Sister Helene Ven Horst and Helen Tang Marycrest College, Davenport, IOWO
Use of the Densitometer for Photometric Measurements
The densitometer is generally considered to be somewhat of a specialized instrument due to its limited use chiefly in the areas of paper chromatography, electrophoresis, and in the analysis of films. The authors have made use of this instrument for these purposes but have likewise used it with success in such analyses where the photometer is commonly used. The advantages of this modified photometer include the following: (1) a minimum amount of solution is required for any one analysis; (2) no special cuvettes are required; (3) minimum amount of time and work are required in converting the densitometer into a photometer; and (4) the time for an analysis does not exceed that required in the usual photometric procedure. A model 525 Photovolt densitometer, a Varian recorder, and a decade box were connected according to the wiring diagram in Figure 1. The densitometer
rubber tubing which led to a waste jar. By placing a pinchcock on the rubber tubing it was possible to regulate or stop the flow of solution, either for purposes of analysis or in order to introduce a new solution. A diagram of this apparatus is given in Figure 2. In the analysis of any particular solution, appropriate narrow band-pass filters were inserted into the search unit of the densitometer containing the photoelectric cell. The effectiveness of this modified calorimeter was determined by analyzing serial dilutions of various colored solutions according to standard procedures. The results are given in Table 1. Table 1. Recorder Response for Serial Dilutions -
Concentration Wave length range of filtern (pammaslml) (mu)
Solution Nickel with dimethylglyoxime Iron with 1-10 ~henanthroline Manganese as permanganate Phosphorus as molybdophosphate
Figure 1. W i r i n g d i a g r o m for denritometer, recorder, and deco d e box.
was modified in the following manner in order to accommodate a short piece of 7-mm 0.d. glass tubing which contained the solution being analyzed. The original paper guide and aperture plate were replaced with a piece of heavy cardboard which fit closely against the arm rest and evenly with the edges of the top of the densitometer. A 7-mm square was cut in the cardboard so as to be directly above the light source and in line with the photoelectric cell in the search unit of the densitometer. Two 1-cm thick felt strips, 10-cm long (cut from a blackboard eraser), were glued to the cardboard to form a kind of channel for the tubing. This framework was covered with another piece of cardboard which was also provided with a 7-mm square opening positioned in line with the aperture in the lower cardboard. In order to eliminate any stray light a piece of closely woven black cloth covered the entire surface of this apparatus with the exception of the square aperture. Such an arrange ment also provided an easy means for changing the tube without disturbing the position of the aperture. One end of the glass tubing was sealed to a small funnel and the other was connected to a piece of
--
Range of recorder remon~e
0.3 to 5.0
445
6 to 92
1.0to 5.0
505
2 to 07
1 . 7 t o 8.5
550
12 to 91
0.8to 13.3
615
8 to 93
In all these cases there resulted a lmear relationship by plotting the recorder response for the serial dilutions of the standards versus concentration. The order in which the serial dilutions were analyzed did not affect the results. In some cases the order of concentrations was increased; in others the order was reversed. Flushing the tube with the solvent did not alter the
'if
Figure 2. Attachment used in cowerting denritometer for use in liquid photometry
results, provided that when the solution was being analyzed a sufficient amount (3 ml) of the solution was added to bring the recorder response to a constant reading. In an effort to compare the readings obtained on the Bausch and Lomb spectrophotometer with those of the modified densitometer, standard curves were prepared from the data shown in Table 2. Equal volumes (3 ml) of serial dilutions of iron with 1-10 phenanthroline were analyzed a t a wave length of 505 mp. Since no special effort was made to duplicate the read'mgs by adjusting the resistance of the modified Volume 36, Number 2, Februory 1959 / 81
densitometer, the results may not seem to he in good agreement; however, it is to be noted that the slopes of these curves are nearly identical. It is apparent that with a minimum change in the densitometer this instrument is readily adapted for use in photometric analyses. This modified instrument has its advantages, primarily that of its extended use in the laboratory and the absence of any cost in making the modification. The authors gratefully acknowledge the financial assistance of the American Academy of Arts and Sciences.
82
/
Journal of Chemical Education
Table 2. Comparison of Absorbance of Spectrophotometer and Recorder Response of Modified Densitometer in Analysis of Iron with 1-10 Phenonthroline
Concentration (gammas/ml) 1
2 3
4
Absorbanoea 0.24 0.45 0.68 0.90
Recorderb response 0.28 0.53 0.74
0.97
" Slope of ourve plotting absorbance vs. concentration 0.22i.
*Slope of curve plotting recorder response vs. concentration
0.225.
