of formaldoxime provided EDTA wm added before formaldoxime, and this fact was utilized to develop an automated blanking system. Technicon AAII colorimeters are dual-beam, and permit synchronizing a blank or reference stream with the colordevelopment stream; the output signal is directly related to the difference in absorbance between the two streams. For the subject system, the reference stream included the sample and all the reagents at the same concentrations found in the color stream, but the order of addition was altered so that EDTA was introduced before formaldoxime. The validity of
Table I. Manganese-Calibration Showing Suppression of Ferrous Ion Interference TheoretMeasured Mn concn, mg/L ical Mn concn, 5mg 10mg 1mg 2mg 0 mg Fe/L Fe/L Fe/L Fe/L Fe/L mg/L 0.025 0.025 0.024 0.025 0.023 0.025 0.050 0.048 0.049 0.048 0.048 0.049 0.100 0.100 0.100 0.098 0.096 0.098 0.1 50 0.150 0.150 0.150 0.150 0.150 0.180 0.180 0.179 0.180 0.179 0.182 this blank was tested as follows. First, the response of the color stream was measured when distilled water was pumped through the reference stream: the peak heights for a 0.100 mg/L Mn standard and for a 5 mg/L Fe solution corresponded to 0.100 mg/L Mn and 0.015 mg/L Mn, respectively. Second, the response of the reference stream (inverse mode setting on the AutoAnalyzer I1 colorimeter) was measured when distilled water was pumped through the color stream: the peak heights for a 0.100 mg/L Mn standard were equivalent to
