TKN: Why are they called Missouri tablets?
As suggested in the comments, the predecessor of the method that uses &quot;Missouri&quot; tablets was developed at the University of Missouri (Columbia, Missouri, USA). I believe the first report was from 1972 and named the method as the &quot;Missouri Automated Nitrogen Method (MANM)&quot;. They also refer to it as the &quot;Missouri Technicon&quot; method because it uses a Technicon AutoAnalyzer instrument. Many variations of this method have been described, and the specific formulation of Missouri tablets is for one of them.

An automated spectrophotometric method, utilizing Technicon AutoAnalyzer modules, has been developed to determine total nitrogen in fertilizers containing only ammoniacal, nitrate, and urea nitrogen. This colorimetric system employs the Berthelot ammoniaphenate-hypochlorite reaction. A homogeneous chromous/titanous reduction system for the automatic reduction of nitrates was interfaced with the digestion unit and color development manifold, resulting in a total nitrogen system for fertilizers with an effective analysis rate of $\pu{20-25 samples/hr}$. A new sample retrieval system removes digested samples from the helix. The totally automated system was optimized to obtain the highest sensitivity for nitrogen. Data were compared for $\pu{458 samples}$ analyzed by the automated method and the comprehensive nitrogen method (CNM), 2.053–2.054. The average difference between the 2 methods (MANM – CNM) was +0.04% nitrogen. The average relative difference was 1.12%. The average value for $\pu{140 samples}$ of $\ce{KNO3}$ primary standard analyzed by the MANM was 13.84% nitrogen (theoretical content 13.85%). The results of these standards ranged from 13.58 to 14.14% nitrogen and the standard deviation was 0.11%. The relative standard deviation was 0.80%. Of fertilizer samples received for analysis, less than 1% contained insoluble organic nitrogenous material which was unsuitable for analysis by the MANM. These few samples were not included in the statistical evaluation. The MANM is rapid, accurate, and precise and is a convenient and dependable automated method for the determination of total nitrogen in fertilizers containing only ammoniacal, nitrate, and urea nitrogen.
 Source: Charles W. Gehrke, John S. Killingley, Larry L. Wall, Sr. (1972) Missouri Automated Nitrogen Method (MANM) for Fertilizers. Journal of Association of Official Analytical Chemists 55:467-480. https://doi.org/10.1093/jaoac/55.3.467


A rapid automated method for total protein nitrogen has been developed, using the Technicon AutoAnalyzer with block digestor. Ammonia is determined by the automated ammoniasalicylate reaction at a rate of $\pu{40 samples/hr}$. A number of variations in digestion parameters have been evaluated. Hydrogen peroxide was used as a digestion accelerator. A salt-acid ratio of 1:1 and a block temperature of $\pu{425 ^\circ C}$ were chosen. The catalysts evaluated included $\ce{HgO, CuSO4, SeO2,}$ and $\ce{TiO2/CuSO4}$. Maximum nitrogen recovery in the shortest time (30 mill) was achieved with HgO as the catalyst. The results from multiple analyses of 10 experimental samples of different refractoriness with the block digestor method, using $\ce{HgO}$ or $\ce{CuSO4}$ as a catalyst, compared well with the results by the official AOAC Kjeldahl method. The accuracy, precision, economy, and saving of space offered by the Missouri-Technicon block digestor method make this an attractive alternative to classical Kjeldahl analysis for large numbers of total protein determinations in samples of different refractoriness. The AutoAnalyzer cartridge manifold is simple and reproducible in its performance, and a large dilution of the sample is made on dialysis which eliminates the matrix effects from the sample digests. At least $\pu{250 samples}$ can be analyzed in $\pu{9 hr}$ with 1 instrumentation setup.
Source: Larry L. Wall, Sr., Charles W. Gehrke (1975) An Automated Total Protein Nitrogen Method. Journal of Association of Official Analytical Chemists 58:1221-1226. https://doi.org/10.1093/jaoac/58.6.1221