Radio Release Determination of Dichromate Ion in Natural Waters HAROLD G. RICHTER’ and ARTHUR S. GILLESPIE, Jr. The Research Triangle Institute, Durham, N.
C.
b A radio release procedure is presented for the measurement of dichromate ion concentrations in natural waters. On reacting the acidified sample with radioactive silver metal, silver ions are stoichiometrically produced in the sample. The resulting count rate of the solution is directly proportional to the original dichromate concentration. The two commonly occurring ions which interfere with the reaction, Fe+3 and CI- are easily accommodated in the reaction system. The limit of sensitivity of the procedure, with the radio silver used for these studies, is 10 pg. of Cr(VI) per liter.
The method of measuring dichromate concentrations in natural waters described here makes use of the “radio release” technique. In this technique, nonradioactive ions (or elements) of interest are caused to react with an insoluble radioactive compound (1 2). The radioactive compound is chosen such that its reaction with the ion of interest releases a stoichiometric quantity of the radioactive species into solution. Measurement of the radioactivity of the solution is then a measure of the concentration of the nonradioactive ion of interest. The technique is useful for measuring very small concentrations of ions by the radioactive species. The chemical reaction employed in the work reported here is that between dichromate ion and silver metal. The following equation describes the reaction in acid solution.
S
is used routinely in France to measure the volume flow rate of streams. The technique is standard and straightforward: a concentrated solution of sodium dichromate is injected into a stream a t a known rate. The waters are sampled downstream where the dichromate concentration has reached a uniform concentration. Heretofore, the concentration of dichromate in the injected solution and in the collected samples has been determined by colorimetry. Knowing these concentrations and the injection rate of the dichromate solution, the volume flow rate can be calculated. The quantity of sodium dichromate injected usually is 1-2 kg. of the salt per each cubic meter per second estimated flow rate for mountain rivers tested in France, and for flow rates below 50 cubic meters per second. Dichromate Concentrations are then of the order of 1 mg. of the ion per liter in the collected samples. This concentration is about the minimum one can measure conveniently by present colorimetric methods. The size or flow rate of the stream which can be measured by the colorimetric technique is limited, however, by both the cost of the dichromate and the logistic problems of getting large quantities of sodium dichromate to a difficultly accessible injection point. A more sensitive technique for measuring dichromate ion concentrations in natural waters is described here. ODIUM DICHROMATE
The Centre d’Etudes Nucleaires de Grenoble (Isere) France. On leave from the Research Triangle Institute. 1 146
ANALYTICAL CHEMISTRY
~
Cr20,-2
+ 6Ag + 14H+ -+ 6Ag+ + 2Cr+3 + 7H20
The radioactive isotope Agliom (half life = 253 days) which emits a mixture of beta and gamma rays, is desirable for this work. If solutions containing dichromate ion are made acidic and then reacted with metallic silver-1 lOm, the solution will contain silver-1 10m ions in direct proportion to the original concentration of the dichromate ion. Counting a portion of the solution enables the concentration of the dichromate to be determined. EXPERIMENTAL
Apparatus and Reagents. .4 scintillation counter having a 2- X 2-inch sodium iodide crystal is used for counting samples. The radioactive silver is 50-mesh gauze in the form of a rectangular stirrer paddle. A central hole is made in a 0.5- X 3-cm. piece of the gauze so that it can be affixed to the end cf a threaded polystyrene stirring shaft by means of two small polystyrene nuts. The gauze is then irradiated in a nuclear reactor. (The silver used for this work was irradiated in the Grenoble reactor “Melusine” for 48 hours a t a flux of approximately 10l2neutrons per square centimeter-second. This should produce a specific activity of Agllomof 1 mc. per gram, but because of the neutron distribution in Melusine, and the ab-
sorption spectrum of Ag109,the specific activity of the ilgllom is approximately 10 mc. per gram.) After irradiation, the gauze is attached to the shaft which can be rotated at 250-300 r.p.m. by a small motor. The NH4F.HF solution was the reagent grade salt with a concentration of 300 mg./ml. The HzSOd and ”,OH used were concentrated, analytical grade reagents. Procedure. Measure 100 ml. of the sample to be analyzed into a 250ml. beaker of Teflon. Add by pipets 1 ml. of N H 4 F . H F solution and 1 ml. of concentrated &SO,. Stir the ~ o l u tion with the silver stirrer for 1 hour. At the end of this time, add by pipet 5 ml. of concentrated XH,OH, stir for 1 minute, and then remove the solution from the stirrer. Place the solution, or a known portion of it, on the scintillation counter and record the counting rate. For absolute measurements, run a standard solution of dichromate under the same conditions. For measuring the flow rate of streams, if the concentrations of the collected sample and the (diluted) injection sample are similar, only the ratio of the concentrations (counting rates) is necessary. I t is more accurate, however, for this latter application, to run a sample of the stream water collected before the dichromate injection and use this (counting rate) as a blank. RESULTS A N D DISCUSSION
I n the development of this procedure for dichromate, simplicity of manipulations was deemed desirable so that it could be used routinely by unskilled technicians. At the same time, it was desired to measure dichromate ion a t very small concentrations. Since there are many other ions in natural water, the two goals seem at first too incompatible. However, preliminary work had shown that there are usually only three ions in unpolluted streams which may interfere with the dichromatesilver reaction : chloride ion, ferric ion, and nitrate ion. Following is a discussion of the reasons for choosing the particular reagents given above to minimize these potentially interfering ions. Chloride Ion. Chloride ion can cause difficulties in the radio release procedure for dichromate ion by precipitating silver chloride. If the silver chloride were to remain suspended
in the solution, there would be no problem because the sample of solution taken for counting would be representative of the original dichromate ion concentration. If, on the other hand, the silver chloride were to remain on the stirrer, or on the walls of the vessel, then a sample of t’he solution would not be a measure of the initial dichromate concentration. This latter situation is the one which occurs so it is necessary to complex the silver so that it remains in the solution. There are two approaches to keeping silver in solution: (1) To complex it as soon as it is formed (thus, silver chloride is never precipitated) (2) To allow the reaction to go to completion after which the precipitated silver chloride is dissolved by a suitable reagent. For the first approach, many reagents were tried, but only two commercially available compounds were successful : thiosinamine and 1,3-diethy1-2-thiourea. These can keep silver complexed in acid chloride solutions. Unfortunately, dichromate in such solutions slowly oxidizes these compounds so that a t lower pH values the results are not reproducible. I t is necessary to adopt the second approach-that of allowing the reaction to be completed-then attempt to dissolve the silver chloride from the silver stirrer. Of the several reagenbs tried for this purpose, ammonia was the most successful. Thiosulfate may be satisfactory in basic solution, only if it is free of sulfide ion, while cyanide in basic solution tends to dissolve some of the silver metal, thus giving high blank values. Coating of the stirrer with a thin impervious layer of silver chloride so that remaining dichromate in the solution cannot react does not appear to be a problem, as quantitative result8s have always been obtained with solutions up to 0.5M in chloride ion. Ferric Ion. Ferric ion can oxidize silver metal in acidic solutions. Most streams do not contain ferric ions as such because the p H values are above those a t which ferric hydroxide precipitates. But when t)hese waters are made acidic and ferric hydroxide may dissolve, or ferric ion may be leached from suspended matter, so t h a t ferric ion may cause difficulties in this radio release technique. Since the solution is quite acidic, and because dichromate is a good oxidizing agent, many organic agents capable of complexing ferric ion are not useful in the solution. Of the inorganic ions capable of complexing ferric ion, only two are satisfactory: phosphate and fluoride. Both are satisfactory, but fluoride seems to be somewhat better than phosphate. For much of the early work in development of this procedure, phosphoric acid was used to make the solutions the desired pH values and at the same time complex
the ferric ion in solution. When it later developed that the use of sulfuric acid was advantageous (discussed below), fluoride ion was added to the solutions to reduce the interference of ferric ion more than phosphate. In solutions prepared as described in the procedure, 10 mg. of Fe+3 per liter produce the same activity of silver-ll0m as does 50 pg. of Cr+6 per liter. Nitrate Ion. S i t r a t e ion was thought to be a potential problem because it can oxidize silver metal in acidic solutions. I t could be present in streams either from biological action or by having been leached from fertilized fields. Experiments using K H 4 N 0 3solutions show t h a t 5 mg. of NO3- per liter produce an activity of silver-ll0m equal to less than 3 pg. of Cr+6 per liter. Nitrate ion then does not interfere unless it appears in higher concentrations. Organic Material. Suspended organic materials, such as algae, decayed plant material, etc., may interfere by reducing the Cr+B ion. This interference is not serious except at very low levels of dichromate concentration or for extended delays between stream dosing and sample assaying. Methods of circumventing this problem are now being investigated in a separate study. Effect of pH and Stirring Time. The reaction between dichromate ion and silver metal is complete in reasonably short times only if the p H is less than 3.0. Many experiments were carried out at p H 2.5, for which stirring times of 2.5 hours are required. These experiments were with H3P04. Lower p H values required correspondingly larger amounts of the acid, for which volume corrections became necessary. With HzS04, however, much smaller volumes of acid are required to achieve a given p H value. It was found that identical results are obtained for a given solution of dichromate ion in stream water from pH 2.5 down to p H 1.0 using either HzSOc or H3P04. It was further found convenient to add 1 ml. of HzS04to a 100-ml. volume of stream water (containing 300 mg. NH&’ H F ) , and the pH was then 1.5 f 0.1 units. It is thus not necessary t o use a pH meter. At 1.5, 1 hour of stirring time is sufficient for complete reaction. Reproducibility, Precision, and Sensitivity. It is possible to present here details of the reproducibility and precision using samples of natural waters to which known quantities of dichromate ion have been added. Table I shows the counting rates obtained when 2 ml. of a standard solution of dichromate were diluted to 500 ml. with various natural waters, and 100 ml. of the resulting solution was then treated with and NH4F H F according to the method given in the
Table Rates
I. for
Reproducibility of Counting a Standard Dichromate Solution
Run No.
Counts per sec.
1 2
3 4 5
Av .
1670 1640 1654 ~
Table II. Reproducibility of Counting Rates for Blank Samples
Run N o . 1 2
3 4 5
Av .
Counts per 103 107 107 105 109 -
sec.
106
procedure. These results were obtained over a period of several weeks, using both HzS04and H3P04in the pH range 1.0-2.5. Each run was made on a different standard solution made up at a different time with different water. Standard counting time was 100 seconds which represented a total of about 160,000 counts. Table I1 shows the counting rates of various “acid blanks” (samples of stream waters containing N H 4 F .H F made to p H 1.5-2.5 with HzS04 or HBPo4 and with 1 to 3 hours of stirring. The counting rates for the blank samples are high. I t must be mentioned that no attempt has been made to optimize the counting arrangement. The scintillation counter used for these studies is in a chemical laboratory in which are also other radioactive sources more or less well shielded in lead bricks. The counting rate of the instrument with no sample is about 60 counts per second; the net blank counting rate is therefore about 46 counts per second. The numbers from Table I and Table 11, together with the concentration of the standard dichromate solution used for obtaining data of Table I , enable one to calculate the sensitivity of the procedure : The dichromate solution is 0.0500 mg. of Cr(V1) per milliliter, 2 ml. of which diluted to 500 ml. gives a solution of 0.200 mg. of Cr(V1) per liter. This concentration of Cr(V1) produces a net counting rate of 1548 (= 1550) counts per second. A solution containing 0.01 mg. of Cr(V1) per liter therefore produces 77.5 counts per second above the acid blank. In one field experiment, the measured dichromate concentration was 0.03 mg. of Cr(V1) per liter, a concentration which, from calculation, gave a volume flow rate for the stream within 3% of that determined by colorimetric means. The VOL. 37, NO. 9, AUGUST 1965
1 147
stream flow was 7 cubic meters per second. I t is convenient to clean the silver gauze stirrer between experiments with a KCN solution. Stirring of the K C N (3 grams of the salt per 100 ml. of HzO) for 1 or 2 minutes, followed by three 5-minute rinses with water, has always produced a bright, active stirrer which gives reproducible results. Actually, in the experiments described here, 2 pieces of gauze were irradiated simultaneously in the reactor. Their activities are identical and a sample and an “acid blank” can be carried out
together. These stirrers show no signs of deterioration after having been used for more than 100 samples or blanks with KCN cleanings between each use. ACKNOWLEDGMENT
We acknowledge the assistance of K. L. Fletcher who helped with many of the preliminary measurements. LITERATURE CITED
(1) Gillespie, A. S., Richter, H. G., ANAL. CHEM.36,2473 (1964). ( 2 ) Gillespie, A. S., Richter, H. G.,
Trans. Am. Nuclear SOC. 5 , No. 2, 273 (1963).
RECEIVEDfor review April 15, 1965. Accepted May 24, 1965. Part of the work was carried out at the Research Triangle Institnte, Durham, S . C., under IJ. S.Atomic Energy Commission, Division of Isotopes Developments Contract No. AT-(40-1)-2513. The remainder of the work was performed at le Centre d’Etitdes Sucleaires de Grenoble, in the Section d’Application des Radioelements, where one of 11s ( I f . G . R . ) was given the freedom to pursrie the atitdy and records here his appreciation of the cooperation from that groiip. H. G. Richter also expresses his appreciation for a 1964-65 Fulbright Grant.
Solvent Extraction Studies with a Hydrochloric Aci d-2 - Ethy I hexa noI Systern K. A. ORLANDINI, M. A. WAHLGREN, and J. BARCLAY‘ Argonne National Laboratory, Argonne, 111.
b The distribution of 45 elements between 83% 2-ethylhexanol-1770 petroleum ether and hydrochloric acid solutions has been determined. A graphic presentation of the extraction data as a function of acid concentration is given, Very effKient extraction of Ti(lV), V(V), Ru(VIII), Sn(lV), Pa(V), Nb(V), Ta(V), Ga(lll), Fe(lll1, Sb(V), Au(lll), As(lll), Np(VI), and Mo(VI) was achieved. O f the remaining elements studied, only W(VI), Zn(ll), U(VI), Tc(VII), Cd(ll), Hg(ll), and Pd(ll) were appreciably extracted. Separations within the group of well extracted elements can b e accomplished by back-extraction utilizing selective reduction, complexation, and variation of acid concentration. The extracted elements are readily stripped into dilute acid solution or water. A number of applications of the system to activation analysis as well as other radiochemical separations, including preparation of some carrier-free samples, are given.
I
ALCOHOLS have not been widely applied as extractants for inorganic species as have ethers, ketones, and more basic extractants such as the high molecular weight amines. Alcohols are often used as diluents for other extractants (2) and as solvents for chelate extractions. Distribution studies between hydrochloric acid and a n immiscible alcohol have been used to determine ionic species in hydrochloric acid solution ( 1 , 3 ) . I n many analyses the samples are solubilized in strong mineral acid MMISCIBLE
1 Present address, University of California, Berkeley, Calif.
1148
ANALYTICAL CHEMISTRY
solution, while the optimum conditions for specific separations occur in the p H range 0-12 \\here masking and p H control are most effective I n radiochemical procedures, a preliminary separation step is often used to eliminate the bulk of interfering activity and consequent radiation hazard in a simple, remote operation. The hydrochloric acid-alcohol system t o be described is very suitable for this type of separation because of the rapid equilibrium, ease of manipulation, and high solute capacity. I n the case of sequential multielement analysis, the separated fractions are free in most cases of excess reagents and salts which may interfere in a following analytical step Many of the elements covered in the present study extract into various oxygenated solvents such as ethers and ketones, but 2-ethylhexanol is superior to these reagents in several respects. The low solubility (0.33y0 in concentrated HCl) of the alcohol is a distinct advantage over ether and ketone systems. Having a flash point of 85” C., and an azeotrope with water boiling at 99” C., gives the alcohol system stability as well as ease of evaporation. The low vapor pressure a t room temperature permits one to carry on experiments without significant solvent loss and with low flammability hazard. The latter is of particular concern when working with highly radioactive samples. Of some 45 elements studied, 14 were well extracted (75-9975) while nine were extracted in the 30-60Oj, range and would be considered interferences. The alkali metals, alkaline earths, and rare earths were not extracted appreciably. Nickel, bismuth, indium, scandium, silver, and copper were extracted 10%
and less; iridium (IV), ruthenium (111), chromium (111), manganese (11), and yttrium, 1% and less. EXPERIMENTAL
Reagents. 2 Ethyl-1-hexanol (practical grade) was obtained from RIatheson, Coleman, and Bell. This grade has a n assay of 98yc by gas chromatography, the remainder consisting of isomers. -411 other chemicals and solvents were reagent grade or equivalent except n-hexanol which was Eastman practical grade. Procedure. Distributions were determined using purified radioactive tracers. All extractions and Ire-equilibrations were carried out in 40-ml. centrifuge cones using battery-driven mechanical stirrers. Although phases separated well, centrifugation was employed to eliminate any suspended droplets in either phase. Aliquots were pipetted into small polyethylene bottles or dried on planchets for counting. Single channel and niultichannel analyzers were used for gamma counting, and gas proportional detectors for alpha and beta counting. Petroleum ether was used as diluent for all alcohols studied. The 2-ethylhexanol for all extractions was diluted to 83% by volume for ease of phase separation, unless otherwise noted. A small amount of cryhtalline KMnOc (30-40 mg.) was added three times during the evtraction of V, Ru, Xi), Au, Mo, and Sb tracers in order to oxidize these elements to their hi her oxidation states. Cr, Ce, hln, an! Bi could not be maintained in the maximum valence state in the alcohol-HC1 system. The organic phase was pre-equilibrated in all cases. Equal volumes (10 ml.) of organic and aqueous phases were used for the extractions. The distribution coefficients (D)given are the ratio of radioactive count rates of equal
