Patent GB-1057442-A (GB)
Title: A method of and apparatus for the photometric analysis of liquids
IPC: G01N21/75
Filed: 1963-09-27
Abstract: 1,057,442. Photo-electric analysis of liquids. K. LADENDORF, E. LADENDORF, W. LADENDORF, E. LADENDORF, P. LADENDORF and K. F. LADENDORF, [trading as FIRMA BRAN & LBBE. Sept. 27, 1963 [Oct. 1, 1964], No.38193/63. Heading G1A. [Also in Division G2] A method of photo-metric analysis of liquids consists of measuring quantities of a test liquid, an oxidizing or reducing agent or complexforming substance, and an indicator, passing them into a mixing container 28 (Fig. 1), in a first predetermined sequence by means of a programme control, to form a first reaction solution, which is passed into a measuring cuvette 31, the photoelectric measuring apparatus being used to compare the transmissivity of the measurement cuvette (plus solution) with that of a reference cuvette to obtain a "zero value" measurement for the first reaction solution, after which this first reaction solution is removed and a second reaction solution, formed by mixing previously measured quantities of test liquid and reagents in a second predetermined sequence, is passed into the measuring cuvette to obtain a measurement of the transmissivity of the second reaction solution whereby the amount of the component in the test liquid to be investigated, is determined from the value of the colour or turbidity change as a change in transmissivity compared with the stored zero value. The actual zero value may change due to contamination of the measuring cuvette and so the stored zero value may be altered to allow for this. In the measurement of, for example, the silicic acid content of a liquid, using molybdenum blue indication, the test liquid, a reducing agent and ammonium molybdenate solution (the indicator) are stored in stock containers 10, 11, 12 (Fig. 1). Quantities of each are measured by dosage cylinders 19, 20, 21. To obtain a solution for setting the measuring apparatus to zero, the reducing solution and ammonium molybdate solution are initially mixed in chamber 28. They react and any trace of silicic acid is indicated by a slight blue coloration. Addition of the test liquid has no effect except to dilute the solution to the required volume of the measuring cuvette 31, into which the prepared solution is passed. The light from source 37 passes through cuvette 31 and a reference cuvette 36 into two photo-electric cells, 35, 34, which together with resistors 40, 41, Fig. 2 (not shown), form a Wheatstone Bridge arrangement. An A. C. voltage 42 is applied to the bridge at 43, 44 and the bridge is balanced by potentiometers 48, 49, driven by motors 50, 51 respectively. In the zeroing procedure, potentiometer 49 is set to approximately a middle position and potentiometer 48 to its extreme left position, and the switch 57 connects - the amplifier 52 to point 46. If the bridge is out of balance due to the presence of slightly-coloured liquid in cuvette 31, the balance is restored by rightward movement of the potentiometer 48, this movement being transmitted to a recorder by a coupled potentiometer 53. The solution in cuvette 31 is emptied to be replaced by a second solution, prepared by the reaction of the reducing solution with a silicon molybdate complex of ammonium molybdate and test solution, the transmissivity of which depends on the amount of silicic acid in the test liquid. Measurement is carried out as before, the potentiometer 48 moving further to the right to obtain a balance. When the zero is to be checked in order that contamination of the cuvette 31 may be compensated for, a fresh "zero-value solution" is passed into cuvette 31 and the switch 57 connected to point 47 thus short-circuiting the potentiometer 48. A balance is obtained by movement of potentiometer 49 and the original setting of potentiometer 48 is thus retained. Contamination of the cuvette 31 may be indicated by a signal element 45 influenced by the tap 49', and cleaning of the apparatus may be considered necessary. In a second embodiment, Fig. 3 (not shown), compensation is carried out by the swing- ing of a diaphragm between the reference cuvette 36 and the photo-electric cell 34, and controlled by motor 51. A third embodiment, Fig.4 4 (not shown), uses only one cuvette, for measurement and for reference, the photo-electric cell being replaced by a resistor; zero point compensation is again provided by the potentiometer 49. A fourth embodiment, Fig. 5 (not shown), is similar to the third except that a diaphragm is used for zero-point compensation. The method, which can also determine turbidity variations, can be used for analytical measurement of other liquids, of which examples are given. Continuous flow of the solutions through the measuring cuvette 31 can be provided, by use of magnetically controlled valves, controlled in a predetermined time sequence, and this prevents contamination of the cuvette, obviating zero-point compensation.