13"
with and without the ellipse [in the latter case, the flame and source were kept in the same position as in the former, except that a lens was used to produce an image (1 :1 magnification) on the monochromator entrance slit].
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RESULTS AND DISCUSSION 7"
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Figure 1. Schematic diagram of experimental design
Analytical curves for both Cd and Hg were obtained with and without the ellipse. The signal levels obtained with the ellipse were about 10-fold greater than the signals obtained without the ellipse. The Cd analytical curve was linear from about to 1 pg/ml with and 10-4 to 10 pg/ml without the ellipse. The Hg analytical curve was linear from about 10-l to 100 pg/ml with and 1 to 1000 pg/ml without the ellipse. The limits of detection (defined as that concentration resulting in a signal-to-noise ratio of 2 when the instrumental time constant was 1 sec) for Cd were 0.000002 pg/ml with and 0.00002 pg/ml without the ellipse and for Hg were 0.03 pg/ml with and 0.3 without the ellipse. Therefore, with the present rather crude cylindrical ellipse, an approximate ten-fold increase in signal and an approximate ten-fold reduction in limit of detection result for both Cd and Hg. RECEIVED for review January 15, 1971. Accepted February 9, 1971. This work was supported by AFOSR (SRC) OAR, U.S.A.F., AF-AFOSR 70-1880B.
Modification of the Kjeldahl Nitrogen Determination Method William C. Urban 2765 Glen Mawr Road, Jacksonville, Fla. 32207
THREE DRAWBACKS of the usual Kjeldahl receiving solution (standard sulfuric or hydrochloric acid) are that it must be quantitatively standardized, quantitatively measured into the receiving flask, and that additional titrations and calculations are required if a blank is run. Also, when using this solution, it is necessary to have on hand standard sodium hydroxide solution (which can be difficult to keep) for the forward titration to the methyl red end point. Two drawbacks of the boric acid solutions which are frequently used are that the boric acid is volatile, introducing an error due to acid evaporation, and that additional titrations and calculations are required if a blank is run. A solution was developed to eliminate the drawbacks of both these solutions. The solution consists of an aqueous solution of p-hydroxybenzoic acid (& = 2.9 x 10P). The low ionization constant of the p-hydroxybenzoic acid makes negligible any pH differences between the blank comparison and sample receiving solutions due to small differences in the amount of liquid distilled over, and also (partly by keeping the pH high enough) makes negligible any pH increase in the sample receiving solutions due to increase in volume because of titration. The calculated pH (before distillation) of the experimental receiving solution is 2.92 at 25 O C . One hundred milliliters of this solution contains more than 0.005 mole of hydrogen ion and un-ionized acid, enough for all but the most extraordinary determinations. The final determination is done by back titrating with standard strong acid, the end point being the pH of either a simple or a blank comparison solution. 800
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
EXPERIMENTAL
The experimental receiving solution was prepared by placing into a large beaker 21.21 grams of para-hydroxy benzoic acid, then adding enough distilled water to make 3 liters of solution. (A somewhat weaker solution may be more desirable, because in a cool room the solution may become supersaturated, causing a small amount of acid to precipitate.) The simple comparison solution (which can be used when the ammonia given off by impurities in the reagents used may be disregarded) is made by pouring into a flask (the same size and shape as the sample receiving flasks) the same amount of experimental receiving solution used for the samples, then diluting it with distilled water to the final volume of the sample receiving solutions after distillation. The resulting pH is the end point pH. The blank comparison solution is made simply by running a blank. The increase in pH of the blank receiving solution due to ammonia absorbed from the blank solution will be in direct logarithmic proportion to the amount of ammonia absorbed. Therefore, using the final pH of the blank receiving solution as an end point for back titration of the sample receiving solutions automatically corrects the amount of titrant used, making unnecessary any blank titrations or calculations. To test the experimental receiving solution, ammonium sulfate was analyzed for nitrogen. In the usual manner. two samples and one blank were run. Each sample was dissolved in 250 ml of distilled water, and the solution made basic with 60 ml of 10M sodium hydroxide solution. Approximately 200 ml was distilled over into 100 ml of experimental receiving solution. The final determination was done
by back titrating with 0.1165M hydrochloric acid. The pH was determined with a Digicord “Photovolt” model pH meter.
ACKNOWLEDGMENT
The author is indebted to the Wilson & Toomer Fertilizer Company; 1161 Tallyrand Avenue, Jacksonville, Fla. 32206, for the use of their laboratory for the ammonium sulfate analysis.
RESULTS AND DISCUSSION
The experiment found a 21.15% nitrogen content for ammonium sulfate. The calculated value is 21.20%, so the error was -0.24%. In addition to requiring only one standard solution, this method offers a single space-saving procedure both for original research and for industrial laboratory use.
RECEIVED for review December 15, 1970. Accepted February 9, 1971.
Inexpensive Method for Obtaining Deuterium Nuclear Magnetic Resonance Spectra on the XL-100 NMR Spectrometer Robert E . Santini Department of Chemistry, Purdue University, Lafayette, Ind. 47907
WE HAVE DEVELOPED a method for obtaining deuterium (2H) NMR spectra from the lock channel of a Varian XL-100-15 NMR spectrometer system. The circuit enables the accumulation of spectra in a Varian C-1024 time averaging computer (TAC) while the spectrometer is operated in an HR mode. With careful adjustment of the flux stabilizer for minimum drift, up to one hundred 25-second scans may be obtained in the computer memory. The scanning ramp generated by the TAC is interfaced to the field sweep coils in the magnet gap cia the existing calibrated sweep controls. In this way it is possible to obtain calibrated sweep widths over a four decade range. Sweep times may be selected via the TAC, or alternatively, by the X-Y recorder digital electronics in the XL-100 system. The circuit provides additional variable amplification of the time averaged analog output of the TAC in order that full scale recorder traces may be produced. This circuit may be constructed for about $100 parts cost, removing the need for a substantially greater investment in custom made high frequency components for deuterium NMR. The completed circuit, including the power supply, may be housed in a 6-inch X 6-inch X 6-inch enclosure. CIRCUIT DESCRIPTION
The circuit which interfaces the sweep signal generated by the TAC is shown in Figure 1. In operation, the full 25-Vp, ramp generated by the C-1024 (J-8) is applied to the input
networks consisting of R , , R1, and A l . The output of A1 is a 5-Vp, analog to the scanning ramp. This 5-V,, ramp is applied to ABwhere it is offset symmetrically about zero volts. The offset may be precisely adjusted by RI. This adjustment procedure is necessary to ensure that the center of the sweep ramp corresponds exactly to the center of the display on the monitor oscilloscope in the XL-100 system. The sweep ramp is inverted in A S . AQand A3 operate at unity gain, thus, SImay be used to choose the direction in which the magnetic field is swept. The output of As is applied, via R l cand R l l ,to the horizontal input of the monitor oscilloscope. The horizontal sweep of the monitor oscilloscope is therefore a linear function of the field sweep. The sweep waveform is also applied to the linear sweep module cia R12,R 1 3 and , A d . The exact ratio of RU to R13 is adjusted such that the sweep ramp is about 0.5 V,,. This value is adjusted to precisely match the sweep waveform produced by the normal circuitry in the linear sweep module. This measure maintains the calibration of that portion of the linear sweep module which is utilized for deuterium NMR experiments. The modification to the linear sweep module which is required for deuterium NMR is shown in Figure 2 (see also Varian schematic No. 87-126-840). A DPDT switch is installed such that in the lower position the sweep module operates normally. In the upper position the output of A4 is applied to the sweep width divider network and a 1.9-kQ load is applied to the existing ramp generator. If the adjustment of the ratio of Rll to R13is properly done, the calibration of the sweep width divider network is preserved.
Figure 1. Sweep ramp interface circuit and A 4 = Fairchild UA-741-C or equivalent RI = 20KQ,1% R P ,Rs, Ra = 5KQ, 1 % R3, R4,R8, Rs,and RPa= lOKQ, 1 % R7 = 5KO, 10 turn, cement type pot. Rio = 4KQ, 1% RII, Ria = IKQ, 1 % Riz = 9KQ, 1 % Power supply (for complete circuit) = Analog Devices No. 902 Al,A2,A3,
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SCOPE HORIZONTAL INPUT
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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