INSTRUMENTATION
instrument abstracts
Cary
Applied Physics Corp./362 W. Colorado
Street/Pasadena/California
at the Richfield Laboratories
Cary Model 14 Spectrophotometer enables determination of lead concentrations to one part per billion
The destructive effect of lead on the activity of costly catalysts makes accurate determinations of even minute amounts extremely important. With improved techniques now in use at the new Research Laboratories of the Richfield Oil Corporation, Anaheim, California, chemists can determine lead concentrations in naptha charge stocks within one part per billion. The conventional dithizone colorimetric procedures can be used to estimate the lead concentration to an accuracy of approximately 10 parts per billion. A refinement of this procedure, employing the Cary Model 14 Recording Spectrophotometer, is used to more precisely determine the concentration. The color intensity of the lead dithizone solution is measured at 5100 Angstroms for the unknown sample and for two standard solutions whose concentrations are respectively a little more and a
little less than the estimated concentration of the unknown. By interpolating, the analyst can then determine the lead concentration of the unknown to an accuracy of one part per billion or better. In this procedure, the high photometric accuracy of the Model 14 is of primary importance in reliably recording the minute differences in absorbance values between the sample and standards. This high photometric accuracy is one of several performance features provided in each Cary Recording Spectrophotometer to a degree not found in any other similar instruments. Perhaps these advantages can lead to new breakthroughs in your analytical techniques. Complete information on both Cary Spectrophotometers, Model 11 and Model 14, is contained in a bulletin which is available upon request. Ask for Data File A10-98.
BRIEF SPECIFICATIONS OF CARY
SPECTROPHOTOMETERS
MODEL 11
MODEL 14
RANGE
2100A to 8000Â
1860Î to 2.6 microns.
STRAY LIGHT
Less than 0.0001% over most of working range.
Less than 0.0001% between 2100A1 and 1.8 microns; less than 0 . 1 % at 1860/5 and 2.6 microns.
SCANNING SPEEDS
From l.OÂ/sec. to 125S/sec.
From 0.5Â7sec to 500Â7sec.
RESOLUTION
Better than 1.0A throughout range.
Better than 1.0Â" U.V.-visible region and 3.0Â" near-infrared.
WAVELENGTH ACCURACY
Better than 5.0Â" U.V. region and 10.0Â visible region.
Better than 4. 0Â throughout range.
REPRODUCIBILITY
Better than 0.5Â U.V. region and 3.0X visible region.
PHOTOMETRIC REPRODUCIBILITY
0.004 in absorbance.
Q
^ ^
rang(,
0.002 in absorbance.
For further information, circle number 80 A on Readers' Service Card, page 101 A 80 A
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ANALYTICAL CHEMISTRY
proper polarization of the instrument and avoids "hold-back" with negative overloads on the mid-zero ranges. Another useful instrument is Millivac's sensitive precision RMS Millivoltmeter which uses an electronically protected thermocouple as its Κ MS responsive meter-rectifier. An impor tant feature is the possibility of analyz ing the crest factor of odd wave shapes. The instrument is shown in Figure 2. 1. In operation the toggle switch is raised and by turning the calibrator vernier control until the needle is on the calibration check mark near full scale, the calibration voltage is thus chosen to within 0.2%. 2. After this operation, the toggle switch is released and the range switch is turned to a position where a needle deflection becomes visible. 3. The reference vernier control is now adjusted until the needle is ex actly on one of the five available reference points (A to E ) . 4. Finally, the toggle switch is de pressed which transfers the complete measuring circuit (range attenuator, amplifier, thermocouple and meter) to the calibrator. The Dekapot-calibrator is then adjusted until the needle re turns to the chosen reference point. The correct voltage reading now ap pears on the dials of the Dekapot. When dealing with other than sinus oidal wave shapes, particularly with waves which contain sharp spikes, seri ous errors can arise in ordinary RMS voltmeters. The Millivac instrument avoids this by providing the several reference points on the dial which are associated with various amplifier load ing conditions. By repeating measure ments with different reference points, having different permissible crest fac tors, a definite limit on the crest factor scale of the instrument can be found beyond which the amplifier begins to be overloaded. Readings begin to appear too small. On the other side of this limit, all readings are identical, indicat ing· that no overloading exists and that the measurements are valid. Alterna tive methods of estimating crest fac tors would involve elaborate oscillo graphic equipment and planimeter anal ysis. This instrument has 13 ranges from 300 microvolts to 1 KV at frequencies from 10 cycles to 500 kilocycles for sine waves and 30 cps to 125 Kc for square waves. The crest factor range (square waves) is 1 to 20, the crest factor being the ratio between peak and RMS values. The output is 0.25 to 3 volts and it is possible to monitor the wave shape on an oscilloscope while measurements are being made.
