Abstract:
A method for automatically determining the concentration of nitrite in a liquid sample includes:
       determination of the extinction of the liquid sample at a wavelength λ of 150-250 nm,   addition of a nitrite reducing agent to the liquid sample,   determination of the extinction of the reduced liquid sample at a wavelength λ of 150-250 nm, and   determination of the nitrite concentration from the difference between the concentration values obtained from the extinctions of the non-reduced and the reduced liquid samples.

Description:
BACKGROUND 
     The invention relates to a method and a device for automatically determining the concentration of nitrite in a liquid sample. 
     Such methods and devices are employed, inter alia, in waste water clarification plants for monitoring and controlling the clarifications process. In the known measuring methods and devices, the extinction of the liquid sample of the waste water is determined with the aid of UV radiation. Using the extinction value obtained in this manner, the concentration of the sum of nitrite and nitrate is calculated. Since the spectral curve shapes of the extinction of nitrite and nitrate show large similarities, photometric means virtually do not allow for an exact differentiated determination of nitrite or nitrate in a liquid sample containing both nitrite and nitrate. However, monitoring or controlling the correct process of nitrification, i.e. the microbiological oxidation of ammonium over nitrite using Nitrosomonas, and the subsequent microbiological oxidation of nitrite to nitrate using Nitrobacter, requires individual determination of both the concentration of nitrite and the concentration of nitrate. 
     It is thus an object of the invention to provide a method and a device for automatically determining the concentration of nitrite in a liquid sample possibly containing nitrate. 
     SUMMARY 
     The method for automatically determining the concentration of nitrite in a liquid sample according to one aspect comprises the following method steps:
         determination of the extinction of the liquid sample at a wavelength λ of 150-250 nm,   addition of a nitrite reducing agent to the liquid sample,   determination of the extinction of the reduced liquid sample at a wavelength λ of 150-250 nm, and   determination of the nitrite concentration from the difference between the extinctions of the non-reduced and the reduced liquid sample.       

     During the first extinction determination step, the concentration of the sum of nitrite and nitrate in the liquid sample is determined with the aid of the UV photometry, as is known from prior art. Subsequently, an adequate amount of a nitrite reducing agent is added to the liquid sample, i.e. in a amount at which a complete nitrite reduction takes place. Thereby the nitrite is completely expelled as nitrogen from the liquid sample. 
     Preferably, this process also allows the nitrate concentration to be determined on the basis of the extinction determination of the reduced liquid sample. The reduced liquid sample does no longer contain nitrite, but exclusively contains nitrate. Therefore the extinction determination of the reduced liquid sample shows the concentration of nitrate in the liquid sample. 
     Generally, different nitrite reducing agents can be used for reducing purposes, for example ammonia, hydrazoic acid, urea, amidosulphuric acid, etc. Preferably, amidosulphuric acid is used as a nitrite reducing agent since said acid does not show any self-extinction in the monitored spectrum, is not volatile, is not explosive, and is relatively stable. Amidosulphuric acid is thus suitable particularly in an automatic process where the nitrite reducing agent must be stored in a storage tank for an extended period of time. 
     Preferably, the liquid sample is mixed in a suitable mixer after the addition of the nitrite reducing agent. Thereby the reduction of the nitrite is accelerated, and a homogeneous mixing of the liquid sample with the nitrite reducing agent is reliably ensured. Preferably, the photometric determination of the extinction takes place at wavelengths λ=213 nm and λ=223 nm. Two measurements at different wavelengths differentiate between nitrate and/or nitrite, and other substances. The spectrum of nitrite and nitrate shows its largest upward slope between approximately λ=210 nm and λ=230 nm. Two measurements in the region of this upward slope allow for a reliable differentiation between nitrite and nitrate, and other substances which are photometrically active in this region. 
     The determination device according to one aspect for automatically determining nitrite in a liquid sample comprises a measuring chamber for receiving the liquid sample, a sample transporting device for supplying and discharging the liquid sample to and from the measuring chamber, a photometer for determining the extinction of the liquid sample in the measuring chamber, and a reducing agent adding device for feeding the nitrite reducing agent to the measuring chamber. By provision of the reducing agent adding device, a nitrite reducing agent can automatically be added to the liquid sample in the measuring chamber subsequent to the first extinction determination step, said nitrite reducing agent fully expelling the nitrite from the liquid sample. The determination device thus allows the described process for determining nitrite in a liquid sample to be carried out automatically. 
     According to a preferred embodiment, the measuring chamber is defined by a pivoting fork movable in a gap, and the gap walls, wherein the pivoting fork is adapted to be pivoted out of the gap for the purpose of receiving a new liquid sample. The pivoting fork comprises two fork arms defining an open space between said arms which extends perpendicularly to the base plane of the pivoting fork. The fork arms may be unconnected with each other at their free arm ends, but may also be connected with each other such that they define a closed ring around the measuring chamber. The two sides extending perpendicularly to the base plane of the pivoting fork are defined by the opposing fixed gap walls. This measuring chamber structure is particularly suitable for determination devices configured as immersion probes which are adapted to be directly immersed into a clarification basin for the purpose of continuously determining the concentration of nitrite and nitrate. 
     When the pivoting fork is pivoted out of the gap, the moving ambient liquid causes the liquid sample surrounded by the pivoting fork to be automatically exchanged for a new liquid sample, said new liquid sample being isolated in the measuring chamber when the pivoting fork is pivoted back into the gap. Thus pumps susceptible to malfunction are not required for supplying and discharging a liquid sample. 
     Preferably, the two opposing gap walls each comprise a photometer window of quartz glass. The distance between the two photometer windows is the measuring length. The measuring radiation enters into the measuring chamber through the one photometer window, wherein the liquid sample partially absorbs the measuring radiation depending on the constituents in the liquid sample. The measuring radiation leaves the measuring chamber through the opposing photometer window, and impinges onto a wavelength-selective receiver of the photometer which determines the extinction in a wavelength-selective manner, e.g. for two different wavelengths in the UV range. 
     Preferably, the pivoting fork comprises a mixing tongue which is elastically movable relative to the pivoting fork. The mixing tongue is adapted to be deflected and moved relative to the pivoting fork by its inertia and/or by suitable snap-in elements arranged at the gap walls. Relatively small movements of the pivoting fork after the addition of the nitrite reducing agent cause the liquid sample to be rapidly mixed by the moving mixing tongue, and the nitrite in the overall volume of the liquid sample to be quickly expelled. In this manner, the concentration of nitrite can be quickly determined, and a high measuring frequency, i.e. a rapid measuring sequence, is ensured. 
     According to a preferred embodiment, the windows are arranged in the base plane of the gap walls, and the pivoting fork cleans the windows while moving past them. Each time the pivoting fork receives a new liquid sample, it cleans the two optical windows, thus ensuring that the windows are clean during the next extinction determination step, and that the determination can be carried out in a trouble-free and faultless manner. The pivoting fork is made from a relatively soft material, e.g. a plastic material, which does not scratch the windows, but cleans them without leaving any residues. 
     Preferably, the pivoting range of the pivoting fork is shielded by a liquid-permeable cage. Thus larger particles of the liquid are prevented for entering into the pivoting range such that it is nearly precluded that the pivoting fork gets damaged or jammed in the gap. 
     Alternatively or additionally, the pivoting fork, in its pivoted-out sample exchange position, may be arranged in an open gap defined by two gap walls, said gap preventing to a large extent larger solid particles from entering into the interspace defined by the fork. 
     Preferably, the device for automatic determination is configured as an immersion probe which may be directly and permanently arranged in a liquid tank, such as a clarification basin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described in greater detail with reference to the drawings. The drawings are presented for illustrative purposes and are not to be taken as limiting the claims. 
         FIG. 1  shows a longitudinal section of a determination device according to one embodiment of the invention configured as an immersion probe, 
         FIG. 2  shows a cross section of the determination device of  FIG. 1  with the pivoting fork pivoted back into the gap, 
         FIG. 3  shows the determination device of  FIG. 2  with the pivoting fork pivoted out of the gap, and 
         FIG. 4  shows a graphic representation of the extinction spectrum of nitrite and nitrate in the UV range. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a determination device  10  configured as an immersion probe which is immersed into a liquid  12 . The liquid  12  is waste water in a clarification basin. The determination device  10  serves for quasi-continuous monitoring of the nitrite and the nitrate content in the liquid  12 . 
     The determination device  10  comprises a housing  14  which is essentially configured as an upright cylinder and comprises a gap  16  located in the cylinder transverse plane and approximately in center of the cylinder, said gap  16  being defined by an upper gap wall  18 , a lower gap wall  20  and a gap side wall  22 . The housing  14  of the determination device  10  is made from metal. 
     In the housing  14  a control device  24 , a UV light source  26 , a photometer  28 , a pivoting fork pivot motor  30  and a reducing agent adding device composed of a reducing agent tank  32  and a reducing agent valve  34  are arranged. In the gap  16  a pivoting fork  38  defining a sample transporting device is pivotably supported. The pivoting fork  38  comprises two fork arms  40 , 42  which are arranged at an angle of approximately 80° relative to each other. The pivoting fork  38  is pivotably supported in a transverse plane, i.e. the slot plane, by a shaft  44  driven by the pivot motor  30 . In  FIG. 2  the pivoting fork  38  is shown in the measuring position, i.e. pivoted into the gap  16 , and in  FIG. 3  the pivoting fork  38  is shown in the sample exchange position, i.e. pivoted out of the gap  16 . 
     The two arms  40 , 42  of the pivoting fork  38  and the three walls  18 , 20 , 22  of the gap  16  define a measuring chamber  46 . 
     The two opposing gap walls  18 , 20  each comprise windows  50 , 52  of quartz glass. The windows  50 , 52  are arranged in the plane of the two opposing gap walls  18 , 20 , and define the measuring chamber  46  when the pivoting fork  38  is in the measuring position shown in  FIG. 2 . In the region of the root of the pivoting fork  38  a resilient mixing tongue  56  is arranged which has a slightly larger radial length than the two arms  40 , 42  of the pivoting fork  38 . During a pivoting movement of the pivoting fork  38 , the mixing tongue  56  snaps into a snap-in recess  58  provided in the region of the gap wall  22 . It is also possible to provide a plurality of snap-in recesses. 
     The disclosure has been described with reference to an exemplary embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
     In the upper gap wall  20  a reducing agent inlet opening  62  is provided through which the reducing agent is supplied from the reducing agent tank  32  to the measuring chamber  46  via the reducing agent valve  34 . Alternatively or additionally to the reducing agent valve  34 , a microdosing pump may be provided. 
     The pivoting fork  38  is made from a plastic material and has such a height that no interspace remains between the pivoting fork  38  and the walls  18 , 20 ,  22  of the gap  16  through which the liquid sample can flow out of the measuring chamber  46 , such that the arms  40 , 42  of the pivoting fork  38  wipe the gap walls  18 , 20 . Since the two windows  50 , 52  are arranged in the plane of the respective walls  18 , 20  of the gap  16 , the two windows  50 , 52  are also wiped and cleaned during each pivoting movement of the pivoting fork  38 . The same may apply to the mixing tongue  56 . 
     On the outside of the housing  14  a liquid-permeable metal cage  60  shielding the pivoting range of the pivoting fork  38  is provided. The cage  60  may be made of a closed-meshed wire mesh, or the like. The cage  60  prevents larger solid particles from entering into the pivoting range of the pivoting fork  38 . In this manner, a high mechanical operational safety is ensured since it is nearly precluded that the pivoting fork  38  gets jammed in the gap  16 . 
     A measuring process is performed as follows: 
     First, the pivoting fork  38  is pivoted out of the gap  16 , as shown in  FIG. 3 , and is subsequently pivoted back into the gap  16 , as shown in  FIG. 2 . In this manner, a liquid sample is supplied to the measuring chamber  46 . Now a first photometric determination of the extinction of the liquid sample is carried out, namely at the wavelengths λ=213 nm and λ=223 nm. On this basis, the concentration of the sum of nitrite and nitrate in the liquid sample is calculated. 
     Now the reducing agent valve  34  is opened, and a defined amount of reducing agent is supplied to the measuring chamber  46  via the opening  62 . By slightly pivoting the pivoting fork  38 , the mixing tongue  56  is set into movement relative to the pivoting fork  38 , and the supplied reducing agent is thus mixed with the liquid sample. 
     Amidosulphuric acid is used as the reducing agent. Nitrite can be expelled from the liquid sample according to the following chemical equation:
 
HNO 2 +(NH 2 )HSO 3 →N 2 +H 2 SO 4 +H 2 O
 
     The gap height is 1-2 mm such that the measuring chamber volume ranges approximately between 1 ml and 10 ml. An amount of less than 10 μl of the reducing agent is added. 
     The reducing agent completely expels the nitrite from the liquid sample within a few seconds. Subsequently, a second photometric determination of the extinction of the liquid sample at the same wavelengths as stated above is carried out, and the nitrate concentration is determined from the measured extinction values. Then the nitrite concentration is obtained from the difference between the sum extinction measured first and the nitrite concentration. 
     With the aid of the described process, the concentration of both nitrite and nitrate in a liquid sample can be exactly determined. 
       FIG. 4  shows a nitrate extinction curve  70  and a nitrite extinction curve  72 . As can be seen, the maxima of the two curves are spectrally very close to each other, and the two curves  70 , 72  show only a very small extinction or no extinction at all above 240 nm. This indicates that the photometric extinction determination alone does not allow for a differentiation or allows only for a very inaccurate differentiation between the concentrations of nitrate and nitrite.